the reznor gas-fired space heating handbook gas fired space h… · heating with natural gas and...

SPACE HEATINGHANDBOOK

The Reznor Gas-Fired

$15.00

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CONTENTS

1. BASIC CONCEPTSHeat transfer 1-3

Convection 1-3Conduction 1-3Radiation 1-3Building heat loss and insulation 1-4

CombustionProcess of combustion 1-5Requirements for combustion 1-5Composition and combustion of

natural gas 1-5Complete and incomplete combustion1-5Controlled and explosive combustion 1-6Odorants 1-6Flammability limits 1-7

2. PRINCIPLES OF COMBUSTION HEATERS

Orifices 2-7A good burner flame 2-7

Flashback and extinction pop 2-7Burner operation at high altitude 2-7

Heat exchangers 2-8Indirect-fired 2-8Direct-fired 2-8

Ignition systems 2-9Manual pilots 2-9Automatic pilots 2-9Carryover 2-9

Controls 2-9Space-temperature controls 2-9Thermostats 2-10

Location of thermostats 2-10Thermostat adjustment 2-10Safety controls 2-10Flame-proving systems 2-10Safety valves and gas valves 2-11

Solenoid valves 2-11Hot-wire valves 2-11Motorized valves 2-12Diaphragm valves 2-12High-temperature limit controls 2-13Indirect-fired systems 2-13Gas-pressure regulators 2-13Proving switches 2-13

Venting systems 2-14Flue products 2-14Flue collectors 2-14Vent dampers 2-14Gravity and power venting 2-14Multiple venting 2-14

3. GENERAL APPLICATION AND SELECTION CRITERIA

Duct systems for unit heaters 3-7Duct furnaces 3-7

Combustion air 3-7Duct systems 3-8

Air-flow & heat-distributionconsiderations 3-8

By-pass ducts 3-8Insulation 3-8Controls 3-8

Gas-control systems 3-8Drying applications 3-9

Packaged units 3-10Combustion air 3-10Duct systems 3-10Insulation 3-10Filter requirements 3-10Heating-only use 3-10Sizing 3-10Pilot ignition 3-10Gas-control systems 3-11Air controls 3-11

Heating and/or makeup-air use 3-11Makeup air 3-11Sizing 3-11Pilot ignition 3-11Gas-control systems 3-11Air controls 3-12

Use with cooling systems 3-12Mechanical cooling 3-12Evaporative cooling 3-12

Direct-fired combustion 3-13Makeup air 3-13Combustion/dilution air 3-13Duct systems 3-13Exhaust/relief 3-13Makeup-air-only 3-13Sizing 3-13Process makeup air 3-13Building makeup air 3-14Gas controls 3-14Air controls 3-15

Heat PhysicsThe nature of heatTemperature 1-1Temperature scales 1-1Temperature conversion formulas 1-2Heat quantity 1-2Measuring heat quantity 1-2The calorie 1-2The British Thermal Unit 1-3

Gas measurementVolume 1-7Pressure 1-7Specific gravity 1-7Heat content 1-8Heating value 1-8

Three basic types of heatersIndirect-fired heaters 2-1Direct-fired heaters 2-1Infrared radiant heaters 2-1

High intensity 2-2Low intensity 2-2

Relative efficiencies 2-2Basic system components

Gas burners 2-3Yellow-flame burners 2-3Blue-flame (Bunsen) burners 2-3Atmospheric burners 2-4Power burners 2-5

Forced draft and induced draft 2-5Burner designs 2-5

Indirect-fired 2-5Direct-fired 2-6

Infrared 2-6High-intensity 2-6Low-intensity 2-6

Selection considerationsFuel availability 3-1Occupancy 3-1Building function 3-1

Application and equipment considerationsIndirect-fired combustion 3-1Vent systems 3-1Combustion air 3-1Unit heaters 3-2

Fans and blowers 3-3Location 3-3Sizing 3-3Typical air throw (unit heaters) 3-4Multiple-unit systems 3-5Paddle fans 3-5Controls 3-5

Pilot-ignition systems 3-6Gas-control systems 3-6

Unit heaters in low ambienttemperatures 3-6

Heating- and makeup-air use 3-15Sizing 3-15Gas controls 3-15Air controls 3-15

Heating units with cooling systems 3-15Mechanical cooling 3-15Evaporative cooling 3-15

Infrared heating 3-16Combustion air 3-17Exhaust/relief 3-17High-intensity 3-17

Sizing 3-18Location 3-18Whole building or spot heating 3-18Controls 3-18

Low-intensity 3-18Sizing 3-18Location 3-18Controls 3-18Whole-building or spot heating 3-18

Example applicationsIndirect-fired heating 3-19Unit heaters 3-19

Manufacturing plant 3-19Greenhouse 3-19School building 3-20

Duct Furnaces 3-20Commercial 3-20

Packaged systems 3-21Manufacturing, heating only 3-21Manufacturing, heating and

makeup-air with cooling 3-21Separated combustion 3-22

Direct-fired combustion 3-23Kitchen makeup air 3-23Factory heating & makeupair 3-23

Infared radiation 3-24High intensity 3-24

Manufacturing plant 3-24Spot heating 3-24

Low intensity 3-25Whole-building heating 3-25Assembly-line heating 3-26

4. TECHNICAL GIUDE

Optimizing distribution systems 4-7Optimum forced-air equipment sizing 4-7Optimum infrared equipment sizing 4-7Zoning 4-7Cost/benefits 4-7Optimum equipment location 4-7Coverage 4-7Optimum duct system 4-8Control system requirements 4-8Selecting equipment 4-8

Safety and maintenanceStandards, codes and testing 3-27Industry standards 3-27

ANSI 3-27CSA 3-17

Codes 3-27National Fire Protection

Association 3-27National Fuel Gas Code 3-27Insurance codes 3-27Certification testing 3-27

AGA 3-27UL 3-28ETL 3-28CGA 3-28

Independent labs 3-28Local code requirements 3-28Clearances 3-28Cautions and limitations 3-28Hazard intensity levels 3-28Common precautionary

statements 3-29Miscellaneous precautions 3-29General safety tips 3-29

Maintenance 3-30Good housekeeping 3-30General maintenance requirements 3-30Inspection 3-30Forced-air equipment 3-30Radiant heaters 3-31General troubleshooting guide 3-31Troubleshooting chart 3-32

Heat system selectionHeat requirements 4-1Design temperatures 4-1Degree days 4-1Infiltration 4-2Ventilation 4-2Makeup-air requirements 4-3Heat loss formulas and U-Factors 4-4U-Factor table 4-4

Air-flow requirements 4-5Ventilation & makeup-air 4-5Entering-air temperature 4-5Temperature rise 4-5Static pressure 4-5Relief, exhaust 4-6

Technical dataPipe sizes 4-9Pipe pressure drops 4-9Equivalent lengths of fittings & valves 4-10Gas-meter flow time 4-10Orifice/pressure tables

Natual gas 4-11LP gases 4-12Specific gravity for other gases 4-12

5. GLOSSARY

Space heating terms 5-1 to 5-4

This text represents the combined efforts of several talented engineers, technical support personneland others associated with Reznor®, HVAC Division of Thomas & Betts Corporation. This handbookhas been prepared to provide an all-in-one-place source of information about natural-gas and pro-pane space heating and space-heating equipment for industrial and commercial use. We have triedto make this manual useful to a broad range of people who deal with heating of industrial and com-mercial buildings. We believe architects and HVAC engineers will find the “Principles of Heater Op-eration”, “Practical Applications”, and “Technical Data” sections particularly helpful. We have includeda “Basic Concepts” section which we hope will prove useful to engineering and technical studentsand which can also serve as a review of fundamentals for others.

As in most technical fields, terms in the heating trade have grown into use somewhat haphazardly,sometimes with ambiguous or multiple meanings. To ensure clarity and avoid possible misunder-standing, we have included a glossary of terms so that the reader may be sure of the meaning in-tended in this handbook.

We have aimed throughout the book to present, in a condensed and readable form, as much accu-rate and up-to-date information as possible. We welcome comments from readers and will appreciatesuggestions for improvement in future reprints or editions of this handbook.

PREFACE

INTRODUCTION

Natural gas is just one of the sources ofheat energy available for space heatingtoday; the others are fuel oil, manufacturedgas, propane, butane, coal, electricity, andwood. Of all the available fuels, naturalgas offers the best combination of conve-nience, availability, economy, cleanliness,non-pollution characteristics, and reliabil-ity of supply. As a result, it has become amost widely used fuel for commercial andindustrial (and residential) space heatingin the U.S. and other industrial nations.

The wide use of natural gas in the UnitedStates has come about in just a little over acentury. Although it is possible to tracehumankind’s awareness of the existenceof natural gas back about 2,500 years tothe ancient Chinese (who piped it to thepoint of use through bamboo poles), mod-ern natural-gas history in the United Statesdid not begin until 1821. It was then thatWilliam Hart dug the first gas well (27 feet

deep) outside Fredonia, New York. For thenext 37 years, use of natural gas and thedrilling of wells spread through westernNew York State and Pennsylvania, andnorthern Ohio and Indiana. The youngnatural gas industry grew quickly, and by1900, there were natural gas wells in 17states. Today, only a few states do no haveany gas wells within their borders, but allhave natural gas available because of theseveral million miles of gas transmissionand distribution lines that now crisscrossthe entire country.

This vast underground network and thetransportability of natural gas account forthe convenience and reliability of naturalgas as a fuel. Only electricity approachesthe convenience and reliability of pipe de-livered natural gas, but it is more costlyand more subject to interruption due toweather. Coal, although less expensive, isawkward to transport and deliver to space

heating equipment and more difficult tocontrol to get the uniform heating ex-pected of today’s space heating equip-ment. Although fuel oil competes withnatural gas in price and in controllabilityat the space heater, it must be transportedand delivered by truck or rail and storedon site. Propane, although generally avail-able, is used where natural-gas lines arenot in place and is stored on site. All fac-tors considered, natural gas offers the bestcombination of characteristics.

This handbook deals strictly with the heat-ing of commercial and industrial build-ings and facilities. It is also limited to spaceheating with natural gas and propane gas.For the most part, natural gas and propanegas are similar, and most of the technicaltext applies to both. Where major differ-ences exist, propane will be mentioned;otherwise, the reader may assume that bothare meant even if only natural gas is men-tioned for convenience and brevity of ex-pression.

Heat Physics

1-1

1-1 Molecules are in constant motion, inall directions

The Nature of Heat. In 1799 Sir HumphreyDavy melted two pieces of ice by rubbingthem together in freezing weather, prov-ing that heat is really a form of energy andnot the weightless fluid scientists of thetime believed it to be. Today, we knowthat heat is the energy of molecular mo-tion. (Figure 1-1). How fast the moleculesof matter in an object are moving deter-mines our experience of the object as “hot”or “cold”. The faster the molecules aremoving (that is, the more energy theyhave) the hotter the object seems to us.

Temperature. The words “hot” and “cold”are common terms we use to describe theintensity of our feeling of heat. But theyare not precise terms because our sensa-tion of heat is not reliable. For example, aroom that feels warm to an active personwill seem cool to a person who has beensleeping or resting. For accurate measure-

ment and for detection of small changesand differences in the intensity of heat,scientists long ago invented the conceptof temperature and thermometer for mea-suring it.

Temperature Scales. Persons interested inheaters and space heating should be fa-miliar with four temperature scales: Fahr-enheit, Celsius (formerly called Centi-grade), Kelvin, and Rankine (Figure 1-2).The Fahrenheit thermometer is in commonuse in the United States. On this scale theboiling temperature of water is 212 degreesand the freezing temperature of water is32 degrees.

In scientific work and in most parts of theworld, the Celsius scale is used. This scalewas set up to have the convenient figuresof 100 degrees for the boiling point of wa-ter and zero for the freezing point.

Kelvin and Rankine are absolute scales.The Fahrenheit and Celsius scales have adisadvantage: it does not have to get verycold to result in negative numbers. Thismeans that at zero on either scale there isstill some heat present (because it can getcolder that zero, there is heat to be re-moved. Lord Kelvin in England took aCelsius scale and changed it so that zerorepresented the complete absence of heat.

The zero point on the kelvin scale is knownas absolute zero--it is not possible to getany colder than zero degrees Kelvin. Inpractice, it is not even possible to actuallycool anything down to absolute zero, butscientists have succeeded in cooling he-lium gas down to within several hun-dredths of a degree of absolute zero. Simi-larly, the Rankine scale is a Fahrenheitscale that start at absolute zero.

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1-2 Four scales for measuring temperature: Fahrenheit, Rankine, Celcius, andKelvin.

1 BASIC CONCEPTS

Temperature Conversion Formulas. Thefollowing formulas can be used to convertfrom one temperature scale to another:

F = (9 ÷ 5 x C) +32

C = 5 ÷9 x (F - 32)

C = K - 273

K = C + 273

Heat Quantity. Temperature differs fromheat quantity. The temperature of an ob-ject is no indication of the quantity of heatpresent. For example, the flame of a burn-ing candle has a much higher temperaturethan a hot-water baseboard heater, but youcouldn’t heat a room with the candle.Here’s another way of thinking about this:dip a cupful of boiling water from a bath-

tub full of boiling water and both will havethe same temperature but not the sameamount of heat. (Figure 1-3). The cupfulof hot water might melt only an ice cube,but the tubful could melt several poundsof ice. It is possible for a body to have: 1.low temperature and little heat; 2. low tem-perature and a lot of heat; 3. high tempera-ture and a small amount of heat; 4. hightemperature and a large quantity of heat.Figure 1-4 shows a basic concept impor-tant for understanding and working withspace heaters: the temperature of matter(gas, liquid or solid) can be increased byadding heat (energy) or lowered by remov-ing heat (energy).

Measuring Heat Quantity. Heat quantityis usually measured in calories or in Brit-

ish Thermal Units (BTU). BTUs are thecommonly used measurement in engineer-ing in the United States. The calorie is themetric system unit used in the rest of theworld for engineering purposes and it isused for scientific work universally, includ-ing in the United States.

The Calorie. The calorie used in engineer-ing is defined as the quantity of heatneeded to warm one gram of water onedegree Celsius. That is, when one gram ofwater becomes one degree warmer, it picksup one calorie of heat energy and if it coolsby one degree, it loses one calorie. Note:This “engineering” calorie is not the sameas the one people count when dieting. Thefood Calorie is called the large Calorie andis equal to 1,000 “engineering” calories.

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1-3 At the same temperature (degrees)the larger volume holds a greater heatquantity (BTU).

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1-4 Temperature indicates intensity (not quantity) of heat.

Heat Physics

1 BASIC CONCEPTS

The British Thermal Unit (BTU). Theheat quantity unit of the English system isdefined as the amount of heat needed towarm one pound of water by one degreeFahrenheit. See Figure 1-5. One BTUequals 252 calories.

Heat Transfer. A property of heat that isimportant in space heating is that heat canmove from one object or place to another.From personal experience since early inlife, we know that when we bring two sub-stances of different temperatures together,the warmer one cools and the cooler onewarms up. This experience is stated inphysics as the Law of Heat Exchange: inall cases of heat exchange between circu-lating currents as shown in substances thetotal number of BTUs or calories gained

by the cooler substance equals the totalnumber of BTUs or calories lost by thewarmer substance.

Space heating engineers and techniciansare usually interested in distributing heatthroughout the space of a building or en-closure. This distribution can involve threemethods of heat transfer: conduction, con-vection and radiation.

Convection. Convection takes place in liq-uids and gases because liquids and gasesexpand and become lighter when heated.When the heating is uneven, the lighter(hotter) gas or liquid rises, setting up Fig-ure 1-6. This movement (convection) is animportant factor in the distribution ofheated air in buildings.

Conduction. In conduction, heat transfertakes place through a solid material (seeFigure 1-7). For example, only the bottomof a pot is heated in cooking food, but themetal handle soon becomes hot. Gener-ally, metals are excellent conductors ofheat (some better than others), while wood,stone, cloth, liquids, and gases are poorconductors.

Radiation. Radiation is the transfer of heatenergy by rays or waves. Radiant heat doesnot need matter on which to ride from oneplace to another (Figure 1-8). Heat rayscan travel through a vacuum: that is howheat from the sun reaches the earth.

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1-5 Calorie and British Thermal Unit (BTU) - two ways of measuring heat quantity. 1-6 Convection: gas or liquid rises whenheated and sinks when cooled.

Heat rays that strike matter can be trans-mitted, reflected or absorbed, or some com-bination of the three depending upon thenature of the material and the condition ofits surface. Just as light passes throughglass, radiant heat can pass through air.Heat rays are reflected when they meetbright smooth surfaces like highly pol-ished metals. Rough dark surfaces absorb

heat and become warmer in doing so (ob-jects with black surfaces absorb heat raysbetter than objects having white surfacesbecause white surfaces reflect heat).

Building Heat Loss. Heat loss from abuilding can occur through conduction,convection and radiation. It can also oc-cur through infiltration--the leakage of airinto a building through cracks, poorly fit-

ted windows and doors, chimneys andother breaks in the continuity of the en-closure. To minimize heat loss, buildingsare provided with insulation which mayconsist of nonconductive materials likespun fiberglass, reflective materials likealuminum foil. Pliable compounds areused for sealing joints and cracks.

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1-8 All radiant energy, like heat from the sun, travels through empty space, needs noconducting material. Light colors reflect heat; dark colors absorb heat.

1-7 Heat travels “downhill” - from the hotter object to the cooler one.

The Process of Combustion. The burningof wood, paper or gas in the kitchen stoveis a common everyday experience, so com-mon we give little thought to the processtaking place that produces heat. Actually,what takes place in this burning process -combustion - is a chemical reaction. Thehydrogen and carbon atoms of the wood,paper or gas combine with oxygen atomsfrom the air and form carbon dioxide gasand water vapor. Chemists and chemicalengineers call this an oxidation reaction.

1 BASIC CONCEPTS

Combustion

1-5Requirements for Combustion. Threethings are required to support combustion:fuel, air (for oxygen), and heat (to raise thefuel to its ignition temperature). All threeelements must be present to start combus-tion and to keep it going (in most casesheat produced by the combustion process,once started, keeps the combustion pro-cess going). See Figure 1-9.

Composition and Combustion of Natu-ral Gas. Natural gas consists chiefly of agas called methane whose chemical for-mula is CH

4. This formula shows that each

molecule of methane is made up of oneatom of carbon and 4 atoms of hydrogen.Natural gas also contains small amountsof related gases like ethane (C

2H

6) and pro-

pane (C3H

8). Notice that all three gases

contain only carbon and hydrogen in theirmolecules. Figure 1-10 gives a visual ideaof the molecules of these gases. Using thechief component of natural gas, methane,Figure 1 -11 illustrates the rearrangementof atoms and molecules that takes place inthe chemical reaction we call combustion.

Complete and Incomplete Combustion.Figure 1-11 illustrates complete combus-tion which takes place when there areenough oxygen atoms to match up withall the carbon and hydrogen atoms present.This illustration of complete combustionpoints out three important facts: 1. burn-ing fuel gas completely produces onlyharmless carbon dioxide and water vapor;2. a definite amount of fuel requires a defi-nite amount of oxygen for complete com-bustion; 3. definite amounts of combus-tion products are formed in burning a fuelgas completely.

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1-9 Three factors are needed forcombustion.

1-10 Fuel gases consist of carbon andhydrogen atoms.

1-11 Complete combustion - forms only water and carbon dioxide.

When there is not enough oxygen for allthe fuel present, the result is dangerousand costly incomplete combustion. Fig-ure 1-12 shows what happens during in-complete combustion. Some atoms of car-bon get to combine with only one atom ofoxygen forming carbon monoxide--a poi-sonous gas that can cause death whenbreathed.

Carbon monoxide can be particularly dan-gerous because it has no odor, color or tasteand has increasingly serious effects at in-creasing concentrations and exposuretimes.

Various devices are available for detect-ing and measuring carbon monoxide. Fig-ure 1-13 shows the effects of various car-bon monoxide concentrations and expo-sure times. Levels of concentration aregiven in parts per million (PPM).

Incomplete combustion is also costly be-cause not all the heat available in the fuelis released when carbon monoxide isformed, so there is a waste of fuel and con-sequent inefficiency.

Controlled and Explosive Combustion.Since complete combustion is necessaryfor both safety and economy in space heat-ing, it is important to supply air and fuelgas to heating devices in the properamounts and at proper rates to ensure com-plete burning of the gas in a steady con-trolled flame.

Explosive combustion can result when anair-gas mixture does not ignite the instantit leaves the burner. The air-gas mixturemay then collect in the combustion cham-ber and burn almost instantaneously (ex-plode).

The gas burners of heaters and furnacesare designed and equipped for safe andeconomical controlled combustion, whenthey are correctly adjusted and operated.

Odorants. For further protection againstexplosion, gas utilities add very smallamounts of compounds that give naturalgas a detectable odor. The “rotten-egg” orgarlic-like smell of these odorants calls im-mediate attention to gas leaks so that cor-rective measures can be taken.

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Carbon Monoxide: (CO) - Product of Incomplete

100 PPM - Safe for continuous exposure

200 PPM - Slight effect after six (6) hours

400 PPM - Headache after three (3) hours

900 PPM - Headache and nausea after one (1) hour

1,000 PPM - Death on Long Exposure

1,500 PPM - Death after one (1) hour

Most codes specify that CO Concentrations shall not exceed 50 PPM

1-12 Incomplete combustion - causes some carbon monoxide to be formed. 1-13 Effects of carbon monoxide.

1 BASIC CONCEPTS

Combustion Gas Measurement

Flammability Limits. The percentage ofgas in a gas-air mixture determineswhether a mixture will burn. Mixtures withless than 4% natural gas in air have toolittle gas to burn or explode. Mixtures con-taining 4% to 14% natural gas can burnwith a controlled flame in a properly de-signed and adjusted burner or they canexplode if they collect in an enclosedspace before ignition. Mixtures contain-ing more than 14% natural gas up to 100%cannot burn or explode. Figure 1-14 showsthese ranges; 4% is known as the LowerFlammability Limit (LFL) and 14% iscalled the Upper Flammability Limit(UFL). These points are also known as theLower Explosive Limit (LEL), and theUpper Explosive Limit (UEL) respectively.The flammability limits for propane areslightly different than for natural gas andare shown in Figure 1-15.

Gas Measurement

Use of natural gas as a fuel involves fivefactors that can be measured to describethe gas and put it to use.

Volume. At the retail level utilities andpipelines deliver and sell gas by volume.The basic unit of measurement in theUnited States is the cubic foot--the vol-ume of a space one foot high by one footwide by one foot deep. For conveniencein recordkeeping, most utilities haveadopted a larger unit, the MCF, whichequals 1,000 cubic feet; e.g., 96 MCF isthe same as 96,000 cubic feet and3,000,000 cubic feet is the same as 3,000MCF.

Pressure. To be useful as a fuel, gas mustbe moved from its source at the wellheadthrough transmission and distributionpipes into the heater or furnace. The forcefor gas movement is the pressure impartedto gas by pumps and compressors.

Three levels of pressure measurementsapply to gas. In the high pressures of long-distance transmission pipelines, pressuresare often several hundred pounds persquare inch (PSI). For commercial and in-dustrial use, natural gas is usually deliv-ered at medium or low pressures. Mediumpressure range is from 1/2 psi to 5 psi and

is usually measured in inches of mercury(Hg). Pressure below 1/2 psi are consid-ered low pressures and are commonly usedfor residential distribution. These low pres-sures are measured in inches of water(W.C.). In these cases the pressure of thegas is expressed as the height of a columnof water or mercury (see Figure 1-16) bal-anced by the gas against gravity in a ma-nometer. Two examples of measurementsin this low-pressure range are inches ofwater (often abbreviated as W.C.) andinches of mercury (abbreviated as Hg).

Specific Gravity. Specific gravity is an im-portant characteristic in the use of naturalgas as a fuel. Knowing its specific gravity

lets you know whether the gas will rise orfall when released into air, how well it willmix with air, and how it will flow throughpipes and the orifices of the gas burners ofheaters and furnaces. For example: becausebutane gas has a specific gravity of 2.0(twice as heavy as air) it falls when releasedinto the air and can drift into low areasand collect there, creating a possible fireor explosion hazard.

Specific gravity is the ratio of the weightof a gas compared to the same volume ofdry air at the same temperature and pres-sure. For example, a cubic foot of naturalgas that has a specific gravity of 0.5 willweigh just half as much as a cubic foot of

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1-14 Flammability limits of natural gas.

1-15 Flammability limits of propane gas.

air. Actually, natural gas specific gravityranges form 0.4 to 0.8, with most deliv-ered in the U.S. having a specific gravityof 0.6. This range exists because naturalgas contains varying amounts of methane,propane, etc., depending upon the sourceof the gas. Propane has a specific gravityof 1.52.

Specific gravity is one of the factors thatthe heater manufacturer must know to pro-vide properly designed and factory-ad-justed heating equipment. This character-istic affects flow of gas, particularlythrough orifices. See Basic System Com-ponents.

Heat Content. Another measure of inter-est to buyers and users of fuel gas is theenergy or heat content.

Large-volume users, energy economists,and scientists who deal with large blocksof energy talk about therms, dekathermsand quads. These are very large units.

therm = 100,000 BTUs

dekahterm = 1,000,000,000 BTUs

quad = 1,000,000,000,000,000 BTUs

A quad is one million dekatherms or onequadrillion BTUs, hence the name “quad”.

Heating Value. The term “heating value”refers to the amount of heat generated byburning gas completely. Heating value isexpressed in BTUs per cubic foot of gasmeasured at standard temperature and pres-sure.

Natural gas delivered for heating purposestypically has a heating value somewherebetween 800 and 1150 BTUs per cubicfoot. The exact heating value for a spe-cific gas can be obtained from the supply-ing utility. The heating value for propaneis approximately 2550 BTUs per cubicfoot of gas.

The heating value of natural gas showsconsiderable variation for several reasons.First, natural gas from different sources cancontain different proportions of methane,ethane, propane, and butane. The morecarbon and hydrogen atoms there are in amolecule of gas, the greater its heatingvalue. This means that, as the percentageof ethane, propane, and butane increases,the heating value increases. Second, natu-ral gas may contain varying amounts ofnitrogen--an inert gas that does not burnat all. The greater the amount of nitrogenpresent, the lower the heating value willbe.

The heating value for the natural gas to beused is a necessary piece of informationfor : 1. sizing a heater or furnace and thegas-supply piping for it; 2. estimating gasconsumption and the cost of operation.

For heating equipment to do its job, it mustreceive the necessary number of BTUs perhour. This required BTU/Hour input rateusually appears on the nameplate of thedevice. It can be converted to gas flow ratefor the particular gas available by usingthe heating value (obtainable form thesupplying utility) and the following for-mula:

R = I ÷ H

where:

R = gas flow rate in cubic feet per hour;

I = input rate in BTUs per hour;

H = heating value of gas in BTUs percubic foot.

Example calculation: A heater with an in-put rating of 125,000 BTUs per hour is tobe supplied with gas having a heat valueof 990 BTUs per cubic foot. The heaterwill use approximately 127 cubic feet ofnatural gas per hour (125,000÷990 =126.263).

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1-16 Manometer for measuring gas pressure.

A space heater does two basic things. Itburns gas to generate heat and it transfersthe heat to the room or other air-space tobe heated. For most heaters, the heat isgenerated in the following way: a propermixture of gas and air bursts into flamewhen heated to its ignition or combustiontemperature and combustion continues asthe gas-air mixture is steadily supplied tothe flame.

Three Basic Types ofHeaters

Once generated, the heat must be trans-ferred to the space to be heated. There arethree ways in which gas-fired heaters trans-fer their heat. Classified in this way, theyare called indirect-fired heaters, direct-fired heaters, and radiant heaters.

Indirect-fired Heaters. The most com-monly used type of gas-fired space heateris the indirect type. Figure 2-1 shows aschematic diagram of an indirect-firedheater. Combustion occurs at the burnerlocated in a metal-walled chamber, calledthe combustion chamber, which may be abox or a tube (cylinder). The flame heatsthe chamber walls on the inside and theheat moves to the outside surface of thewalls by conduction. The hot outside sur-faces then heat the air of the space to beheated while the mixture of burned gasesfrom the flame, called the combustion prod-ucts, go their separate way to the outsideof the building. This arrangement is calledindirect because there is no direct contactbetween the flame and the heated insideroom air. In most installations however, airfor combustion comes from inside theroom or building. NOTE: Air for combus-tion for indoor units is ultimately obtainedfrom outside the building by infiltrationthrough cracks at doors, windows, etc.

2-1

2 PRINCIPLES OF COMBUSTION HEATERS

Three Basic Systems

Direct-fired Heaters. In a direct firedheater, there is no separation between theair to be heated and the combustion cham-ber, as shown in Figure 2-2. The flame andits combustion gases mix with the air tobe heated, that is, the space air is heateddirectly. Because the heated air containsthe combustion gases, direct heating islimited to certain types of buildings andsituations where large volumes of freshoutside air are admitted to the heated spaceas a normal part of the building’s func-tion. The uses of direct-fired heaters arediscussed in the section on GENERAL AP-PLICATION AND SELECTION CRITE-RIA.

Terminology note on direct and indirect.Some publications and workers in the heat-ing field use the terms “direct-fired” and“indirect-fired” in a different sense thandefined in the preceding paragraphs. Theyapply the term “indirect-fired” to a heaterthat generates the heat away from the pointof use (for example, a remotely locatedboiler generates steam which passesthrough piping to a radiator or other heaterin the room to be heated); they use “di-

rect-fired” to refer to all heaters that gen-erate the heat at the point of use. Readersshould be aware of this possibility for mis-understanding when using other sourcesof information. Throughout this hand-book, the two terms are used only in thesense defined in the two paragraphs titled“indirect-fired heaters” and “direct-firedheaters.”

Infrared Radiant Heaters. Radiant heat-ers operate on an entirely different prin-ciple from indirect- and direct-fired heat-ers. In a radiant gas-fired heater, the flameis used to heat a surface which in turn emitsheat in the form of rays. (See Figure 2-3.)The materials used in this process can with-stand extremely high temperatures. Radi-ant gas-fired heaters are fast and efficientand can be used conveniently in indus-

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2-1 Indired-fired heating.

2-2trial processes like the drying of paper,paint, and textiles as well as space heat-ing.

High-intensity Infrared Heaters. In high-intensity heaters, the burner (see Figure 2-3) is cast from ceramic material and pos-sesses a great number of ports of orificesin which the flame resides. The surfacebetween these ports or orifices reachesabout 1500-1950°F. At this temperature,the surface flows cherry red and becomesan infrared radiator. Then, like the sun, itheats objects in the room through radia-tion, without heating the air in between.

Low-intensity Radiant Heaters. Radiantheaters designed for low-intensity opera-tion are based on the same principles ashigh-intensity heaters. The main differ-ence is that the radiating plate is heated toa lower temperature (200-900°F). At this

temperature, the plate does not glow butemits long-wave heat energy more like theheat from an electric iron or steam radia-tor.

Relative Efficiencies. Each type of heaterhas its own set of operating characteristicsand applications, including efficiencycharacteristics. In thermal (combustion)efficiency, all three types of gas-fired heat-ers are about the same, that is, their burn-ers extract about the same amount of heatfrom a cubic foot of gas. However, in over-all system efficiency, there are differences,mostly due to installation and heat trans-mission methods.

Indirect-fired duct-furnace system efficien-cies are lower due to duct heat losses (heatlost from warm-air ducts to the unheatedspaces through which the ducts may bedirected), casing losses (heat lost from theheater to the space surrounding the heater),flue loss (heat lost through walls and ceil-ings due to movement of heated air). Be-

cause non-duct indirect-fired systems (seeunit heaters, page 3-2) eliminate duct andcasing losses, their system efficiency issomewhat higher.

Direct-fired systems may not have ductloses and generally do not have casinglosses. They also minimize flue losses (be-cause they are not vented to the outdoors.The hot combustion gases directly warmthe circulated air that is being heated), sotheir efficiency tends to be higher than forindirect-fired systems (theoretically100%). However, efficiency is limited toabout 90% because buildings heated inthis manner must be ventilated at the rateof 3.5 to 4.5 CFM for each 1000 BTUHinput.

Finally, radiant heaters have very goodsystem efficiencies for the following rea-sons:

1. Radiant heaters do not heat the air; theyheat the objects in the space by radia-tion. Occupants can be comfortable at alower air temperature--and a lower airtemperature means lower losses throughthe building’s walls;

2. Radiant heaters require no forced air cir-culation, minimizing convection lossesthrough the building’s walls;

3. Radiant heaters normally are unventedbut buildings must be ventilated at therate of 3.5 to 4.5 CFM for each 1000BTUH input. Such ventilation is usu-ally accomplished with powered ex-hausters installed at the highest pointof the roof.

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A modern gas-fired space heater is morethan a flame in a metal box. It’s a system ofcomponents and subsystems working to-gether automatically, efficiently, andsafely. A generic forced-air heating systemis shown in Figure 2-4 and its parts andsubsystems are described in the followingparagraphs.

Gas Burners

The burner is a device for burning gas un-der control. It is made of cast iron or press-formed sheet steel. Space heaters employburners designed on the principle of theblue-flame or Bunsen burner. Invented in1842 by Robert Wilhelm von Bensen, thistype of burner has many advantages overthe yellow-flame burner that was in use atthe time.

Yellow-flame Burners. As shown in Fig-ure 2-5, the yellow-flame burner was sim-ply a tube of port fed by gas. In this type ofburner, the gas tends to burn inefficientlyand forms soot (unburned carbon par-ticles). Early in the heating industry, suchburners were used and were called “lumi-nous flame” burners because of the brightyellow appearance of the flame. Specialtipped orifices were used to encourage airto unite with the gas, creating less likeli-hood of sooting. However, by today’s stan-dards, such flames are considered unsafe.

Blue-flame (Bunsen) Burners. The Bun-sen burner was a considerable improve-ment over the yellow-flame burner. Figure2-6 shows the basic design. It consists of amixer tube equipped with an opening andshutter (for admitting a controlled amountof air to mix with the incoming gas) andan adjustable orifice for regulating the gasflow. Air that enters the burner to be mixedwith raw gas is called primary air; air thatenters the flame while combustion is inprogress is called secondary air. The theo-retically exact amount of primary air plussecondary air needed for complete com-bustion is called total air. In practice, someadditional air is required to make sure com-bustion is complete. The air required overand above the ideal total is called excessair.

Air and gas mix in the body tube. The mix-ture burns at the end of the tube with aflame that has four zones. The most clear-cut zone is a thin bright blue cone, calledthe primary cone where the combustionprocess begins. The dark area inside theprimary cone is the raw gas mixture stillbelow the ignition temperature, havingjust left the mixing tube. Air around theflame enters the flame and completes thecombustion, forming the darker outercone. Finally, an almost-invisible outerglow of incandescent combustion prod-ucts surrounds the flame.

Basic System Components

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Temperatures are not the same at all pointsin the flame. The lowest temperature is inthe unburned-gas dark zone and the high-est temperature usually exists just abovethe primary cone tip.

Atmospheric Burners. Burners that takein primary combustion air at atmosphericpressure are known as atmospheric burn-ers. Figure 2-7 shows the cross section of atypical atmospheric burner for gas-firedheating and identifies its 3 functional sec-tions and their principal elements. Themixing head lets in gas through the smallhole (orifice) in the orifice spud and ad-mits primary air through large openingsin the mixing-head face. Flow of air canbe adjusted with shutters that slide overthe air openings; in some burners the rateof gas flow can be adjusted by turning theorifice spud, in others the orifice size isfixed.

The mixing tube mixes the air and gas asit conveys them to the burning head. Themixer is tapered to create a venturi. Thegas jet generates a suction that causes airto enter the burner at the primary opening,the venturi shape causes the air to flowinto the mixing chamber at high velocity.

The burner head distributes the air gas mix-ture uniformly to many small holes calledports. These multiple ports spread theflames to provide good heat transfer andgood access of secondary air to the flame;however, the primary function of the portsis to control the air-gas-mixture velocityso that the flame rests on the port.

Velocities that are too high will allow theflame to lift off the port. Conversely, ve-locities that are too low will often permitthe flame to flash back through the portand end up in the mixing chamber.

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Outer MantleIncandescent Productsof Complete Combustion

Outer ConeComplete Combustion ofIntermediate Products

Inner Cone Zone of PartialCombustion

Unburned Mixture of Gasand Primary Air

Burner Port

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Primary Air Shutter

Primary Air Opening

Gas OrificeOrifice SpudGas Supply

SecondaryAir

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2-6 Bunsen Burner. 2-7 Basic burner for gas-fired heaters.



Power Burners. In most commercial andindustrial applications, gas-fired spaceheating equipment employs atmosphericburners, but in some very large indirect-fired heaters (one million BTU/hour ca-pacity and greater), burners receive theircombustion air under pressure from a fan.

Forced-draft and induced-draft burners.Forced-draft power burners receive theircombustion air at pressures above atmo-spheric pressure from a blower or other airsource located in the burner primary airsupply. (See Figure 2-8). Induced-draftpower burners have combustion air pulledthrough them by a vacuum created by afan located in the flue outlet. (See Figure2-9.)

Burner Designs. Gas burners come in agreat variety of shapes, sizes, and designs.There are several basic types: slotted-port,ribbon-port, single-port, impingement-tar-get, and jet. Each type is subject to varia-tion when designed for specific heatingequipment.

Typical Gas Burners Found in Direct-fired Heaters are unique, in that they areequipped with aeration plates that are ad-jacent to the gas ports and they controlthe circulated air velocities over the burnerto insure efficient burning of the gaseousfuels (Figure 2-10).

Unlike ribbon-type burners, these devicesare so designed that they can burn a widevariety of gas fuels by adjusting gas pres-sure at the orifice.

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2-6Typical Gas Burners Found in Indirect-fired Heaters are the ribbon-port type thatvary in length and in port sizes, and mayemploy a single ribbon or many ribbons,depending on the volume of gas to beburned. Normally they are atmospheri-cally aspirated. Usually, these burners re-quire only primary air adjustments to op-erate efficiently on different gaseous fu-els. (See Figure 2-11). Ribbon-type burn-ers produce a stable flame, allowing use ofa wide variety of gaseous fuels.

Typical Gas Burners Found in Infrared-radiant Heaters are:

• High-intensity infrared units. Thesecontain ceramic plates with multipleports on very close centers. The portsare sized so that the flame is confinedpartially with the ports, creating ex-tremely hot surface temperatures, usu-ally between 1500°F and 1950°F. Thissurface emits infrared energy (see Fig-ure 2-12).

• Low-intensity infrared units. These usea jet-type burner that receives its com-bustion air from a powered blower. Theflame is contained within a tube which,when heated, emits infrared energy (seeFigure 2-13).

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2-7


Orifices. Burner orifices (Figure 2-14) arerequired to meter gas to each burner or pi-lot at a rate determined by burner capac-ity, gas heating value, specific gravity ofthe gas, and the altitude at which theburner is installed. Orifices are machinedfrom brass, aluminum or steel and arethreaded into the gas train for easy instal-lation and removal.

A Good Burner Flame. For most economi-cal operation and best nonpolluting op-eration, the flame should be stable andhave a blue center cone that is not sharplydefined, with minimum yellowing in theouter cone. In any properly designedburner, this flame pattern is accomplishedby a proper balance of primary air and sec-ondary air provided in sufficient amountsto ensure complete combustion (Figure 2-15a).

The usual signs of improper air control arelighting yellow tipping, and floatingflames (illustrated in Figure 2-15). Liftingrefers to the rising of the flame off the portwith no incandescent gases visible be-

tween the flame and the top of the burnerport. This condition is caused by too muchprimary air; it is easily corrected by reduc-ing primary air flow, using the air shutteras a throttling device. Yellow tipping in-dicates too little primary air and can becorrected by increasing primary air flow(by opening the air shutter) until the blueinner cone becomes visible. A sever lackof primary air (or none at all) causes flamesto become completely yellow. Floating de-scribes flames that wander erratically as ifblown by a soft wind. This condition re-sults from too little secondary air.

Flashback and Extinction Pop. Undersome conditions, combustion can proceedbackward through a burner port, resultingin a cone that inverts and extends backthrough the port. This flashback happenswhen the speed at which a gas mixture willburn is higher than the speed at which thegas mixture moves through the gas port.Natural gas does not have a great tendencyto flash back because it is a relatively slow-burning fuel. Flashback will not occur

during normal operation of a properly de-signed and adjusted burner. Turning a gasburner off can temporarily create condi-tions that result in a short-lived flashback--a tiny explosion known as extinction pop.This type of flashback is annoying but nothazardous, and can occur in even the bestdesigned and adjusted burners.

Burner Operation at High Altitude. Inburners operated at atmospheric pressure,altitude is an important factor because in-creased altitude causes air density (weight)to decrease (that is, the air becomes “thin-ner”). At high altitudes, therefore, thelower air density (less oxygen per cubicfoot of air) results in combustion effectsthat are different than those at normal alti-tudes. For this reason, the heating systemdesigner or equipment buyer shouldspecify the altitude of the location wherethe heater will operate, if it is significantlyabove sea level (2000 feet altitude andhigher is considered “high altitude”). Themanufacturer can then make adjustmentsin orifice sizing to accommodate the lessdense air.

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2-15 Symptoms of incorrect air adjustment or overfiring. Burner flame appearances: Good (a); Bad (b, c, d, e).

2-8Heat Exchangers. Heat exchangers areused in indirect-fired gas space heaters.Their purpose is to provide efficient trans-fer of heat from the inside of the combus-tion chamber to the air on the outside. Forgood heat transfer, they have a series oftubes to provide a large surface area, asshown in Figure 2-16.

Heat exchangers see rather sever service.They are subjected to high temperatures,so they require a material that will not blis-ter or peel. Because the heating and cool-ing cycle creates repeated expansion andcontraction, heat exchanger material mustwithstand cyclic stresses. During warm-upor when the air-temperature-rise across theheat exchanger is extremely low, a heatexchanger is subjected to lower tempera-tures. This creates corrosive conditionsthat result when condensed moisture com-bines with combustion products. In addi-tion, chlorine and halogenatedhyrocarbons, if present in the atmosphere,can cause serious and rapid corrosion ofheat exchangers. Manufacturers do notwarrant their equipment when such con-taminants are present.

Indirect-fired Heat Exchangers. Four ma-terials are commonly used in today’s heatexchangers: uncoated 1010 carbon steel,fused-ceramic coated 1010 steel, stainlesssteel, and aluminized steel. Uncoated 1010steel is by far the least expensive but itwill not adequately withstand corrosionor scaling and is not recommended for hightemperatures. Fused-ceramic-coated steelhas excellent corrosion and scaling resis-tance as long as the coating remains in-tact; unfortunately, ceramic coatings aresomewhat brittle and may not stay intact.All things considered, stainless steels arethe best, but they cost much more than theothers. Type 321 stainless steel is one ofthe best (and costliest) heat exchangermaterials available. Type 409 can provideexcellent corrosion resistance at a lowercost.

Aluminum-coated steel (hot-dipped 1010)offers what is probably the best combina-tion of serviceability and cost. It has ex-cellent resistance to corrosion and scalingand costs considerably less than stainlesssteel but only slightly more than uncoated1010 steel.

Aluminized-steel heat exchangers canhandle entering-air temperatures as low as40°F and outlet air temperatures as highas 160°F, with a temperature rise acrossthe heat exchanger in the range of 30 to90°F, under normal atmospheric condi-tions. For lower entering-air temperatures(as low as minus 20°F) and higher outlettemperatures (above 160°F), or where cor-rosive conditions exist, stainless steel isthe recommended heat-exchanger mate-rial. Stainless steel exchangers will toler-ate high inlet-air temperatures, providingthe rise through the unit does not exceed70°F--a highly useful and desirable char-acteristic for some process work.

Heat Exchangers in Infrared Heaters.The heat exchanger in radiant heaters is aradiating, rather than a conducting, device.

Heat-intensity Radiant Heaters. Suchheaters contain a ported ceramic block inwhich the flame burns within the indi-vidual ports to create extreme temperatureson the ceramic surface. Surface tempera-ture reaches approximately 1950°F.

Low-intensity Radiant Heaters. Theseheaters have a metal heat exchanger which,when heated, radiates heat. The surfacetemperature usually does not exceed1,000°F.

Direct-fired Units. Direct-fired equipmentdoes not employ heat exchangers since theflame is burned directly in the circulatedair.

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Ignition Systems. In most space heatingsystems, temperature is controlled by turn-ing a fixed flame on and off rather thanadjusting the size of the flame to vary theheat input to the building. Automatic gaspilot-light systems are used to ignite theheater’s burner every time heat is needed.In addition, these systems have a safetyfeature which shuts off the gas supply tothe burner if the flame at the pilot goes out(See Flame-proving Systems, page 2-10).

There are four general types of ignitionsystems: manual constant pilots, automaticspark-ignited pilots, automatic hot-surfacedirect-ignited systems, and automatic di-rect spark-ignited systems.

Manual Pilots. Manual systems have pi-lot burners that must be lighted by handwith a match and remain “on” continu-ally, ready to ignite the main burners whenthe gas is turned on automatically by othercontrols.


2-9Automatic Pilots. Automatic spark-ig-nited pilots (Figure 2-17) are ignited firstby an electric spark, then the pilot flameignites the main burner. In a hot-surfacesystem, an electric resistance coil heats upto a glow hot enough to ignite the pilotwhich in turn ignites the main burner. Inautomatic direct-spark-ignited systems, anelectric spark ignites the main burner with-out the aid of a gas pilot burner. Hot-sur-face igniters may also ignite the mainburner directly.

All four types of systems have provisionsfor automatic safety gas shutoff in case ofignition failure.

Carryover. To light the main burner, allpilots depend on the phenomenon of“carryover”--the spread of flame from oneport to another. Additionally, where morethan one burner is ignited from a singlepilot, a “carry-over” system is employedto assure that each subsequent burner isignited. This is done either by apparatusdesigned as part of the burner or is addedas an accessory to the burner assembly.

Space-Temperature Controls. The pur-pose of a space heating system is to main-tain a given temperature within a space orstructure. To accomplish this purpose,space heating systems include an electri-cal control system which usually consistsof thermostat safety controls, and relatedpower transformer and wiring; forced airsystems also include fan or blower con-trols.

Figure 2-18 shows a typical forced-air-sys-tem heating cycle in graph form. At point1, the room temperature has dropped be-low the room thermostat’s setpoint, so thethermostat contacts close, energizing theburner circuit. The combustion-chambertemperature begins to rise. After this tem-perature has reached a predetermined level(2) the fan cuts in. When the heated spaceexceeds the setpoint temperature, the ther-mostat contacts open and the burner flamegoes out (3), but the fan continues to oper-ate and extract residual heat from the heatexchanger. Finally, when the combustionchamber temperature cools to the pointwhere moving air would be uncomfort-able, the fan cuts out (4). This cycle re-peats throughout the heating season, asneeded.

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2-10A number of different room temperaturecontrol options are available. The optionsvary from a simple system with ON-OFFcontrol for a warehouse to sophisticatedsystems having a temperature-control ac-curacy of plus or minus one-tenth of a de-gree for storage of temperature-sensitivematerials or laboratories where environ-mental conditions must be precisely con-trolled.

Thermostats. The thermostat is a tempera-ture sensitive electrical switch. The mostcommon temperature-sensitive elementfound in thermostats is a bimetallic stripthat changes shape with temperaturechange. Figure 2-19 shows a basic ther-mostat control system and three basic typesof bimetal elements used in space-heaterthermostats. Attached to the bimetal ele-ment is a set of electrical contacts, con-structed so that when the space tempera-ture drops, the change in shape of the stripcloses the contacts. This contact closurecompletes the circuit to the gas valve,opening the valve and allowing gas to flowto the burner.

Location of Thermostats. Thermostatsmust be carefully located so that they de-tect average conditions and make theequipment respond promptly to heatingrequirements. Thermostats should beplaced out of the path of warm dischargeair and where they will not be affected byradiation from the sun, powerful lights orother heat sources.

In multiple-heater installations, individualthermostats can be used with each heater,but in some installations it may be desir-able to control several heaters with onethermostat. Several types of thermostatsare available to match the control optionswhich are offered with heating equipment.

Thermostat Adjustment. Many types andstyles of thermostats are in use in spaceheating. They can have as many as threeadjustments. Normally, they can be easilyadjusted from the outside of the case to setthe system to the desired room tempera-ture; this temperature is sometimes calledthe setpoint. Some thermostats have a com-

pensator adjustment that can be used toprevent temperature overshoot orunderheating due to the time it takesheated air to reach the thermostat. Suchcompensators are referred to as heat an-ticipators. Some thermostats also have adifferential adjustment. This adjustmentmakes it possible to adjust the differencebetween the temperature at which the ther-mostat opens its contacts and the tempera-ture at which it closes them. Setting thistemperature difference as small as possibleresults in more nearly constant tempera-ture control.

Safety Controls. Safety devices are nec-essary to prevent fire, explosion or dam-age to the heater in case of a malfunctionof an operating part.

Flame-Proving Systems. A safety pilot isa device designed to prevent gas flow whenignition cannot take place. If the pilotflame is not adequate for smooth ignition,a sensing device signals a magneticallyoperated valve or a safety relay circuit toprevent the main gas valve from opening.

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Basic Systems Components

2-11The safety pilots now on the market maycontain one of two types of sensing de-vices to prove the presence of flame at thepilot. The most common sensing devicein use today is the thermocouple--a de-vice that produces a low electrical volt-age and current when heated. It is used inconstant-pilot systems. The thermocoupleis placed so that the tip is heated by thepilot flame and, when heated adequately,generates an electrical current that is di-rected to an electromagnet. This electro-magnet may hold relay contacts closed orit may hold a manually set valve in theopen position. If the pilot flame goes out,the thermocouple’s output ceases--causingthe relay to open or the valve to close,shutting off all gas flow to both the mainburner and the pilot burner.

Spark ignition systems employ a sensorthat does not depend on heat as the ther-mocouple does. In this system, the pilot-burner head and a probe near the pilot-burner head serve as electrodes that allowthe pilot flame between them to be part ofan electrical circuit. (See Figure 2-17). Theelectrical current will not flow betweenthem through air or air-gas mixture, there-fore there is no signal without a flame.When a flame appears, current flows formthe probe through the flame to the pilotburner head, activating the main burnerthrough an electronic device which in turnenergizes a control relay or a solenoid- orhot-wire-operated valve.

Safety and Gas Valves. Gas valves per-form two functions in gas-fired space heat-ing equipment: temperature control andsafety.

Modern space heating equipment employsan automatic or semiautomatic device forturning on and shutting off gas to theburner. This device usually consists of avalve in the gas supply line coupled to anelectro-mechanical, electro-thermal orelectro-pneumatic mechanism for openingand closing the valve.

Redundant valves (two safety shutoffvalves in the same gas train) are currentlyrequired by ANSI (American NationalStandards Institute). These valves areplaced and arranged to shut off the gas tothe main burner and the pilot when cer-tain conditions occur (e.g. pilot light goesout, heat exchanger temperature gets toohigh).

Solenoid Valves. Probably the most com-mon type of control valve is the solenoidvalve (Figure 2-20) in which an electricalcoil is used to create a magnetic field tomove the valve disc. When no electric cur-rent is supplied to the solenoid coil, thespring, gravity and inlet-gas pressure holdthe disc in the closed position, but whencurrent is supplied the coil’s magnetic forceraises the disc to the open position. As withall gas valves, this design meets the re-quirements for fail-safe operations, that is,in case of electric-power failure the valvecloses due to action of the spring, theweight of the disc and plunger, and thepressure of the gas in the supply line. Mostsolenoids act quite rapidly, making a loudclick on opening or closing. Some heat-ing equipment contains solenoids havingan oil bath to slow the action and softennoise. Most solenoid valves operate on ei-ther 24 or 120 volts AC.

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2-20 Solenoid gas valve. 2-21 Hot-wire type of gas valve.

2-12Hot-wire Valves. Figure 2-21 shows onetype of hot-wire valve. In this type ofvalve, the valve disc is connected to a bi-metallic strip that is straight when coldand bends when heated. As the drawingshows, the straight bimetal strip (a) holdsthe valve disc in the closed position whenno electrical current flows through theheating coil that surrounds the strip. Whencurrent is applied to the coil, the bimetalstrip bends, (b) pulling the disc up andallowing gas to flow. In this instance, theheating coil is located where it is in con-tact with gas. The gas cannot be ignitedby the heating coil because there is no airin the gas, therefore combustion cannottake place. Other types of hot-wire valveshave the heater coil mounted outside thevalve body, and utilize a sealed plungerto activate the valve disc.

Motorized Valves. Gas valves for tempera-ture control can also be actuated by anelectric motor or an hydraulic power gen-erated by an electric motor. Such motor-ized valves are used in gas-fired space heat-ing equipment when inputs exceed400,000 BTUH. These motorized valvesuse a heavy spring to hold the valveclosed. The spring usually exerts a forceof approximately 50 pounds.

Diaphragm Valves. Solenoid, hot-wire,and motorized valves require significantamounts of electrical power for operation.Diaphragm valves can be designed to op-erate on very low current because they usethe pressure of the gas to provide the actu-ating force. Figure 2-22 illustrates the op-erating principle. The drawing shows thevalve in the open (electromagnet-ener-gized) position. For operation as a safetyshutoff valve, the electromagnet needsonly to hold the rather small armature in

the bleed-port-closed position. In this con-dition there is gas pressure below the dia-phragm and atmospheric pressure aboveit and the higher gas pressure below holdsthe diaphragm up, keeping the disc un-seated and allowing gas to flow. If the pi-lot light goes out, the thermocouple stopsproducing a current and voltage, the mag-net de-energizes, the spring pulls the ar-mature to close the bleed port and openthe bypass. This allows gas to enter abovethe diaphragm, equalizing the pressure onboth sides, and the weight of the disc anddiaphragm seats the disc and stops gasflow.

When used as safety shutoff valves, somediaphragm valves are reset manually to re-store gas flow after shutoff. Most dia-phragm valves are designed to open andclose automatically and are sometimesused as combination control and automaticpilot safety valves.

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2-13High-temperature Limit Controls. Vari-ous thermostatic devices can be used toprevent heaters from reaching abnormallyhigh temperatures. These high-temperaturelimit controls usually resemble operatingcontrols in design and principle of opera-tion, except that they shut off the gas onlywhen operating temperatures approachunsafe levels.

Indirect-fired Systems. The most com-mon limit control found in indirect-firedheaters is the combination fan-and-limitswitch. This two purpose device is a ther-mostat with 2 sets of contacts: operatingcontacts and limit contacts. One set turnsthe fan or blower on and off according totemperatures reached by the air comingout of the heater. To avoid blowing coldair when a heater first comes on, heatertechnicians adjust this set of contacts sothe fan or blower “cuts in” only after thetemperature at the heater has reached alevel for comfort in the heated space. Con-versely, for more economical operation,this set of contacts is set so the fan or blowerdoes not “cut out” when the burner turnsoff, but remains on until the heater hascooled down somewhat. These cutin andcutout points are normally adjustable be-tween 100 and 150° F.

The second set of contacts on this controldevice protects the heater from damagefrom overheating. Such overheating canoccur if the air flowing across the heat ex-changer is interrupted or reduced appre-ciably. The limit contacts open the elec-trical circuit to the gas valve and causethe valve to close when the heater’s com-bustion chamber or heat exchangerreaches a preset maximum temperature.

Gas-pressure Regulators. For continu-ously proper operation of a correctly de-signed and adjusted burner, the gas pres-sure at the burner orifice must be main-tained at a given value, or within certainlimits above and below a given value. Pres-sure Regulators are special valves used formaintaining pressure at a nearly constantlevel. All types of pressure regulators con-tain two basic parts: a pressure-standarddevice (spring or dead weight to whichthe outlet pressure is compared), and avalve with a continuously variable gaspassage. The photo in figure 2-23 shows atypical regulator used in space heaters. The

adjustment screw in the top of the regula-tor housing is used to establish the desiredgas pressure to the burner by changing thepressure of the spring against the dia-phragm. On indirect-fired equipment, regu-lators are usually adjusted to 3.0 to 3.5inches W.C. for natural gas and approxi-mately 10.5 inches W.C. on propane gas.

Combustion-Air Proving Switches.Power-vented units depend on combustionair as provided by a powered combustion-air blower. There are several devices thatcan be used to determine if this air is avail-able to the burner. The most common isthe pressure activated diaphragm switch,used on indoor equipment. This devicemonitors for correct air flow volume bysensing differential pressure of the air as itflows either to the burner or from the flueexhaust. Heating equipment installed out-doors usually has a sail switch in the airstream or a centrifugal switch on theblower motor, but also may employ a pres-sure switch as on indoor equipment.

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2-14Venting Systems. The purpose of ventingis twofold: 1. to remove the waste prod-ucts of gas combustion; 2. to ensure effi-cient combustion in the heater.

Flue Products. In the case of completecombustion, the unwanted combustionproducts are water vapor, carbon dioxide,nitrogen, and possibly a trace of sulfurcompounds. In case of incomplete com-bustion, the combustion-product mixturewill also contain carbon monoxide. In ei-ther case these waste gases must be drawnor forced from the heater’s combustionchamber and heat exchanger.

Flue Collectors. Venting is accomplishedby connecting a heater’s combustion sec-tion to the outer air through a ventpipe orflue. Flues are installed through thebuilding’s roof, wall or into a chimney.Flue pipes are usually made of 20-gauge,or thicker, galvanized steel, aluminum orother non-corroding metal.

A proper venting system consists of morethan a pipe to discharge gases to the out-doors. An important item is the diverter,also called a draft hood (Figure 2-24). Thisdevice usually comes built into the heateror furnace by the manufacturer. However,most hot water heaters require an addeddraft hood. They must be supplied sepa-rately as part of the flue system and shouldbe installed in strict conformance with theheater manufacturer’s instructions. Alldiverters serve a number of important pur-poses:

1. provide a balance of draft over theburner to prevent unit efficiency fromvarying beyond acceptable limits;

2. prevent any down drafts in the chimneyfrom entering the combustion zone;

3. prevent widely fluctuating wind veloci-ties from creating varying drafts in theheater combustion chamber;

4. dilute the flue gas with interior air toreduce the likelihood of condensationin the flue pipe or chimney.

Because the condensate contains traces ofsulfurous and carbonic acids, it is highlycorrosive. Condensate collecting in theflue and chimney can drain back downinto the heater and, in a short time, canform an acid that “eats through” the draftdiverter, heat exchanger walls, and otherparts of the heater.

Venting systems should be installed ac-cording to the heater manufacturer’s in-structions. After October 1987, ventingsystems have been required to be installedaccording to one of four categories:

TemperatureBurner above

Category type dew point

1 atmospheric more than 140°F

2 atmospheric less than 140°F

3 pressurized more than 140°F

4 pressurized less than 140°F

They should be inspected periodically forobstructions that cause improper ventingand damage or deterioration that couldcause improper venting or leakage of com-bustion products. A blocked flue will causethe release of combustion products intothe area being heated. At the same time, itwill reduce the intake of combustion air,causing incomplete combustion and pro-duction of carbon monoxide. The net re-sult may be the “spilling” of combustionproducts containing carbon monoxideinto the heated area.

Vent Dampers. Dampers have been devel-oped to automatically close a vent duringburner “off” cycles to prevent heat lossesby escape of room air through the vent.Although installation of such dampers re-duces off-cycle losses, on-cycle losses arenot affected since the damper is driven toa full-open position during burner “on”

cycles. If a vent damper is to be added to aheater vent system, the heater manufac-turer should be consulted as to whether aparticular damper model has been testedwith the heater unit in question. Onlydampers listed by AGA should be consid-ered for use. Although AGA has certifiedseveral models for use with standing pi-lots, Reznor strongly advises against in-stalling dampers on systems having stand-ing pilots. Only a spark-ignited intermit-tent pilot should be used in conjunctionwith a vent damper.

Gravity and Power Venting. When a fluesystem depends on convection to draw thehot flue gases out of the heater and intothe outdoors, it is said to be a gravity vent.Ordinarily, gravity vents perform quitesatisfactorily, but some circumstances re-quire power venting; that is, flue gasesmust be forced out under the power of anelectric-motor-driven blower.

Multiple Venting. When heaters are grav-ity vented, more than one heater can beconnected to a single vent provided thecommon vent is large enough. ANSI speci-fications should be followed for calculat-ing the size of the common flue.Power-vented heaters should never bemultiple-vented to a common flue whenthe system falls into category 1 or 2 in thetable above. The danger is that combus-tion products from one heater could beforced into the heated space through oneof the other heaters connected to the com-mon flue. Venting should always followthe installation manual instructionsclosely.

The above discussion of venting appliesto indirect-fired heating equipment only.Because direct-fired heaters allow theburned gas to mix with the air in the heatedarea, they are not vented but rely onfresh-air mixing with the products ofcombustion to meet health standards.

Selection Considerations

3 GENERAL APPLICATION AND SELECTION CRITERIA

3-1In applying heating equipment for indus-trial and commercial purposes, three fac-tors should be considered: fuel availabil-ity, occupancy, and building function.

Fuel Availability. The first step in the pro-cess of selecting space heating equipmentinvolves determining the fuel to be used.Obviously, if only one fuel is available,no decision is necessary. If several fuelsare available and one is natural gas, thechoice is made simpler because natural gasoffers the best combination of ready avail-ability, reliability, continuity of supply,low cost, nonpolluting combustion, andgood control for accurate uniform heat-ing. Where pipeline natural gas is not avail-able, propane makes a sound alternativechoice--it is a clean-burning, nonpollut-ing and controllable as natural gas. Al-though its cost may be higher on a per-cubic-foot basis, propane has a higher heat-ing value and is competitive on a per-BTUbasis.

Occupancy. A number of questions needto be answered about how the space to beheated is occupied and used. How mayhours per day and how may days per weekdo people regularly use the space? Do thepeople usually sit (e.g. office) or do theyengage in heavy physical work (factory?)Does the space store materials that are per-ishable? Does the space to be heated in-clude sleeping quarters? (Equipment cer-tified to ANSI Z83 must not be used toheat any space where people sleep.) An-swers to these questions will determinewhat equipment to use, what temperatureshould be maintained and how much tem-perature variation is tolerable.

Building Function. A number of questionsare related to the function, use and con-struction of a building. What are the di-mensions of the space to be heated? Whatis the total window area? What is the ma-terial of construction of the walls and roof,and how are they insulated? Are walls ex-posed directly to outdoor conditions orare they shielded by another heated build-ing?

The choice of type of system to best fit thejob can quickly be narrowed downthrough the process of elimination. Canthe situation use a direct-fired heater? Doesthe use of the space require exhaust fansto remove large quantities of air, i.e., isheated makeup air indicated or advisable?Must air or heat be distributed uniformly?Is dust removal necessary or desirable? Dodifferent rooms or areas in the space havedifferent heating requirements? Answersto these questions quickly rule out certaintypes of equipment, for example radiantheaters do not provide any dust filtration.

In addition to being categorized by theirmethod of heat transfer (indirect-fired, di-rect-fired, radiant), heaters are categorizedaccording to their construction-applica-tion features. Forced-air heaters fall intothree types in this category: unit heaters,duct furnaces, and packaged heaters. Logi-cally, radiant heaters could be classifiedas unit heaters, but the heating industrytraditionally has put them into a categoryall their own because they are radicallydifferent in principle. There are no radiantheaters comparable to duct furnaces andpackaged units.

Application and Equipment Considerations

INDIRECT-FIRED COMBUSTION

Vent Systems. Modern indirect-firedequipment is designed to operate safelyand efficiently with single-wall vent pipein runs 5 feet to 50 feet long. Double-wallpipe is often substituted for single-wallpipe and in many areas is required by lo-cal standards. For power-vented heatingequipment, the vent pipe can run verti-cally or horizontally. (See Figure 3-1.)Horizontal vent pipe allows less room heatto escape through the flue than verticalpipe, saving fuel.

Gravity-vented equipment must have ver-tical venting, but may have horizontal runsthat do not exceed 75% of the vertical flueheight dimension.

The heating-equipment manufacturer’sventing directions should be followedclosely as to terminal arrangement, pipesize, pipe connections, pipe support, etc.All pipes must be sealed to prevent leak-age of flue gas. Aluminum or Teflon taperated 550°F service is recommended.

Combustion Air. Gas-fired heating equip-ment needs adequate air for proper and ef-ficient combustion. For this reason, heat-ing equipment must not be installed intight enclosures that do not allow suffi-cient air volumes to enter to replace theair exhausted through the vent system.When a building’s construction includesextensive insulation, vapor barriers, tight-fitting and gasketed windows and doors,and weather stripping, the need for com-bustion air requires the introduction ofoutside air through properly sized wallopenings and ducts.

Under all conditions, enough air must beprovided to make sure there will not be anegative pressure within the equipmentroom. When the space containing a gas-fired unit is confined (less than 50 cubicfeet of space for each 1000 BTUH input),special openings must be included to per-mit outside air to be introduced to supportcombustion. Size and location of theseopenings are specifically described in themanufacturer’s installation instructionmanual.

3-2

UNIT HEATERS

The unit heater is the workhorse of com-mercial and industrial space heating. Unitheaters (Figure 3-2) are compact, versatile,completely self-contained, and are de-signed for mounting near the ceiling inthe space to be heated. A wide range offacilities can be heated for occupant com-fort with unit heaters: factories, garages,storerooms, supermarkets, gymnasiums,swimming-pool enclosures, auto show-rooms, etc.

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3-2 Blower-type (top) and fan-type unitheaters.


Fans and Blowers. Unit heaters are avail-able with either fan-powered orblower-powered recirculated air movers.A fan is a propeller-type air mover; ablower is a centrifugal device. (See Figure3-3.) Fans deliver air at large flow ratesagainst low resistance. Blowers are usedto distribute air through ductwork whereresistance is high and lower volumes arerequired or to “throw” heated air a consid-erable distance in a ductless system. Mostunit heaters employ a fan, which is cycledon and off in a sequence related to theon-and-off cycling of the burner.

Location of Unit Heaters. Heaters shouldalways be located as close to the ceilingas possible, within the above-the-floormounting- height range recommended bythe manufacturer for the size and designof the specific heater. Mounting at heightsgreater than recommended results in poorheat distribution and stratification at thefloor and ceiling levels, which can causeoccupant discomfort. On the other hand,mounting the units too far below the ceil-ing results in accumulation of warm air atthe ceiling, increasing heat loss and heateroperating costs. Obviously, the ceilingheight is an important consideration in se-lecting and sizing a unit heater.

Indirect-fired Combustion, Unit Heaters

3-3Heaters with standard horizontal louvers,mounted at a height of 8 to 16 feet, canprovide adequate mixing and coverage atthe floor level. Vertical mixing becomesdifficult and requires careful requires care-ful size selection of optional vertical lou-vers or some other nozzle option mountedon the outlet supply air opening of theheater.

NOTE: Be certain to maintain the fire haz-ard clearances that are required for theseunits. These distances, determinedthrough test, are found in the installationinstructions and on the rating plate on theunit.

Sizing of Unit Heaters. The tables in Fig-ure 3-4 give the approximate coverage dis-tances in feet for fan and blower heatersequipped with standard adjustable lou-vers, 30° nozzles, and 60° nozzles. Oncethe heat loss of the building has been cal-culated (see Section 4), this table can helpin selection of number, size and type ofunit heater for the desired heating patternand the ceiling height of the space to beheated.

Heater size should be determined accord-ing to the ambient temperature to be main-tained and whether the heater will operateovernight or will be shut down. Overnightshutdown may require a larger unit heaterwith greater air “throw” to ensure breakupof the cold-air layer that develops at thefloor during shut down. A rule-of thumbfor applying the tables in Figure 3-4 forovernight shutdown conditions is to as-sume a mounting height 2 feet greater thanactual for each 5 degrees that the start-upambient temperature is below 50°F.

3-3 Propeller-type fans (left) are more common on unit heaters than centrifugalblowers (right)

NOTE: Data based on 80° entering air and 60° rise through unit, standard louver effective as indicated when ceiling height aboveheater is not over 4 feet. For high mounting or where spot heating is required, choose outlet and mounting height giving cover-age to floor. Mounting close to ceiling provides maximum heat utilization.

3-4. Unit-heater throw pattern and capacity tables. Such tables can be used for selecting heater sizes and locations.

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NOTE:O-X = Feet from heater to start of floor coverageO-Y = Feet to end of floor coverageO-Z = Feet to end of throw (50 feet per minute velocity).Table data based on louvers set at maximum deflection.

Mounting Heater SizeHeight 125 165 200 250 300 400to Heater X Y Z X Y Z X Y Z X Y Z X Y Z X Y ZBottom With Standard Horizontal Louvers 8 ft 14 24 65 14 35 75 13 38 83 12 44 94 12 36 105 12 55 11810 ft 16 22 58 16 32 72 15 36 78 15 40 88 14 38 96 14 53 11212 ft 18 20 54 18 30 66 17 34 72 17 38 84 16 35 90 16 49 10814 ft -- -- -- -- -- -- 20 31 68 19 33 77 18 30 85 18 45 10016 ft -- -- -- -- -- -- -- -- -- 22 30 72 20 27 80 20 40 9218 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 23 35 85

With Downturn Nozzle with 25-65 degrees Range of Air Deflection (30 degree Nozzle)10 ft 10 22 28 10 24 33 10 31 45 8 40 53 8 38 5112 ft 13 18 26 12 22 30 12 29 43 10 38 50 10 36 48 10 50 7014 ft 16 16 22 15 20 25 14 26 40 12 36 47 13 34 44 12 47 6616 ft -- -- -- -- -- -- 16 23 36 14 33 42 15 31 40 14 43 6218 ft -- -- -- -- -- -- 18 20 30 16 28 36 18 26 34 16 38 5820 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 18 34 5322 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 21 30 46

With Downturn Nozzle with 50-90 degrees Range of Air Deflection (60 degree Nozzle)12 ft 0 8 20 0 8 22 0 8 25 0 12 30 0 10 2816 ft 0 10 18 0 10 20 0 10 23 0 14 28 0 12 26 0 12 3220 ft 0 14 16 0 14 18 0 12 21 0 16 26 0 14 24 0 14 3024 ft -- -- -- -- -- -- 0 14 18 0 18 24 0 16 20 0 16 2828 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0 18 2632 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0 20 2436 ft -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0 22 22



Multiple-unit Heaters. Most commercialand industrial spaces require more thanone heater for both adequate heating ca-pability and properly uniform heat distri-bution throughout the heated space. Mul-tiple heaters should be located and spacedso that the air stream “wipes” the outerwalls as it moves in a circular patternaround the space. See Figure 3-5. For mini-mum turbulence, uniform heating, andproper air flow through the space, the air-flow pattern of each heater should overlapthe pattern of the heaters adjacent to it, assuggested in Figures 3-6, 3-7, and 3-8.

Paddle Fans. Most unit heaters now onthe market are designed to drive heatedair down from fairly high ceilings. But,even with downturn nozzles, these heat-ers may not force the air down to the floorfrom excessive heights. When units areunable to drive air down to floor level,paddle fans (also known as ceiling fans)can be used to help push hot air down toreduce the temperature difference betweenfloor and ceiling.

NOTE: When used in conjunction with at-mospheric unit heaters, paddle fans mustbe located so as not to affect the drafthoodrelief opening.

Paddle fans can work effectively fromheights up to 80 feet. With paddle fans aspart of the air-circulation system, unit heat-ers can be mounted at any height that doesnot interfere with headroom. Unit heatersand paddle fans working together can re-duce fuel consumption by as much as 30%and create a more comfortable heated en-vironment.

Unit Heater Controls. In unit heater ap-plications for small areas (or multiple unitheaters for larger open areas) individualroom thermostats for ON-OFF operationprovide an adequate and simple means ofcontrol. Unit heaters are available withsummer/winter switches to permit fan op-eration without burner operation duringwarm weather when only air circulation isdesired.

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3-5 Air flow of simple multiple-heater system (plan view).

3-6 Throw pattern with 30° outlet nozzles (side view).

Pilot-ignition Control Systems. Unit heat-ers contain one of the following types ofignition systems:

1. Manual match-lit gas pilot with 100%shut off;

2. Spark-ignited intermittent gas pilot;

3. Spark-ignited gas pilot with timed lock-out;

4. Direct spark--an electrical spark ignitesthe main burner;

5. Hot surface ignited gas pilot;

6. Hot surface ignited main burner.

Gas-control Systems. Unit heatergas-control systems may contain asingle-stage gas valve (which providesfull fire on a call for heat) or a two-stagevalve (which fires at either 100% or ap-proximately 50% as required by a remotetwo-stage thermostat).

Unit Heaters in Low Ambient Tempera-tures. Gas equipment for indoor use is de-signed and tested for ambient temperaturesof about 70°F. When such equipment isput to use at significantly lower tempera-tures, unusual performance (and some-times equipment damage) may occur.

The lower the ambient temperature, themore likely problems are to arise. First,trouble with the fan operation may showup at about 60°F or below. The fan switchmay cycle the fan during the heating cycle:when the fan first comes on in answer toheat demand signals, the low entering-airtemperature may cool the fan-switch ther-mostat below the fan cutout temperature,shutting the fan off; then, with the fan offthe thermostat element warms up again andrestarts the fan. This on-and-off cyclingevery few seconds may persist as long asthe incoming-air temperature remains at60°F or less.

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3-8 Throw pattern with 60° outlet nozzles (side view).

3-7 Throw pattern with standard louvers (side view).


Duct Furnaces

3-7More serious, less obvious trouble mayoccur if the ambient temperature is con-trolled to even lower temperatures like50°F or below. Condensation may occurinside (combustion side) of the heat ex-changer. The most common site for suchcondensation is at the top of the exchangerand on the side nearest the fan. This con-dition will eventually corrode the heat ex-changer wall because an acid forms bychemical reaction between the condensedwater and the combustion products. Theacid will actually eat holes in the ex-changer wall, rendering the unit unsafe.

A third possible problem resulting fromlow ambient temperature is that automaticspark-ignited pilots may be slow to light.With the combination of low ambient andlow stack temperatures, a downdraft canoccur over the burner and pilot, coolingthe pilot and igniter sufficiently to inter-fere with ignition. After long shutdowns,the pilot may fail to light altogether andmay require manual resetting if the igniteris the lockout type. Gas sometimes diffusesout of the pilot tubing over a long periodand the ignition system can go into lock-out before the air can bleed out of the pi-lot tubing. Lockout systems of more re-cent design have longer timing (120 sec-

onds) that should preclude this problemfrom occurring. There are few remedies forthese problems when equipment is alreadyin place. However, they can be avoided iflow ambient temperature is anticipated inthe selection of the heating equipment. Toavoid the corrosion problem, the buyershould insist on a stainless steel heat ex-changer. The buyer should specify that thethermostatic fan switch have a manualoverride switch to make the fan run con-tinuously when necessary. The fan cyclingproblem can also be solved with a time-delay relay whose contacts are wired inparallel with the fan control contacts.

Duct Systems for Unit Heaters. As mostcommonly applied, unit heaters are fanmodels and are not suitable for connec-tion to duct systems, however blower mod-els may be equipped with limited duct-work. The limitations are generally con-trolled by available static pressure as itrelates to the air volume in use. For infor-mation about sizing duct systems, see page4-8.

Duct Furnaces

Heaters specifically designed for use withduct systems (Figure 3-9) are called ductfurnaces. Although duct furnaces are de-signed for forced-air circulation only,manufactures supply them without blowerand filter sections which must be selectedand purchased separately or provided aspart of the air-distribution network. Ductfurnaces are available for installation in-doors or outdoors, depending on their de-sign and their certification through na-tional standards tests.

Combustion Air. Duct furnaces are avail-able only as indirect-fired models. Al-though combustion products are separatedfrom the heated air, combustion air is usu-ally supplied from the heated space onunits installed indoors. When the heatedspace contains contaminants, combustionair should be drawn from outdoors throughspecial air-intake ducts (see separatedcombustion application, page 3-22).

3-9 Typical duct furnace.


3-8Duct Systems. A duct furnace is a part of arather complex air heating and distribu-tion system. For efficiency and satisfac-tory heating results, its selection must becoordinated with the rest of the system. Inother words, the furnace, blower, filters,and the duct network should be designedand engineered to work as a system for thespecific structure or facility to be heated.

Airflow and Heat-distribution Consider-ations. Basically, a duct furnace must meettwo requirements. It must have sufficientheating capacity to offset the heat loss ofthe structure to be heated; it must be ca-pable of transferring its heat at a sufficientair flow rate. Manufacturers provide tech-nical data giving relevant ratings of ther-mal input and output capacity and maxi-mum and minimum airfoil rates. The tablein Figure 3-10 is an example of such datafor ONLY ONE STYLE of duct furnace.The requirements for airfoil capacity de-pend on the structure and the design ofthe duct network of which there are an in-finite number of possible variations. Manu-facturers’ installation instructions shouldbe reviewed because they recommendtypes of duct connections for good air cov-erage of the heat exchanger.

Duct furnaces must always be installeddownstream from the blower. NationalStandards do not permit upstream instal-lation because the negative pressure at theheat exchanger could contaminate theroom air with combustion products if the

heat exchanger develops a crack. More-over, cracks are much more likely in a“pull-through” system.

Bypass Ducts. Some duct furnaces requirebypass ducts to avoid unnecessary pres-sure drop through the heat exchanger whenlarge volumes of air must be circulated,especially in combination heating/cool-ing systems.

Insulation. Warm air ducts should notcome into contact with masonry walls orbe otherwise exposed to cold surfaces.Ducts that pass through masonry wallsshould be covered with insulation of 1/2inch thickness or more. Ducts that passthrough unheated space should also becovered with insulation that is at least 1/2inch thick.

Controls. Duct furnaces contain one offive types of ignition systems:

1. Manual match-lit pilot with 100% shutoff;

2. Spark-ignited intermittent safety pilot;

3. Spark-ignited safety pilot with timedlockout;

4. Direct hot-surface-ignition of mainburner with timed lockout;

5. Direct spark ignition of main burnerwith timed lockout.

Gas-control Systems. Duct furnaces maybe equipped with any of several gas-con-trol systems:

1. Single-stage control--a single stage gasvalve (which provides full fire on a callfor heat);

2. Two-stage control for heating applica-tions--a two-stage valve (which fires ateither 100% or approximately 50% asrequired by a remote two-stage thermo-stat);

3. Two-stage control for makeup-air appli-cations--a two-stage valve (which firesat either 100% or approximately 50%as required by a two-stage duct-mountedthermostat);

4. Electronically modulated control forheating applications--close temperaturecontrol with solid-state circuitry thatsmoothly varies the gas input between50% and 100% of the unit’s rated ca-pacity, as required by a call for heat froma wall-mounted room sensor. Such sys-tems may or may not be certified for usewith propane gas;

5. Electronically modulated control formakeup-air--close temperature controlwith solid-state circuitry that smoothlyvaries the gas manifold pressure between50% and 100% firing, as required by acall for heat from a unit-mounted ductsensor (such systems may or may not becertified for use with propane gas);

3-10. Example of a duct-furnace capacity and air-flow table.

SIZE 75 100 125 140 170 200 225 250 300 350 400BTUH input 75,000 100,000 125,000 140,000 170,000 200,000 225,000 250,000 300,000 350,000 400,000Thermal Output Capacity 60,000 80,000 100,000 109,200 132,600 156,000 175,500 195,000 234,000 273,000 312,000Full Load Amps (115V) 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2Full Load Watts 120 120 120 120 120 120 120 120 120 120 120Unit Control Amps (24V) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8Maximum CFM 1850 2465 3085 3455 4195 4935 5555 6170 7405 8840 9875Minimum CFM 615 820 1030 1150 1395 1645 1850 2055 2470 2880 3290Net Weight (lbs) 104 104 126 128 150 172 194 216 262 306 328Shipping Weight (lbs) 128 128 142 144 168 192 216 240 292 338 362Gas Connection (in) 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 3/4 3/4 3/4Venter Outlet Size (in) 4 4 4 4 4 4 5 5 6 6 6

3-9


Indirect-fired Combustion, Duct Furnaces

6. Electronically modulated control formakeup air with remote temperature se-lection--close temperature control withsolid-state circuitry that smoothly var-ies the gas input between 50% and100% of the unit’s rated capacity, as re-quired by a call for heat from a unit-mounted duct sensor which can be setfor a desired temperature from a remoteselector (such systems may not be certi-fied for use with propane gas).

NOTE: Electronic modulation is avail-able for both direct-fired and indirect-fired equipment. Check themanufacturer’s specifications for avail-ability with certain gaseous fuels;

7. Mechanically modulated control--anonelectric, capillary-actuated systemwhich smoothly varies the gas manifoldpressure between 50% and 100% firingas required on a call from a thermostaticsensor. This system cannot be used onpower-vented equipment unless the airmover is operated continuously.

Duct Furnaces in Drying Applications.Duct furnaces are used for drying in someindustrial processes. In each such applica-tion, a certain amount of ventilation andexhaust is needed to carry away the mois-ture driven from the material being dried.Practice has shown that, for most applica-tions, ventilation (makeup air) should beapproximately 105% of the total volumeof heated air.

Sizing of the furnace is important to en-sure both efficient and timely drying. Thefurnace must supply sufficient heat to off-set the normal radiational heat losses ofthe drying oven as well as to dry the mate-rial. In addition, allowance must be madeto heat the air added for ventilation to pro-vide the 10% ventilation required to re-move the process moisture, the total ofoven BTU losses and the drying BTUlosses should be multiplied by a factor of1.30 as shown by the formula below. Theresult is the total number of BTUs neededto complete the drying process in one hour.

H = 1.30 (0 + D)

where

O = Oven radiational losses in BTUH

D = BTUH required to dry material

H = Total BTUH required to complete drying in one hour, including 10% ventila-tion.

For drying times other than one hour, ven-tilating and oven-loss requirements are re-duced or increased proportionally for thedrying time desired, but the drying-heatrequirement is not changed because the

heat required to dry a given batch of mate-rial stays the same regardless of the dryingtime. The following example shows thecomplete calculation of required furnacesize for 1/2 hour drying time:

Drying oven losses = 5,000 BTUH

Heat required to dry material = 40,000 BTUH

Heat requirement = 165,000 BTUH

Multiplier to add ventilation = x1.3

Total heat requirement = 214,500 BTUH

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3-10

PACKAGED UNITS

Packaged units are complete, pre-engi-neered forced-air heaters (Figure 3-11)consisting of one or more duct furnaces,with blowers and filters in appropriate cabi-net enclosures, complete with air-duct,gas-supply and vent-pipe connections.Manufacturers offer a wide variety of fac-tory-installed and field-installed accesso-ries and options. Most packaged heatersare available for use with either naturalgas or propane, in heat input capacitiesranging up to 1,200,000 BTUs per hour.Packaged units are available as both indi-rect-and direct-fired models.

Packaged heaters (Figure 3-11) are avail-able in models for indoor, outdoor, androoftop installation. They are used for con-ventional indoor heating or for the heat-ing of makeup air in heating systems ofschools, churches, office buildings, shop-ping centers, laboratories, and a variety ofindustrial buildings.

Combustion Air. Indoor indirect-firedpackaged systems normally draw combus-tion air from the heated space. Outdoorand rooftop systems draw combustion airfrom outdoors.

Heat required for ventilation (214,500 - 165,000) = 49,500 BTUH

BTUs required to dry material in 1/2 hour:

Drying oven losses = 25,000 x 0.5 = 12,500

Heat required to dry material = 140,000

Heat for ventilation = 49,500 x 0.5 = 24,750

Total BTUs for 1/2 hour drying time = 177,250

Output Required:

Total BTUs for 1 hour = 177,.250/0.5 = 354,500

Furnace capacity required = output/efficiency = 354,500/0.77

= 460,389 BTUH

Duct Systems. A packaged unit is a part ofa rather complex air heating and distribu-tion system. For efficiency and satisfac-tory heating results, it is designed by themanufacturer so the furnace, blower, fil-ters and ducts operate as a system. A pack-aged unit should be carefully selected tomatch the specific structure or facility tobe heated, and a proper duct networkshould be designed for it. Two importantselections are the horsepower and drive tobe used. These choices are based on theair flow rate (CFM) and total static pres-sure (inches, W.C.) involved in the system.Each manufacturer publishes performancedata charts or curves for making these se-lections.

Insulation. Warm air ducts should notcome into contact with masonry walls orbe otherwise exposed to cold surfaces.Ducts that pass through masonry wallsshould be covered with insulation of 1/2inch thickness or more. Ducts that passthrough unheated space should also becovered with insulation at least 1/2 inchthick. Usually, the packaged unit has beeninsulated by the manufacturer accordingto its end use or its location. (Outdoor unitsare always insulated--indoor units may beinsulated upon request.)

Filter Requirements. Package systemshave filter sections ahead of the blowersections to remove dust particles from theair stream. These filters are usually 1 or 2inches thick, and come in both throwawayand washable types. Bag filters and otherspecial types are available for high filtra-tion rates.

Filters should be replaced or washed fre-quently because filters, especiallydust-laden filters, offer considerable resis-tance to air flow. Dust and lint accumula-tion can double the original airflow resis-tance of a clean filter.

Filter resistance is the largest single vary-ing resistance in a duct network. It shouldbe included in airflow calculations in de-signing ductwork for a packaged system.

Heating-only Use

Sizing. If a single unit is to be used to sup-ply all the required BTUs to the space, thetotal heat loss of the space must be dividedby the efficiency of the unit to be used todetermine the heat input.

(Load/Efficiency = Input)

The efficiency is usually listed in themanufacturer’s specifications.

Pilot-ignition Control Systems. The fur-naces of packaged systems may beequipped with one of six types of pilotignition systems:

1. Manual match-lit pilot with 100%shutoff;



4. Direct spark of main burner (with timedlockout).

5. Hot surface ignition of gas pilot (withtimed lockout);

6. Hot surface ignition of main burner withtimed lockout.


Indirect-fired Combustion, Packaged Units

3-11Gas-control Systems. Packaged systemsfor heating-only use may be equipped forone of three types of gas control:

1. Single-stage control--a single-stage gasvalve (which provides 100% fire on acall for heat);

2. Two-stage control--a two-stage valvewhich fires at either 100% or 50% asrequired by a remote two-stage thermo-stat;

3. Electronically modulated control--closetemperature control with solid-state cir-cuitry that smoothly varies the gas in-put between 50% and 100% of the unit’srated capacity, as required by a remoteelectronic sensor. Most of these systemsare not certified for propane gas.

Air Controls. In packaged systems, a heatsensitive device turns the blower on andoff when the heat exchanger temperaturereaches a preset temperature, normallyabout 125°F. This control provides a de-lay on startup (to prevent blowing coldair) and a delay after burner shutdown (toextract residual heat).

Heating and/or Makeup Air Use

Makeup Air. In some facilities, fairly largeamounts of air are lost to the outside ei-ther through an exhaust fan used to removesmoke, fumes, dust or objectionable odorsand undesirable gases or vapors from aprocess (e.g. restaurant kitchen range andoven hoods) or by the nature of the opera-tion (e.g. large doors remaining open forpassage of railroad cars or trucks). Airbrought in from outdoors to replace thelost air is known as makeup air. Packagedunits are frequently used to supply heatedmakeup air.

Sizing. When a unit is to be used to intro-duce makeup air and provide the heat re-quired to offset radiational losses of thebuilding, the volume (CFM) of air to beintroduced provides one load while theradiational loss of the building provides asecond load. These two BTUH input loadsmust both be accounted for to properlysize the unit. A convenient formula for de-termining the BTUs required to heat theoutside air, is as follows:

HR = (MAX 1.085 X T) / E

where

HR = Heat required for unit (BTUH)

MA = Makeup air (CFM)

T = Indoor design temperature minusoutdoor design temperature

E = Unit efficiency

If the makeup air unit is to supply heat foroffsetting radiational losses, this quantitymust be added to the “HR” determinedabove.

Pilot-ignition control systems. Packagedsystems may employ one of six types ofpilot ignition systems:

1. Manual match-lit pilot with 100%shutoff;



4. Hot surface pilot ignition;

5. Hot surface burner ignition (with timedlockout);

6. Spark ignited main burner (with timedlockout).

Gas-control Systems. Packaged systemsfor heating and/or makeup applicationsmay be arranged into many types of gascontrol systems. Some examples are:

1. Two-stage control--a two-stage valve(which fires at either 100% or 50% asrequired by a two-stage duct-mountedthermostat (ductstat).

2. Two-stage control using two furnaces--in such systems, each furnace isequipped with a single-stage valve. Thetwo valves are operated by a two-stageduct-mounted thermostat (ductstat). Thefurnace nearest the blower turns on firstand the downstream furnace comes onsecond when additional heat is calledfor.

3. Three-stage control using three fur-naces--in such systems, each furnace isequipped with a single-stage valve. Thethree valves are staged by two unit-mounted two-stage ductstats. The fur-nace nearest the blower turns on first,the center furnace comes on second, andthe downstream furnace operates last.

4. Four-stage control using two furnaces--each furnace is equipped with a two-stage valve. These valves are staged bytwo unit-mounted two-stage ductstats:the furnace nearest the blower is stagedfirst on low fire, second on high fire. Inthe third stage the downstream furnaceoperates on low fire followed by highfire for the fourth stage.

5. Six stage control using three furnaces--in such systems, each furnace isequipped with a two-stage valve. Thethree valves are staged by three unit-mounted two-stage ductstats and theoperating sequence is similar to the se-quence for the four-stage-two-furnacesystem: the furnace nearest the blowerturns on first, the center furnace second,and the downstream furnace last.

3-126. Electronically modulated control--close

temperature control with solid-state cir-cuitry that smoothly varies the gas in-put between 50% and 100% of the unit’srated capacity, as required by a call forheat from a unit-mounted duct sensor.

7. Electronically modulated control airwith remote temperature selection--close temperature control withsolid-state circuitry that smoothly var-ies the gas input between 50% and100% of the unit’s rated capacity, as re-quired by a unit-mounted duct sensorwhich can be set for a desired tempera-ture from a remote selector.

8. Mechanically modulated control—anonelectric, capillary-actuated systemwhich smoothly varies the gas input be-tween 50% and 100% of the unit’s ratedcapacity, as required on a call from athermostatic sensor. This system cannotbe used on power-vented equipment un-less the exhauster is operated continu-ously.

Air Controls. Air control systems formakeup-air application consist of somecombination of outside air damper,return-air damper, damper motors,mixed-air controller, and electrical con-trols such as relays, potentiometers,switches, and thermostats. These elementsare combined in a great variety of ways tomeet specific makeup-air requirements.Systems can be designed to provide 100%makeup air (continuously or intermit-tently) or they can be arranged to providevariable volumes of makeup air geared tovarying requirements. These systems mayuse: 2-stage, 3-stage, or continuouslymodulated dampers; time-clock ormanual-switch startup and shutdown; orautomatic opening and closing of outsideair dampers.

Use with Cooling Systems. When coolingis required, packaged units include a cool-ing unit, which may be either mechanical(compressor) or evaporative. In combina-tion heating-cooling systems of eithertype, the airflow requirement for coolingis greater than the requirement for heat-ing. Therefore, air flow requirements shouldbe based on the maximum CFM, and themotor horsepower and drive should be se-lected for such capacities.

Mechanical Cooling. Cooling through theuse of DX (Direct Expansion) coils orchilled water coils is possible with pack-aged systems. However, in most cases, thecoil must be added to the ductwork, down-stream from the package. Care should beused in sizing coils so that air velocitiesare acceptable for the duty to which thecoils are to be placed. Low velocities aredesired when cooling outside air, whilegreater velocities may be employed whencooling recirculated air.

Evaporative Cooling. Most areas of theUnited States have the climatic conditionsfor effective evaporative cooling (i.e. hightemperatures and low relative humidity).This type of cooling is applicable wherethe difference between dry-bulb tempera-ture and wet-bulb temperature is 17 to 19degrees. In most applications, evaporativecoolers operate at efficiencies of 75 to 85percent.

Evaporative coolers use the heat-absorb-ing action of the water-evaporation pro-cess to remove heat from air for circula-tion in a building. The coolers in pack-aged systems consist of nozzles that spraywater onto evaporation pads, a water res-ervoir with float and bleed valve, a water-recirculation and spray pump, and air fil-ter (See Figure 3-12). In addition to pro-viding a large surface area for water evapo-ration, the evaporation pads act as a heatexchanger--air for the building cools as itpasses on inside walls of the pads as evapo-ration takes place on the outside.

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3-12 Evaporative cooling system

1. Evap-pad filters2. Spray nozzles3. Float Valve

4. Overflow5. Float bulb6. Drain

7. On-off switch for pump8. Junction box9. Spray pump


Direct-Fired Combustion

3-13Makeup Air. In some facilities, fairly largeamounts of air are lost to the outside ei-ther through an exhaust fan used to removesmoke, fumes, dust or objectionable odorsand undesirable gasses or vapors from aprocess (e.g. restaurant-kitchen range andoven hoods) or by the nature of the opera-tion (e.g. large doors remaining open forpassage of railroad cars or trucks). Airbrought in to replace the lost air is knownas makeup air.

Direct-fired packaged units are frequentlyused to supply heated makeup air. Sincethe combustion products mix with the cir-culated air, direct-fired packaged heatersusually are limited to makeup air applica-tions. Some examples are lumberyard andbuilders’ supply warehouses, foundries,shipping and receiving buildings of heavyindustrial plants, large aircraft hangars, etc.Although indirect-fired packaged heaterscan be used for heating makeup air, direct-fired package units are more widely usedfor this purpose and offer the more eco-nomical way to provide makeup air.

Forcing heated makeup air into a build-ing offsets the negative pressure createdby exhaust fans and creates a slightly posi-tive pressure inside the building. This posi-tive pressure eliminates infiltration, drafts,and makes an exhaust system more effec-tive. At the same time, the heated makeupair helps maintain a more uniform, com-fortable temperature inside the buildingeven though inside air leaves and fresh airenters. Because heated makeup air elimi-nates uncomfortable drafts, it saves fuelby eliminating “thermostat jockeying” thefrequent changing of the thermostat set-ting by occupants to try to improve com-fort.

Combustion/Dilution Air. As the term, di-rect-fired implies, the gas is burned in thecirculated air stream. This can be accom-plished through the use of special burnerdesigns which contain the flame within asmall zone of the air stream. The air/gasratio is 4 to 1 in the flame but the air down-stream from the burner has an air-to-com-bustion-gas ratio of 600 to 1. This highratio results in approximately 5 PPM ofcarbon monoxide and a maximum of 2000PPM of carbon dioxide in the deliveredair stream.

Duct Systems. Direct-fired makeup-airpackages may be attached to distributionduct systems. As in the indirect-fired pack-ages, the airflow rate (CFM) and the exter-nal static pressures must be projected tosize the horsepower and drive properly.

Exhaust/Relief. While some direct-firedunits are used for pressurized heating, mostmakeup-air systems are accompanied withmechanical exhausters or gravity-reliefdampers. The mechanical exhausters areusually the reason for needing makeup-air; power-relief dampers normally are usedin pressurized buildings where ventilationis the prime reason for the makeup-air unit.

Makeup-air-only Systems. Figure 3-13 il-lustrates one of three basic arrangementsfor which packaged heaters are availablefor providing makeup air. This arrange-ment provides 100% makeup air at con-stant airflow rate to match the constant rateof exhaust of a specific process (restau-rant-kitchen hood, for example) or a di-versified industrial process which mightbe operated on an intermittent basis.

Units are also available to provide 100%makeup air at a variable air flow rate (Fig-ure 3-14). This arrangement is often usedto supply general makeup air to a spacehaving variable rates of exhaust. In thistype of system, a building-pressure sensorcontrols the makeup air to keep the build-ing slightly pressurized even though theexhaust systems operate intermittently.This is accomplished by a dischargedamper which may be positioned to ad-just the quantity of air needed to matchthe air being exhausted. This system is themost efficient because it eliminates theneed to heat excess amounts of makeupair and reduces blower-motor power con-sumption, as the air throughput is reduced.Forward-curved blowers are generallyused, therefore as the air is throttled, thebrake horsepower requirements diminish.In the system illustrated in Figures 3-14and 3-15, a bypass damper is also reposi-tioned by internal controls, so the air ve-locity (volume) over the burner remainsconstant.

Sizing. Direct-fired packaged systems aresized to heat and admit just the rightamount of air to either pressurize thebuilding slightly or to create a slightvacuum within the space, depending uponthe type and amounts of contaminantspresent and the use of the space.

Process Makeup Air. If a building isserved by a single separate packaged sys-tem and a single fan exhausts process con-taminants that should not be allowed tomigrate to other parts of the building,makeup air volume should provide about90% of the exhaust-air volume. This ratioof makeup air to exhaust air would createthe slight vacuum needed to ensure thatcontaminants do not seep into the adjoin-ing spaces. If a building is equipped witha number of exhausters to evacuate con-taminants that are not threatening to theoccupants, the makeup-air volume shouldbe 110% of the exhaust-air volume. Thisratio will provide the mild pressurizationneeded to reduce infiltration. To reduce oreliminate infiltration, this part of the cal-culated heat loss may be omitted from thetotal building heat loss, reducing theBTUH requirements for the convected heatloss.

3-12To develop the most efficient makeup-airsystem, the exhaust CFMs must be care-fully calculated so that the makeup-air vol-ume can be accurately identified and suf-ficient capacity incorporated into themakeup-air equipment. Duct, diffuser, andfilter losses must also be determined toestablish the blower-motor horsepowerand speed required.

Building Makeup Air. Full-building heat-ing can be accomplished with makeup air.Makeup-air volume should be at least150% of the building’s infiltration rate.This ratio will result in pressurization suf-ficient to completely eliminate infiltrationand distribute heated air throughout thebuilding. To determine the total BTU out-put required of the packaged unit, add theheat losses of the building (excluding in-filtration losses) to the BTUs necessary toheat the makeup air.

Gas Controls. Gas controls on direct-firedpackages are usually of the modulating“A” type where the input to the mainburner can be reduced to 4% of the ratedburner input when the unit is at full fire(i.e. maximum BTU input for the length ofthe burner). This allows for nearly infinitecontrol of the discharge temperature. Ap-propriate safety controls are always sup-plied and are suitable to meet any of thefollowing codes: ANSI, FM or IRI.

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3-13 System for providing 100% makeup air at constant air-flow rate.

3-14 System for providing 100% makeup air with a variable air-flow rate.

3-15 This makeup air system varies the volume of makeup air while keeping therecirculated air-flow rate constant.

3-15


Direct Fired Combustion

Air Controls. Variable air volume systemsinclude a modulating discharge damperwhich is controlled by either a manual po-tentiometer or a pressure-sensing controlfor monitoring space pressures.

A trim damper is used. It is located in anair duct which bypasses the burner duct.This trim damper operates independentlyfrom the discharge damper and automati-cally controls the velocity of air over theburner.

Heating and Makeup Air Use. Packagedsystems are available for providing bothmakeup air and heated recirculating air.Figure 3-15 illustrates a system that com-bines variable-flow-rate makeup air withvariable-rate recirculation of inside air. Byproviding variable amounts of heatedmakeup air while reheating the inside air,such a system eliminates the need for sepa-rate space heating equipment.

Sizing. When sizing a makeup air unit forboth heating and makeup air, the designermust be certain that the unit can furnishall the BTUs necessary to temper the airand heat the building when the air pass-ing through the unit is delivered from theoutside. Therefore he must calculate theBTUH needed to heat all the air from out-side and must then add the BTUH neededto actually heat the building.

Gas Controls. Since the unit is operatedto create a desired temperature level withinthe space, it is recommended that a spacethermostat be used. Space thermostats areavailable to reset the discharge tempera-ture of the heating package to either addheat as required or to supply air at lowertemperatures to accommodate coolingwhen heat gain in the space has driven thetemperature above the setting of the spacethermostat.

Air Controls. Recirculating units areequipped with return air dampers that arecontrolled by a manual potentiometer orby a pressure-monitoring device locatedin the space. This system also contains atrim damper that is located in a duct justabove the burner. The trim damper oper-ates independently of the recirculated-airdamper and automatically maintains theproper air velocity across the burner.

Use of Heating Units with CoolingSystems

Mechanical Cooling. In mechanical cool-ing through the use of DX (Direct Expan-sion) on chilled water, the coils can be lo-cated in the discharge duct. Care must beused in sizing the motor and drive sinceall coils resist air flow and will add exter-nal losses that must be overcome.

Evaporative Cooling. Use of evaporativecooling with direct-fired equipment is aspecial case. The heating equipment manu-facturer should be consulted to see ifevaporative cooling is applicable to spe-cific equipment.

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3-16 Example of commercial infrared heater.

3-17 Usual methods of suspending infrared heaters from ceiling.

3-16Infrared Radiation Heating

Radiant heaters are comparable in someways to unit heaters. These compact, self-contained units (Figure 3-16) are usuallymounted near the ceiling. They are bettersuited to spot heating than unit heatersbut are also excellent for space heating. Awide range of facilities can be heated foroccupant comfort with radiant heaters:factories, auto repair shops, storerooms, su-permarkets, gymnasiums, swimming-poolenclosures, auto showrooms, public park-ing garages, etc. Unlike unit heaters, ra-dian heaters require no fans or blowers.They are quiet and incur no significantelectrical energy cost. Because they con-tain a minimal number of moving parts,service requirements are lower than withcomparable forced air units. Installationcosts are low because their light weightallows them to be easily suspended fromthe ceiling (Figure 3-17) or mounted onwalls (Figure 3-18).

Wall Mount

I-Beam Mount

Wall Mount

Wall Mount

3-18 Methods of mounting infrared heaters on walls and othervertical surfaces.


3-17

Infrared Radiation Heating

High-intensity Infrared. Heater manufac-turers offer high-intensity radiant heatersin capacities commonly ranging from22,000 BTUs per hour to 100,000 BTUsper hour for natural gas or propane, butheaters well above 100,000 BTU per hourare also available. Because they transferheat by radiation, infrared heaters warmpeople, floors, walls, machinery, and othersurfaces without heating the air in be-tween. As a result, they eliminate the costlyceiling heat losses and the discomforts ofcold-air stratification that can occur withforced air systems. Fuel savings are some-times available (compared to forced-airsystem costs) because roof and wall lossesgenerally are insignificant in sizing for fullbuilding heating with radiant heaters.

Combustion Air. An advanced infraredburner is shown in figure 3-19. The fuelgas is metered through the orifice (1) tothe venturi mixing tube (2) where the gasstream draws in the primary air requiredfor combustion. The enrichment port sup-plies a rich gas mixture to the ignition areaon startup for improved light-off. The ori-fice and venturi are designed to supply100% primary air for complete combus-

tion. If insufficient air were aspirated, in-complete combustion would result; if toomuch air were aspirated, burner efficiencywould be reduced by the heat loss of thisexcess air.

Infrared burners may need to be re-orificedfor operation at high altitudes. You shouldcheck with the individual manufacturer forthis information. Usually a smaller orificeoperating at higher gas pressures will com-pensate for the thinner air because it willproduce a higher velocity gas streamwhich will aspirate more air, insuring suf-ficient oxygen for complete combustion.

Exhaust/Relief. Radiant heaters are notnormally vented through a flue to the out-side, but certain models require ventingwhich is accomplished with normallyavailable vent pipe.

Generally, exhaust fans are used in con-junction with radiant heaters to meet dilu-tion requirements of 3.78 CFM per 1000BTUH of natural gas or 4.55 CFM per 1000BTUH of propane. Unvented infrared heat-ers require minimum amounts of ventila-

tion air for dilution of the products of com-bustion. The following formula gives theminimum ventilation required to maintainadequate dilution:

V = C/60,000 x k

where

V = minimum ventilation (CFM per1000 BTUH of fuel)

C = total required infrared capacity(BTUH)

k = a constant (227 for natural gas; 273for LP gas)

If ventilation or infiltration rates are notsufficient to meet this requirement, powerventilation should be added to provide thenecessary dilution air and exhaust com-bustion products. If the combustion prod-ucts are not eliminated from the building,there is a definite possibility that they willcondense in the ceiling or on metal framesand structures. Such condensation can beextremely harmful to any materials onwhich it forms.

Unvented infrared heaters should not beused in buildings that conduct degreasingor dry cleaning with chlorinated solventslike perchlorethylene unless all solventoperations are individually and efficientlyvented. Reason: the vapors of such sol-vents decompose to hydrochloric acid andother dangerous and corrosive productswhen they pass through an open flame.

(1) Orifice (2) Mixing Section

Porous Ceramic Plate or Stanless Steel Platewith Minute Drilled Ports Flame

3-19 High-intensity burners have these basic parts.

3-18Sizing. For full building heating the ra-diational losses of the building must beestablished. Then this value is multipliedby 0.85 to ascertain the necessary input.Sizes of units are determined by the areaand the mounting height. For spot heat-ing, special intensity charts must be usedand are available from the manufacturer.

Location. Radiant heaters are inherentlyversatile and can be installed in a varietyof locations and orientations. Most mod-els on the market can be mounted with theradiant ceramic plate horizontal or tiltedthrough any angle within 30 degrees ofthe horizontal position. For horizontalmounting they can be suspended from theceiling by chains, threaded rods, or pipebrackets; angle-mounting from walls maybe made with a wide variety of brackets.

Whole Building or Spot Heating. Aver-age floor-to-burner mounting heights forhigh-intensity infrared heaters range from12 feet to 28 feet for space heating; almostany height for spot heating, as required.Figure 3-18 shows a number of commonmounting methods.

Controls. High-intensity radiant heatersfor commercial and industrial use comecomplete with direct spark ignition, oper-ating and safety valves and controls, gas-supply connections, and mountingflanges. Generally, exhaust fans are usedin conjunction with radiant heaters to meetdilution requirements. In many cases, fansare integrated with thermostats to provideventilation when heaters are on and pre-vent unnecessary ventilation and heat losswhen heaters are not operating.

Low-intensity Infrared. Low-intensity ra-diant heaters are generally constructed ofround pipe. The portion of the pipe near-est the burner is either made of stainlesssteel or is ceramic coated. The tempera-ture of the radiational tubes does not ex-ceed 900°F (1100°F with aluminizedtubes) and may drop as low as 100°F nearthe exhaust end. Low-intensity infraredunits are generally equipped with powerburners and often include powered ex-hausts.

Sizing. Low-intensity radiant heaters aresized much the same as the high-intensityunits, but their mounting height is usu-ally restricted to lower heights than thehigh-intensity units because the sourcetemperature is approximately half that ofthe high-intensity units.

Location. Low-intensity units are avail-able in various lengths and some versionsare connected in series so that fairly longcontinuous runs may be used. In most full-building infrared applications, the unitsare located around the perimeter and theirshades or reflectors are positioned to mini-mize the infrared rays striking the wall.Infrared units are used primarily to heatthe floor; with this in mind the importanceof placement and angles of deflection be-comes clearer.

Controls. Controls for radiant heaters arefairly simple except for the fact that powerblowers are used for burner aeration. Thisdictates the need for other devices such asflow switches and spark ignitors. Flame-rectification, direct-spark ignition systemshave been generally used since standingpilots or even spark-ignited pilots have notbeen considered practical because of thevelocity of the gas-air mixture passingthrough the system. However, hot-surfaceignition systems are becoming increasinglycommon.

Whole-building Heating and Spot Heating.Average floor-to-burner mounting heightsfor low-intensity radiant heaters range from8 feet to 12 feet for space heating; and ap-proximately the same heights for spot heat-ing. Figures 3-17 and 3-18 show a numberof common mounting methods.

3-19 INDIRECT-FIRED HEATING


Example Applications

UNIT HEATERS

Unit heaters can be applied in a great vari-ety of ways. They provide a particularlyconvenient way to replace old heating sys-tems, especially steam and water systems

Manufacturing Plant

Facility: Cutting tool manufacturing shopcovering 40,000 square feet, previouslyequipped with 3-million BTUs,low-pressure, gas-fired steam boiler.

Conditions: severe machine shop condi-tions with metal cutting, grinding, andheat-treating operations carried out rou-tinely.

Equipment installed: Six 300,000 BTU/hour fan-type unit heaters for main shoparea and one 140,000BTU/hour unit heaterfor the separate heat-treating room.

Greenhouse

Facility: Cluster of five new greenhouses,each 42 feet wide by 232 feet long.

Conditions: High humidity, seacoastsalt-air climate, agricultural chemicals en-vironment.

Equipment installed: Fifteen 250,000-BTU/hour blower-type unit heaters withstainless steel heat exchangers, burner,spark pilot and OSHA belt guards. Unitswere specially equipped with galvanizedblowers for corrosion resistance against thechemical environment.

because new ductwork need not be in-stalled in the existing building. The fol-lowing examples show how they havebeen used in both new construction andretrofitting.

3-20School Building

Facility: Two-story school building, 50feet wide by 300 feet long, with 25 class-rooms and laboratories, formerly served by50-year-old oil-fired steam boiler system.

Conditions: separate room

temperature control and maximum energysavings required .

Equipment installed: Twenty-five

125,000-BTU/hour blower-type unit heat-ers.

DUCT FURNACES

Commercial

Facility: Supermarket.

Equipment installed: A 225,000-BTU ductfurnace with blower package and coolingcoils was installed in an enclosed roof-topshelter. The furnace includes a stainlesssteel heat exchanger and power venting.



3-21 PACKAGED SYSTEMS

Manufacturing, Heating Only

Facility: Factory with 40,000 square feetof floor space.

Equipment installed: Six duct furnaces of200,000 BTUH input capacity each, at-tached to blower-filter packages to allevi-ate dust condition created by plant pro-cesses.

Manufacturing, Heating andMakeup Air with Cooling

Facility: Factory engineering sectionwhich required heating, cooling, andfresh-air introduction.

Equipment installed: The packaged sys-tem contained a 225,000-BTUH furnaceand 15-ton air conditioner to handle thefresh-air load.

SEPARATED COMBUSTION 3-22

Manufacturing

Facility: One of the largest plants in theU.S. in which vitreous-china plumbing fix-tures are manufactured under one roof,with a total area of 75,000 square feet anda ceiling height of 18 feet. The originalheating system had 50 sets of steam-heatedcoils and blowers in the ductwork of theair-distribution system.

Conditions: Very dusty atmosphere; pro-cess required area to be maintained at nor-mal room temperatures during the day,then heated up to

110 degrees F overnight to dry the workin process, remove the

moisture from the building, and be readyfor the next day’s operations at normalworking temperatures.

Equipment installed: Fifty400,000-BTUH duct furnaces were in-stalled in place of the steam coils, in thesame space between the two ducts at eachblower location. Separated combustionfurnaces were used for higher efficiencyand because of the dusty atmosphere.


3-23


DIRECT-FIREDCOMBUSTION

B��53�5��$��

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Kitchen Makeup Air

Facility: Commercial kitchen using10,000 CFM of exhaust.

Equipment installed: Roof-mounted unitdesigned to supply 9,000 CFM of fresh airthe lower supply of makeup air comparedto exhaust prevents cooking odors fromentering the dining area.

Factory Heating and Makeup Air

Facility: Factory with exhausts for paintspray booths.

Conditions: 15,000 CFM exhaust volume;13,500 CFM makeup-air volume to pre-vent migration of paint overspray.

Equipment installed: Direct-fired unit in-stalled indoors to eliminate extensive roofrevision that would have been necessarywith roof-mounted unit. Burners sized for1,250,000 BTUH to maintain 70-degreetemperature.

3-24Whole-building Heating

Facility: Manufacturing plant.

Conditions: 27,000 square-foot area; grav-ity vent at roof peak to release flue prod-ucts.

Equipment installed: Thirty-two50,000-BTUH high-intensity heaters,equally spaced on outside wall,cantilever-mounted at 30-degree angle.

Spot Heating

Facility: Factory quality-control area.

Conditions: workspace area measuring 20feet by 16 feet, located next to high-trafficwarehouse door with vinyl curtain.

Equipment installed: A single75,000-BTUH high-intensity infrared unitmounted horizontally at height of 18 feetfor comfort of technicians performing tests.


LOW-INTENSITY HEATERS


3-25

Whole-building Heating

Facility: Warehouse area, approximately25,000 square feet.

Equipment installed: 400 linear feet oflow-intensity infrared heaters with reflec-tors, located at 20-foot height and spacedfor optimum coverage.

3-26

Assembly-line Heating

Facility: Factory production line.

Conditions: Well-Ventilated area, workerslocated within 15 feet of assembly line.

Equipment installed: 1200 feet of low-in-tensity heaters, mounted 12 feet abovefloor, made up of 40 sections spaced towarm floor and stationary objects whereneeded to radiate heat to workers. Work-ers also receive warmth directly when theyare within the radiation pattern of theunits.

3-20. These symbols of theCanadian StandardsAssociation certifies thesafety of gas-fired equip-ment. The symbol abovecertifies that the equipmentcomplies to ANSI Stan-dards for The UnitedStates. The symbol to theright certifies that theequipment is approved foruse in Canada.

3-21. This seal of the Underwriters’Laboratories, Inc., shows that equipmentis listed as having complied withANSI standards.

Standards, Codes and Test-ing

INDUSTRY STANDARDS

Industry standards are voluntarily fol-lowed rules and regulations governing thedesign, manufacture, installation, opera-tion, and use of manufactured products. Inthe United States, a large number oforganizations (engineering societies,manufacturers’ trade associations, federalgovernment agencies, etc.) formulate andmaintain the thousands of industry stan-dards in effect today. Any nationally recog-nized laboratory may test to ANSI stan-dards.

American National Standards Institute(ANSI). A number of organizations issuestandards that apply to gas-fired spaceheating equipment. The American Na-tional Standards Institute (ANSI) performsthe clearinghouse role of coordinating thestandards of these organizations. ANSIcommittee Z83 works with the variousstandards for industrial gas equipment;committee Z223 oversees compliancewith the National Fuel Gas Code. The workof both of these committees applies to in-direct- and direct-fired equipment and toradiant infrared heaters.

CODES

Codes are industry standards given theforce of law by legislative action of fed-eral, state or local government. The legis-lative bill or ordinance may adopt an in-dustry standard by incorporating it as averbatim or modified text; simply refer-ring to the standard by name or number isa common means of adoption. Either way,the standard is no longer voluntary in thejurisdiction of the legislation and failureto comply may make the contractor orowner subject to penalties as prescribedby the legislation.

National Fire Protection Association(NFPA). One of the codes that applies toall types of heating equipment is the NFPA


Safety and Maintenance

code. This code is solely concerned withfire safety and prescribes “reasonable pro-visions based on minimum requirementsfor safety to life and property from fire.”Everyone engaged in installation, main-tenance, servicing, operation, and care ofheating equipment of all types should bethoroughly familiar with the NFPA code.

National Fuel Gas Code (Z223). The in-stallation, maintenance, operation andsafety of gas-burning devices is coveredby the National Fuel Gas Code. In mattersof safety this code is similar to the NFPAcode and the two overlap to some extent.Everyone working with gas-fired equip-ment should be thoroughly familiar withthis code.

Insurance Codes. Insurers of buildingsmay have certain requirements or limita-tions on heating equipment installed inbuildings which they insure. Two such in-surers are

FM - Factory Mutual

IRI - Industrial Risk Insurers (Formerly FIA)

These two agencies often require that ad-ditional controls and accessories be addedto the equipment. If a building will be in-sured by either of these two agencies, it is

essential to contact the agency’s local in-spector to obtain a clear definition of whatis required to meet its standards for anyheating, makeup air or infrared applica-tion that is planned.

CERTIFICATION TESTING The fol-lowing agencies will certify or may influ-ence the general makeup of equipment (asin the case of independent insurers). Alltest agencies observe ANSI standards wheninvolved in investigating individual ap-pliances.

Canadian Standards Association(CSA)

Manufacturers of gas-fired space heatingequipment submit their products to CSAfor detailed examination and testing to de-termine compliance with ANSI (UnitedStates), CSA (Canada) and other nationaland international standards. Scrutiny byCSA is concerned with safety and perfor-mance reliability, efficiency, and durabil-ity. The CSA seal of approval, Figure 3-20, is the buyer’s assurance that the par-ticular model of heater or furnace haspassed rather stringent tests, and that theunit has been found in compliance withANSI approval requirements.

3-27

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3-28Underwriters’ Laboratories, Inc. (UL).Underwriters’ Laboratories tests to ANSIstandards for safeguarding people andproperty from fire and accidents in the useof electrical equipment of all kinds. Thisorganization also tests and evaluates elec-trical equipment and components as tocompliance with ANSI standards when in-stalled according to the National Electri-cal Code. Although UL issues a seal (Fig-ure 3-21) denoting that the equipmentbearing the seal is listed as having com-plied with ANSI standards, the electricalcomponents of space heating equipmentmust be installed to comply with the Na-tional Electrical Code for the UL seal tobe meaningful.

ETE (formerly Electrical Testing Labo-ratories). An independent source of test-ing to determine quality, safety, and per-formance of many types of manufacturedproducts. ETL also verifies conformanceto specifications and provides third partycertifications. In 1986, this organizationchanged its formal name to its initials,ETL, because its scope expanded beyondelectrical testing and into gas combustiontesting.

CGA (Canadian Gas Association). TheCanadian Gas Association is the Canadiancounter part of AGA and operates to cer-tify, by test, all gas appliances, accordingto standards issued by The National Stan-dards of Canada.

Independent Laboratories. Laboratorieslike those listed above (i.e. AGA, UL, ETL,CGA) only test to standards. They do notgenerate standards, although they oftencontribute to standards writing along withrepresentatives from manufacturers, utili-ties, and interested government agencies.

Local Code Requirements. Most majorcities have their own codes covering fireprevention, plumbing and heating and re-lated matters. These codes are applicablein addition to national and industrialcodes and may have special requirementsnot covered in the more broadly appli-cable codes. These jurisdictions usuallyrequire that installed appliances complywith ANSI standards and accept AGA cer-tification of compliance. Local authoritiesshould be consulted about how thesecodes apply to specific installations.

Clearances. In the context of installing orlocating space heating equipment, clear-ance refers to the minimum clear, unob-structed distance between a heater, or aspecific part of it, and the nearest combus-tible object or surface.

Manufacturers specify the clearance fromcombustible objects or surfaces for eachpiece of equipment, and the involved testagency confirms suitability of these clear-ance dimensions. Installation of heatersmust conform to these specified clearancesstrictly and to any applicable local codes.The following table gives typical “clear-ances from combustibles” for the varioustypes of space heating equipment.

CAUTIONS AND LIMITATIONS

Hazard Intensity Levels. I In the interestof clarity of communications where safetyis concerned, the industry has adopted stan-dard definitions for words calling atten-tion to hazards in dealing with heatingfuels and heating equipment. These stan-dard words are defined below; any state-ments or instructions following thesewords in manufacturers’ literature shouldbe heeded exactly and carefully.

1. DANGER: Failure to comply will resultin severe personal injury or death.

2. WARNING: Failure to comply could re-sult in severe personal injury or death.

3. CAUTION: Failure to comply could re-sult in minor personal injury or propertydamage.

Common Precautionary Statements. Ev-ery item of heating equipment will havefeatures that require specific precautionsfor safe use. Anyone installing, servicing,or operating gas-fired heating equipmentshould thoroughly read the manufacturer’smanual covering the specific equipmentfor hazard information peculiar to thatequipment. In addition, they should alsoknow the general safety precautions com-mon to all equipment. Some of the morecommon precautions found in manufac-turers’ literature are quoted below. Mostwarning statements in use today are re-quired by ANSI and must be given verba-tim, therefore they are quoted word-for-word by all manufacturers.

At Top At Rear At Sides At Front At BottomUnit Heaters 6 32 18 + 12Duct Furnaces 6-36** - 6 - 3-6Packaged Units 6-36** - 6 - 3Radiant 30-42 24-36 24-42 54-180# 54-180++

++ Some heaters have optional shades that require greater clearance# With unit at 30 degree tilt

CLEARANCES* FROM COMBUSTIBLES (inches) EXAMPLE ONLY

* Typical, consult manufacturer for specific equipment+ Air flow from front, should not be obstructed at any distance** Varies with size and design



3-29WARNING: Gas-fired appliances are notdesigned for use in hazardous atmospherescontaining flammable vapors, combustibledust or chlorinated or halogenated hydro-carbons.

WARNING: All components of a gas sup-ply system must be leak tested prior toplacing equipment in service. NEVERTEST FOR LEAKS WITH AN OPENFLAME.

WARNING: For best operation gravity-vented outdoor heaters should be locatedon roof or slab with at least a 20-foot ra-dius between the center of the vent capand obstructions such as walls, parapetsor cupolas.

WARNING: Failure to provide properventing could result in death, serious in-jury, and/or property damage. Unit mustbe installed with a flue connection andproper vent to the outside of the building.Follow installation codes and venting rec-ommendations in the installation manualfor this heater. Safe operation of any grav-ity-vented gas equipment requires a prop-erly operating vent system, correct provi-sion for combustion air and regular main-tenance and inspection.

WARNING: The use and storage of gaso-line or other flammable vapors and liq-uids in open containers in the vicinity ofthis appliance is hazardous.

CAUTION: Joints where flue ducts attachto furnace must be sealed securely to pre-vent air leakage into draft hood or burnerrack area. Leakage can cause poor com-bustion, pilot problems, shorten heat ex-changer life and cause poor performance.

CAUTION: Make sure the thermostat hasan adequate volt-ampere rating for the to-tal load to be placed on it. Add up the am-perage ratings of all relay coils, valve-so-lenoid coils or valve hot-wire coils, etc.,and match thermostat rating to the total.

CAUTION: If any of the original wire assupplied with the appliance must be re-placed, it must be replaced with type TEW105°C wire or its equivalent, except forenergy cutoff or sensor lead wire whichmust be type SFF-1 (150°C) wire or itsequivalent.

CAUTION: The appliance is equipped fora maximum gas supply pressure of 1/2pound, 8 ounces or 14 inches water col-umn. NOTE: supply pressure higher than1/2 pound require installation of an addi-tional service regulator external to the unit.

CAUTION: The appliance must be iso-lated from the gas supply piping systemby closing its individual shutoff valveduring any pressure testing of the gas sup-ply piping system at test pressure equal toor more that 1/2 PSIG (3.45 KPa).

CAUTION: Due to high voltage on pilotspark wire and pilot electrode, do not touchwhen energized.

CAUTION: The operating valve is theprime safety shutoff. All gas supply linesmust be free of dirt or scale before con-necting the unit to insure positive closure.

Miscellaneous Precautions. Some heat-ers are equipped with built-in diverterswhich prevent the return of burned gasesto the combustion chamber by releasingthem from the heater, in case their passagethrough the flue is retarded or prevented.This is as far as any manufacturer can goto insure the safety of his product; the re-sponsibility for safely and properly vent-ing such heaters from this point on lieswholly with the installer.

Important Note: Proper operation can beimpaired if drafts are permitted to strikethe relief opening of atmosphericallyvented appliances. Therefore, locate unitin an area that is reasonably free from suchdrafts.

General Safety Tips.

If you smell gas:

1. Open windows.

2. Don’t touch electrical switches.

3. Extinguish any open flame.

4. Immediately call your gas supplier.

Manufacturers’ warranties usually are voidif...

(a) furnaces are used in atmospherescontaining flammable vapors or at-mospheres containing chlorinatedor halogenated hydrocarbons;

(b) wiring is not in accordance with thediagram furnished with the heater;

(c) the unit is installed without properclearances from combustible mate-rials or is located in a confined spacewithout proper ventilation and airfor combustion;

(d) furnace air throughput is not ad-justed to fall within the range speci-fied on the rating plate;

(e) other specific qualifications of war-ranty are not observed by the in-staller or user.

MAINTENANCE

Quality gas-fired heating equipment op-erates reliably with a minimum of mainte-nance. However, to ensure good perfor-mance over a long service life, heating sys-tems should be regularly inspected,cleaned, and (if necessary) appropriatecomponents adjusted and lubricated asrecommended by the manufacturer.

3-30

3-22. A properly tensioned belt should deflect about 1/2 inch when pressed withmoderate effort.

Good Housekeeping is Important. Thesingle most common cause of unsatisfac-tory performance and below-standard op-erating efficiency is the lack of generalcleanliness. Any heating system that hasnot received periodic cleaning will notperform up to standard. Air filters, if used,should be cleaned or changed periodically,and all combustion-chamber and air han-dling components should be brushed orblown clean by compressed air.

Cleaning schedules should be established.In the absence of specific recommenda-tions by the manufacturer, the first clean-ing after installation should be performedabout three months after the equipmenthas been in steady use. Subsequentcleanings should then be scheduled ac-cording to the amount of dirt and dust ob-served at the initial cleaning. Some envi-ronments require cleaning of equipmenton a monthly basis, but in most cases an-nual cleaning is adequate.

Inspection. Most manufacturers recom-mend inspecting equipment annually,preferably at the start of the heating sea-son. If heating equipment serves an areathat puts a high amount of dust, soot orother impurities into the air, inspection andcleaning should be more frequent. Visualinspection can reveal many of the causesof poor performance.

Equipment will perform poorly if is hasany of the following defects: cracks,warpage, deterioration or sooting in theexchanger; clogged, warped or corrodedburners; warped or corroded burners;warped, deteriorated or sooted draft hoods;deteriorated, damaged, clogged or sootedvent pipes; continual downdrafts throughthe vent pipes; and damaged or cloggedvent caps.

Some of the control and operating defectsthat the maintenance personnel shouldcheck for are: dirty thermostat contacts,which can cause short cycling or intermit-tent operation; regulators that do not main-tain a steady inlet pressure to the mainburners; erratic fan controls that createlong delays before energizing the fan; en-crusted fan blades, which can reduce airvolumes; fans or blowers that are not up tospeed, which could indicate imminent

bearing seizure; defective capacitors, wornor loose belts, and possibly low voltage.In duct systems there is always the possi-bility that the distribution openings maybecome obstructed.

Forced-air Equipment, Cleaning andServicing. The fan or blower and the mo-tor should be checked for cleanliness andfor loose parts that might result from vi-bration. Grease and dirt should be removedfrom fan or blower-wheel blades and theoutside of the motor frame, especiallyaround the shaft. In dusty conditions, theinside of the motor should be blown outthrough its ventilation openings with com-pressed air. If the fan or blower is beltdriven, the belt should have the propertension (see Figure 3-22) and the pulleysshould be firmly secure on their shafts.

In the combustion chamber, primary andsecondary air-inlet openings should befree of grease and dirt. The heating systemshould be operated through a heatingcycle for a check of the operation of thepilot, ignition, and burners. The burnershould be checked for a stable flame withno lifting, flashback or floating. Most natu-ral gas units have no air shutters and, sinceorifices are factory sized for the BTUH con-tent and specific gravity of the gas, theyshould require no adjustment. Incorrectflames are usually traceable to non-adjust-ment-related causes like low gas pressure,plugged ports, etc.

Propane units usually have air shutters foradjusting primary air input. Whenever pos-sible, specific adjustment instructions ofthe manufacturer should be followed. Ingeneral, the heater should be allowed toheat up for about 15 minutes before reset-ting the shutters to adjust the flame. Toadjust, close the shutter until the flameturns yellow, then open slowly until theyellow just disappears, then lock the shut-ter into place at this point.

Cleaning of heat exchangers usually re-quires some heater disassembly to gainaccess, especially to the inside (combus-tion side) surfaces. In many cases, a bot-tom panel and the burner rack are remov-able. When disassembling without an in-struction manual and drawings of the unit,the maintenance personnel who is unfa-miliar with the unit should note carefullythe position of parts as they are removedso he/she can replace them in the samemanner. The inside of heat exchanger pas-sages are best cleaned with a stiff long-handled brush of a suitable diameter--small enough to fit into crevices and largeenough to exert sufficient self-pressure toremove soot and dirt. The brush should beworked vigorously up and down until sub-stantially all foreign matter is removed.

�G�F


3-31


Radiant Heaters, Cleaning and Servic-ing. Because of their simpler, more openconstruction, radiant heaters are easier tomaintain than forced-air heating equip-ment. In general, radiant heaters shouldbe inspected and cleaned about once a year,preferably before the heating season starts.In extremely dirty environments, theyshould be inspected more frequently andcleaned as necessary.

There are three main areas of concern inmaintaining radiant heaters: the burner ce-ramic plate, the pilot burner, if used, andthe reflector. Dust deposits can accumu-late on the back of the ceramic plate byentering through the primary air inlet ofthe burner. The best way to remove suchdeposits is by applying a soft stream of air(from a low-pressure air hose) first againstthe face of the ceramic plate and then intothe primary air inlet--alternating a num-ber of times between the plate and inletuntil both the back of the plate and the airinlet passages are clear.

The pilot burner can accumulate dust anddirt that can cause it to burn too low forthe heater to turn on when needed. It shouldbe cleaned with compressed air from anair hose. In extremely dusty conditions, itshould be dismantled occasionally andbrushed thoroughly.

Reflectors have a highly polished alumi-num surface. Accumulations of airbornedust and dirt on this surface can reducereflectivity considerably. Especially invery dusty environments, reflection sur-faces should be cleaned frequently withcommercial aluminum cleaners.

General Troubleshooting Guide.

With proper maintenance, gas-fired spaceheating equipment gives years of trouble-free service. There is very little that can gowrong with the gas-combustion parts ofthe heater. Trouble is much more likely todevelop with the controls and other auxil-iary parts.

Most controls are set at the factory or uponinstallation. These settings should not bechanged except on clear indication thatpoor or improper heater performance re-sults from incorrect control settings.

Trouble that appears to reside in the con-trol system often turns out to be somethingrather simple. So there are some easychecks that the service personnel shouldmake before using the troubleshootingchart on the opposite page. First, the fullcycle of the heater should be observed andthe exact nature of the malfunction noted.Before looking for the fault, check thatthe pilot and burner produce a properflame, make sure that primary air open-ings are clear of lint and dust and that thevent system is clear and working properly.

Electrical contacts are a common and easy-to-correct cause of equipment malfunc-tions. They should be checked andcleaned before any more complex and dif-ficult repairs or adjustments are made. Inservicing space-heating equipment, ser-vice personnel should followmanufacturer’s directions as closely aspossible.

The following table can help to determinethe cause of common equipment prob-lems.

3-32TROUBLESHOOTING CHARTTrouble Probable Cause

1. Manual valve turned off. 5. Extremely high or low gas pressure.2. Air in gas line. 6. Bent or kinked pilot tubing.3. Incorrect lighting procedure. 7. Failed energy cutoff device.4. Dirt in pilot orifice. 8. Gas not turned on.1. 5. Faulty thermostat.

6. Faulty magnetic valve.2. Circuit to magnetic valve open. 7.3. Faulty transformer.4.

1. No power to unit. 3. Venter relay defective.2. No power to venter relay. 4. Defective motor or capacitor.1. Manual valve not open. D. Spark cable shorted to ground.2. Air in gas line. E. Spark electrode shorted to ground.3. Dirt in pilot orifice. F. Drafts affecting pilot.4. Gas pressure too high or too low. G. Faulty ignition control.5. Kinked pilot tubing. H. Ignition control box not grounded.6. Pilot valve does not open. 8.7. No spark:

A. Loose wire connections. 9. Faulty combustion air proving switch.B. Transformer failure. 10. Gas not turned on.C. Incorrect spark gap.

1. Manual valve not open. 3. B.2. Main valve not opening.

A. Defective valve. C. Gas pressure incorrect.B. Loose wire connections. D. Cracked ceramic at sensor.

3. E. Faulty ignition controller.

A. Loose wire connections.1. Dirty filters. 4. Improper thermostat location or

adjustment.No heat (heater operating). 2. Incorrect manifold pressure or orifices. 5. Belt slipping on blower.

3. Cycling on limit control. 6.

1. 3. Blower set for too low a temperature rise.4. Incorrect manifold pressure.

2. Defective fan control.1. Circuit open. 3. Contactor inoperative.2. Fan control inoperative. 4. Defective motor.1. 4.

2. Defective fan control. 5. Ambient temperature too low.3.1. Improper motor pulley adjustment. 3. Low voltage.2. Improper static pressure on duct system. 4. High voltage.

Pilot will not light (match lit system).

Power not turned on or thermostat not calling for heat.

Faulty or dirty thermocouple or safety pilot switch.

High gas pressure (in excess of 1/2 pound).

Pilot lighted but magnetic gas vavle will not open. (All manual valves are open) Match lit system).

Ventor motor will not start.

Pilot will not light (spark ignition system).

Flame sensor grounded; pilot lights - spark continues.

Optional lockout device interrupting control circuit by above causes.

Pilot will light, main valve will not open (spark ignition system). Ignition controller does not power main

valve.

Motor running wrong direction (three phase only.

Cold air is delivered on: a. startup b. during operation

Motor will not run.

Fan control heater element improperly wired.

Three phase motor rotating in opposite direction.

Fan control heater element improperly wired.

Motor overload device cycling on and off.

Motor cycles on and off while burner is operating (see motor cuts out by overload device).

Motor cuts out by overload device.

Table I

OUTSIDE DESIGN TEMPERATURE (°F)CITY

HEATING COOLINGAlbany, NY -10 90Atlanta, GA 10 95Baltimore, MD 10 90Boston, MA 0 85Buffalo, NY -5 85Cheyenne, WY -20 90Chicago, IL -10 95Cleveland, OH -5 90Dallas, TX 10 100Denver, CO -10 95Detroit, Ml -5 90Houston, TX 20 95Jacksonville, FL 30 95Kansas City, MO -10 100Los Angeles, CA 40 90Milwaukee, WI -15 90Miami, FL 45 90Minneapolis, MN 25 90New Orleans, LA 25 95New York, NY 5 90Phoenix, AZ 35 105Pittsburgh, PA -5 90San Francisco, CA 35 80Spokane, WA -15 80Montreal, Que., Canada -10 90Toronto, Ont., Canada 0 90Winnipeg, Man., Canada -30 90

Heat System Selection

4 TECHNICAL GUIDE

4-1The following guide summarizes steps andconsiderations for selecting space-heatingequipment for a forced-air ducted heatingsystem. Not all steps are relevant to radi-ant heaters and other non-ducted systems.

DETERMINING HEATREQUIREMENTS

Calculate the sum of all heat losses of thespace to be heated using ASHRAE designdata and procedures. The following designdata, formulas and information are pro-vided as a convenience for estimating to-tal heat loss for selecting equipment. Thedata and information cover only the mostcommon types of commercial and indus-trial construction and represent the kindof information that the heating engineeror contractor will need. Since each heat-ing installation has its own special condi-tions and requirements, the engineer orcontractor must use experience and judge-ment in applying this data and informa-tion.

Design Temperatures. Table I presentsaverage outside design temperatures forcalculating heat gain.

Selection of inside temperature will varywith the preference of the owner or user ofthe space to be heated. Most commercialapplications find an indoor design tem-perature of 68°F satisfactory. For ware-houses and factories, the most commontemperature specified is 55°F. Indoor tem-peratures must be maintained by the heat-ing equipment when the outdoor tempera-ture is equal to or above the outdoor de-sign temperature. For example, a heater ina retail store in Pittsburgh, Pa., should beable to maintain 68°F indoors with -5°Foutdoors.

Degree Days. The degree-day method isprobably the best way to estimate abuilding’s fuel requirements. It is basedon the fact that an average outdoor24-hour temperature of 65°F results in nodemand for heat in most buildings in theUnited States. For example, if the tempera-ture reaches a high of 70°F and touches a

low of 60°F during the night, the meandaily temperature is 65 °F the tempera-ture at which no demand for heat occurs(on the average for the whole country). Adegree-day is a difference of one Fahren-heit degree between 65°F and the meantemperature maintained for 24 hours. Ex-ample: if the outdoor temperature averages64°F, from midnight to midnight, the dif-ference from 65°F is defined as adegree-day. For a day during which themean daily temperature is 62°F, the num-ber of degree-days for that day would bethree.

Degree-day values are available from theU. S. Weather Bureau and the CanadianMeteorological Division, Department ofTransport.

The yearly total of degree-day values for agiven locality can be used to calculate abuilding’s heat loss for an entire seasonwith the following formula:

(I) Q = 24 x H x D ÷ ∆T

where:

Q = heat loss in BTUs per heating sea-son

D = total number of degree days in sea-son

∆T = the design temperature differencebetween indoor and outdoor tem-peratures

H = design heat loss in BTU H

4-2Infiltration Load. Wind pressure outsideof a building continually forces a certainamount of air to infiltrate through open-ings around doors and windows. Additionalair rushes in each time a window or dooropens. This cold air must be included aspart of the heat load and extra equipmentcapacity must be added to accommodatethis extra load. The formula to be used is:

(II) Q = CFM X 1 .085 X ∆T

where:

Q = infiltration heat load, in BTUH

CFM=outside air that enters, in cubic feetper minute

1 .085 = a constant

∆T = temperature difference between in-side and outside, °F

The rate of air infiltration is difficult todetermine. For calculation purposes it canbe estimated by using what is known asthe “air change” method:

(III) R = VxCx1/60

where:

R = infiltration rate in cubic feet perminute

V = room volume in cubic feet

C = air changes per hour

Air changes are based on judgment, previ-ous experience, and empirical data (see airchange requirements, page 4-3).

When specific data and information arenot available, infiltration may be assumedto average one air change per hour.

Ventilation Load. To expel smoke andodors, and to provide general ventilation,it is sometimes desirable to introduce out-side air with a fan or blower rather thanrely on air to “leak in”. Often this forced-air ventilation is required by local codes--especially in spaces that are used for pub-lic purposes. It is also necessary to pro-vide for the amount of air that is deliber-ately exhausted from spaces like wash-rooms, kitchens, spray booths, laundryrooms, etc.

When ducts bring outside air in directlyto the intake side of a heater, this addi-tional heat load must be figured into theoverall determination of total heat capac-ity required. Both the ventilation load(Formula IV) and the infiltration load (For-mula II) should be calculated but only thegreater of the two should be included in

the total heat-load estimate. In applyingunit heaters for general applications, manycontractors allow about 10% of total airquantity for outside air. Values of ventila-tion load for given air temperatures arelisted in Table II.

The formula for calculating ventilationload is:

(IV) V = CFM x 1.085 x ∆T

where:

V = ventilation load, BTUH

CFM=outside air brought in through thesystem, cubic feet per minute

∆T = temperature difference between in-side and outside air, °F

1,085 = a constant

TABLE II. HEATING LOAD DUE TO OUTSIDE AIR

Initial Air Temperature, °FFinal Air

Temperature -10 0 10 20 30 40 50°F

TOTAL BTUH PER CFM

60 75.9 65.0 54.2 43.4 32.6 21.7 10.862 78.0 67.3 56.4 45.6 34.8 23.9 13.064 80.3 69.4 58.6 47.7 36.9 26.0 15.266 82.4 71.6 60.7 49.9 39.0 28.2 17.468 84.5 73.8 62.9 52.0 41.2 30.4 19.570 86.8 75.9 65.0 54.2 43.4 32.6 21.772 89.0 78.0 67.3 56.4 45.6 34.8 23.974 91.1 80.3 69.4 58.6 47.7 36.9 26.076 93.3 82.4 71.6 60.7 49.9 39.0 28.278 95.6 84.5 73.8 62.9 52.0 41.2 30.480 97.8 86.8 75.9 65.0 54.2 43.4 32.6

TABLE III

RECOMMENDED PERIODS OF AIR CHANGE, Minutes

Assembly Halls 5 to 10Auditoriums 5 to 15Bakeries 3 to 5Boiler Rooms 1 to 4Box Annealing 2 to 5Breweries 2 to 8Dry Cleaning Plants 1 to 6Dye Plants 2 to 4Electric Substations 8 to 1 2Engine Rooms 1 to 2Factories (light occupation, no contamination) 5 to 10Factories (dense occupation, fumes, steam) 1 to 5Forge Shops 1 to 3Foundries 1 to 3Furnace Buildings 1 to 2Galvanizing Plants 1 to 3Garages 3 to 5Generator Rooms 2 to 5Glass Plants 1 to 2Gymnasiums 2 to 10Heat Treating Buildings 1/2 to 1Kitchens 1 to 3Laboratories 3 to 10Laundries 2 to 5Machine Shops 3 to 6Offices 4 to 8Packing Plants 2 to 5Paint Shops 1 to 3Paper Mills 2 to 3Pickling Plants 2 to 3Plating Shops 1 to 4Pump Rooms 8 to 12Railroad Round Houses 1 to 3Recreation Rooms 2 to 8Restaurants 5 to 8Shops (general, fabrication) 5 to 10Steel Mills 1 to 5Textile Mills 5 to 12Toilets 2 to 5Transformer Rooms 1 to 5Turbine Rooms 2 to 6Warehouses 10 to 30Wood Shops 3 to 6

For swimming pools, ventilation rates may be based on occupation density: 20 to 25 CFM peroccupant is the rate that should be used. A bare minimum of 15 CFM per occupant may be usedif the occupation density is less than 25 people per 1000 square feet.


4-3

4 TECHNICAL GUIDE

Makeup-air (Ventilation)

Requirements. When air-change specifi-cations are dictated by code, the formulafor determining the CFM requirement issimple. The building or room volume (incubic feet) divided by the number of min-utes per air change yields the makeup airrequired (in CFM). Air change quantitieswill vary as influenced by local, state, andfederal codes. However, when such codesare not in effect, Table III may be helpful,as it recommends ventilation rates for vari-ous buildings.

Heat Loss and U-Factors.

Because of the difference between insideand outside temperatures, heat loss willoccur. The heating engineer must deter-mine the rate at which heat is lost. Thebasic formula used in making this calcu-lation is:

(V) H = A x U x ∆T

where:

H = heat loss, BTU H

A = area of the surface, square feet

U = heat transfer coefficient, BTUH persq ft per °F

∆T = temperature difference between in-side and outside, °F

The rate at which heat will travel througha construction material depends on thethickness and nature of the material. Thisrate is the U-Factor (heat transmission co-efficient) of a material. It indicates howmany BTUs will flow through a materialhaving a one-square-foot surface in onehour for each degree of temperature differ-ence between inside and outside.

4-4

TABLE IV—U-FACTORS

SURFACE AND CONSTRUCTION U-FACTOR**Plaster or

No Plaster Dry WallWALLS Interior on on

Finish Surface Furring

Transite, 3/8" thick on wood ormetal frame 1.18 _ 0.35

Corrugated iron on wood ormetal frame 1 60 _ 0 75

Flat transite on wood or metal frame 1.12 _ 0 42Flat iron on wood or metal frame 1.35 _ 0.70 8" solid brick 0.48 0.45 0.3112" solid brick 0.35 0.34 0 25 4" brick + 8" concrete

block (hollow) 0 32 0 29 0 23

Without WithROOF Insulation Ceiling Ceiling***

Metal deck none 0.95 0 471” 0 24 0 202" 0 15 0.13

Concrete, 2" thick none 0 82 0.432" thick 1” 0.25 0.202" thick 2" 0.15 0 134'’ thick none 0.74 0.414" thick 1” 0.23 0.194" thick 2" 0 14 0.13

Wood, 1" thick none 0 50 0.321” 0 21 0.172" 0.14 0 12

GLASS

Single glazed, plate 1.13Double glazed, plate or single glass (withstorm sash) 0.63Glass block 0.50

DOORS

Glass 1.13Metal 1 .35Wood 0.65

FLOORS

Basement floor on grade 0.04Below grade UdT = 2 *

*The combined U-Factor and temperature difference for floor below grade is figuredat a value of two. Multiply the square feet of floor surface by this factor to arrive at thetotal heat loss for basement floors below grade.**When using special materials not listed above, obtain U-Factor from manufacturer’sliterature.***Ceiling with air space between roof and ceiling. Factors apply to either dropped orconventional ceilings.

Table IV below lists common constructionmaterials and components with their aver-age U-Factors. These U-Factors may beapplied to various types of constructionencountered in commercial and industrial

buildings. Many insulating materials arerated based on “R” factors. To determine“U” factor from “R” factor simply divide1 by R(1/R=U). Conversely, to convert Uto R divide 1 by U(1/U=R).


4 TECHNICAL GUIDE

4-5Determining Air-Flow Requirements. Fora new system, the appliance air-flow ratewill depend on the number of air turnoversper hour desired in the space. One and ahalf to two air changes is average.

Ventilation and Makeup Air. The rate ofair change should be matched to the na-ture and use of the facility to be heated.For example, a drug store requires a morefrequent change of air than a warehouse,and an office will require an even morefrequent change. Table 111 (page 4-3) listsrecommended air -change rates for vari-ous types of facilities.

If a duct furnace is to be installed in anexisting system, check the fan speed, mo-tor current, and duct pressure drop anddetermine the air handling capacity fromblower manufacturer’s data and graphs. See“Determining Optimum Duct System”,page 4-8.

Entering-air Temperature. It is importantto calculate the temperature of the air en-tering the duct furnace. This temperaturecan be calculated as follows:

Temperature Rise. (Not relevant to radi-ant heaters). To select equipment with aheat exchanger made of appropriate mate-rial for the job, it is necessary to know thetemperature rise through the heater re-quired to meet design temperature condi-tions. This is the difference between thetemperature at the cold-air inlet of the

(VI) TM

=TRT

O

where:

TM

= mixed air temperature, °F

TR= % return air x return-air temperature, °F

TO

= % outdoor air x outdoor-air temeprature, °F

heater and the temperature at the hot-airoutlet when the heater is operating undernormal conditions.

It is relatively simple to calculate the re-quired temperature rise of the air flowingthrough the duct furnace with the formula:

This temperature difference will largelydetermine what kind of heat exchanger the

heater should have. See the paragraphs ofheat exchanger, page 2-8.

Static Pressure Requirement

The total frictional losses of the air-han-dling system must be known so that thefan and drive can be selected. The totalstatic pressure drop should include the in-dividual drops through the furnace, duct-work, outlets and grilles, filters and fan.Such losses can be determined from themany charts and formulas published in theASHRAE Guide and Data Book titled“Fundamentals and Equipment”, or frominformation supplied by manufacturers.

If it is necessary to measure the static losswithin an existing duct system, accuratemeasurements can be made with a slopegauge (also called an inclined manometer),provided the slope gauge range covers thepressure range to be measured. If the an-ticipated pressure exceeds the range of theavailable slope gauge, a U-tube manom-eter may be used. (See Figure 4-1.) Of thetwo instruments, the slope gauge is prefer-able because it magnifies readings so thateven very small pressures can be read ac-curately and easily.

(VII) ∆T = (H/Q) x 1.085

where:

∆T = air temeprature rise, °F

H = heat loss, BTUH

Q = air quantity, CFM

1.085 = constant

4-6

4-1. U-tube and slope gauge. 4-2. Pitot tube.

For best results, all static pressure deter-minations should be based on multiple-point readings to ensure a realistic aver-age. It is also important to avoid false read-ings due to air-velocity impact pressure:the sensing device should always be lo-cated where it will not face the air stream.A Pilot tube (Figure 4-2) will eliminatesuch errors if it is positioned with thepointed end (which contains the velocityport) facing the air stream.

Relief, Exhaust Requirements. Exhaustrequirements are established by code orby the fact that harmful dusts, vapors, orodors must be evacuated from the build-ing. Collector hoods usually are used whenthe source of the contaminant is in a spe-cific location. The amount of air neededto collect these contaminants is based onthe velocity across the face of the hood.Capture velocities will vary but a goodrule-of-thumb velocity would be 100 to250 feet per minute (FPM).

Relief dampers are used to relieve build-ing pressures when outside air is admittedunder pressure. Inside pressure results fromfully pressurized heating with 100% out-side air.

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Optimizing Distribution Systems

4-7

4 TECHNICAL GUIDE

Heat distribution planning should includecareful consideration of a number of fac-tors: special requirements of the occu-pants; the location of large windows orother glass-wall areas’ sources of high in-filtration; and the location of partitions,counters, display cases, stacks of storedmaterial, lighting fixtures and other itemsthat might interfere with circulating air orthe rays from a radiant heater.

Determining Optimum Forced-airEquipment Sizing

Theoretically, forced-air heating equip-ment can be sized exactly to meet heatrequirements at the interior and exteriordesign temperatures. Usual practice, how-ever, is to oversize the equipment for fasterpickup after nighttime temperature set-back.

Small sizes are usually selected to haveabout 30% more heat output capacity thanrequired for continuous blower operationand large sizes are oversized by about20%.

Determining Optimum InfraredEquipment Sizing

As with forced-air heating, determinationof the size and number of radiant heatersstarts with a building heat loss survey andcalculation. With radiant heaters, heatlosses due to convection currents at wallsand stratification of warm air at the roofare reduced. Engineering studies haveshown that a properly designed radiantheating system will burn about 85% of thefuel indicated by ASHRAE heat-loss cal-culations. Consequently, total installedinput capacity of radiant heaters can be85% of the capacity calculated byASHRAE methods for forced-air equip-ment.

The size of radiant heaters depends on themaximum practical mounting height andthe spacing required to maintain the de-sired comfort level and to prevent coldspots. Once the mounting height has beendetermined, manufactures’ heater applica-tion tables can be used to select the sizeand placement of heaters. Here, consider-able judgement must be used to place heat-ers so optimum coverage of the floor canbe established. Rays from infrared unitsshould be directed away from walls asmuch as possible and should be concen-trated on the floor area for the best appli-cation. Heaters will not necessarily bespaced at uniform intervals.

Zoning. Central heating or heating-cool-ing systems can be single- or multi-zonedistribution systems. Single-zone systemsusually employ a single furnace and asingle duct to deliver air to the entirespace, under control of a single thermo-stat. A multi-zone system may consist ofone or more heating (or heating-cooling)units serving several separately ductedspace zones. A thermostat in each zonecontrols the temperature of the air deliv-ered to the zone.

The heating engineer or contractor mustexercise judgement in determining thenumber and location of heaters, duct out-lets and cold-air-return inlets. Sometimesthe shape of the space requires a decisionas to whether one heater or several heaterswill provide better service. For example,in an L-shaped structure, it might be desir-able to place one or more unit heaters orradiant heaters in each section. Eachheater would then be sized to meet its zonerequirements and carry its share of the to-tal load.

Cost/Benefits. First cost and operatingcosts of single-zone systems are low butflexibility is limited. Such systems do notreadily met changing space needs or spaceuse.

First cost of multi-zone system is higherthan for an equivalent single-zone systembecause it would contain more ductworkand multiple control dampers and thermo-stats. Operating costs may also be higher(especially for heating-cooling units), butthe flexibility results in greater comfortfor occupants and greater adaptability tochanging needs and use patterns.

Determining Optimum EquipmentLocation

Determine which locations are suitableand available for the equipment. If floorspace is limited (and other conditions per-mit) suspended unit heaters, radiant heat-ers or rooftop packaged units may be thebest choice. For an existing building,check the spaces available for heatingequipment and consult with the owner tomake sure they are not reserved for anotherpurpose.

For new construction, advise the builderor owner as to where the heating equip-ment should be placed for the best heat-ing results, and request allocation of thespace (or spaces) in the building design.

Coverage. Convection-type units,whether unit heaters or packaged units,must be ample in numbers or be attachedto duct systems that can adequately coverthe space to be heated. In connection withcoverage, it has been expressed that theCFM must be adequate to turn over the airin the building at least one and a half times.The turnover rate can be determined byadding the CFM’s from all units and mul-tiplying by this total by 60, and then di-viding this number into the building vol-ume.

Infrared units must be located so that in-frared patterns strike the floor at the pe-rimeter of the building. Care must be ex-ercised in limiting the pattern to as littlewall area as possible. All the availablepattern should be directed onto the floorso that the floor becomes a heat sink andultimately provides heat to the space.

4-8Determining Optimum Duct System

The simplest forced-air system has noducts. A fan blows air directly from theheater into the space to be heated. Thismethod of heat distribution is satisfactoryfor small areas and large areas that can bedivided in small zones. The disadvantageof this method is that air velocities mayneed to be rather high, therefore such sys-tems tend to be noisy and the fast-flowingair may feel uncomfortable to occupants.

For more uniform distribution to larger ar-eas with more remote points of need, anair-distribution duct network is used. Thebasic elements of the system are a centrifu-gal blower belt-driven by an electric mo-tor, a series of main and branch ducts, afilter and duct dampers. A duct systemcan selectively heat a number of separaterooms or areas.

In a duct system, the blower causes coolair to be drawn into the return intake ductand through an air filter. It then pushesthe air through the heat exchanger and onthrough the hot-air ducts to the outlets.This air-circulation pattern continues aslong as the blower operates - and the bloweroperates only when the temperature at theheat exchanger is above the blower “cut-in” and “cut-out” settings. The blowermay be programmed to operate continu-ously where ducted air-distribution sys-tems are in use, providing the air velocityat the discharge opening is not greater than500-700 FPM.

Duct systems need to be carefully andknowledgeable designed. Supply outletsshould be located to give desired air dis-tribution patterns. Air streams leaving thesystem outlets should be directed towardthe area where most heat is desired, butblowing heated air on personnel withinwork zones should be avoided. Horizon-tal and vertical louvers may be used to

direct the heated air where needed. Airshould be kept in continuous but not too-rapid circulation. The use of proper out-lets will minimize variations in air tem-perature throughout the room, and thusresult in the maximum of comfort for theparticular system used.

Duct layout should also provide for return-ing air to the heater, so that there is a com-pletely closed circuit of air being reheatedby the heater or furnace. The closed-cir-cuit arrangement can be more easilyachieved by placing the heater in a cen-tral area such as a corridor or other space,which connects with all adjacent occupiedspaces; otherwise, a special return ductsystem must be used.

Good duct design provides the smallest-size duct that will produce the required airflow at the pressure available. Space andscope limitations of this handbook do notpermit presentation of the large number ofair-distribution-system design methodsthat are available. For best performance ofspace heating equipment, it is worthwhileto retain a qualified consultant. The rec-ognized authority for duct-sizing informa-tion is the Air Conditioning ContractorsAssociation, from whom a detailed manualon duct sizing and design can be pur-chased.

Control System Requirements. In mod-ern space heating equipment, a variety ofautomatic controls are available to cyclethe system in response to thermostat de-mand. The options range from a simpleON/OFF system to electronically modu-lated systems that can control temperaturesaccurately to maintain a temperaturewithin one-tenth of a degree Fahrenheit.See the section on controls, page 2-9.

Selecting the Equipment

After the above mentioned information hasbeen gathered, and the appropriate calcu-lations and decisions have been made,manufacturers’ catalogs and data sheetsshould be consulted to find the best fit.

In addition to heating capacity, air-han-dling capability and efficiency, other fac-tors should be considered in evaluatingand comparing specific equipment. Con-sidering that a heating system is a long-term investment, the purchaser should optfor high quality in design and construc-tion - the cost difference paid for qualitycan be insignificant, especially whenspread over a 20- or 30-year economicpayoff period. Economic justification cal-culations should include consideration ofthe costs of maintenance, repair, down-time, possible loss of income and profit,and inconvenience that can result frompurchasing decisions motivated chiefly byfirst-cost considerations.

The heating equipment buyer should,whenever possible, purchase equipmentbearing the AGA, ETL, or UL symbols andcarrying the assurance that the equipmentmeets the standards of NFPA and NFGC.

4 TECHNICAL GUIDE

Determining Pipe Sizes

To determine proper pipe size for your heatingsystem, add up the longest line required, i.e., thedistance between the main pressure regulator andthe farthest heating unit in the system. Dividethis total pipe length into the total available pres-sure drop.

(VIII) Pa

L x100 = Pb

where:

Pa = Total available pressure drop (Pressure fromutility minus manufacturer’s recommended mini-mum supply pressure);

l = Total equivalent pipe length in feet’

Pb = Allowable pressure drop per 100 feet.

Determine the required CFH of gas in thatbranch line and find this CFH in the lefthandcolumn of Table V. Read across to the properpipe size column and select pipe size based onPb from formula VIII. Bear in mind that fittingsmay add to the total length (L), and you maywish to include these dimensions in a secondevaluation. (See Table VI).

NOTE: For complex systems, it may be advan-tageous to divide the system into major trunklines for closer calculations.

4-9

TABLE V - PRESSURE DROP PER 100 FEET(1000 BTU GAS AT .60 SPECIFIC GRAVITY)

PIPE SIZE, inchesCFH 1/2 3/4 1 1-1/4 1-1/2 2 3 425 0.125 0.029 0.008 0.00250 0.500 0.118 0.033 0.008 0.00375 1.125 0.265 0.074 0.017 0.007 0.002

100 2.000 0.471 0.132 0.031 0.013 0.004125 3.125 0.736 0.206 0.048 0.020 0.006150 4.500 1.060 0.297 0.070 0.029 0.009 0.001175 6.125 1.442 0.404 0.095 0.040 0.012 0.001200 4.62 oz. 1.884 0.528 0.124 0.052 0.015 0.002250 7.23 oz. 2.944 0.825 0.194 0.081 0.024 0.003 0.001300 10.4 oz. 4.239 1.188 0.279 0.117 0.034 0.003 0.001350 14.16 oz. 5.771 1.617 0.380 0.159 0.046 0.006 0.001400 1.16 lb. 7.536 2.112 0.496 0.208 0.060 0.008 0.002500 1.81 lb. 6.81 oz. 3.300 0.775 0.325 0.095 0.012 0.003600 2.6 lb. 9.8 oz. 4.752 1.116 0.468 0.136 0.017 0.004800 4.62 lb. 1.09 lb. 4.88 oz. 1.984 0.833 0.242 0.030 0.007

1,000 7.22 lb. 1.7 lb. 7.63 oz. 3.100 1.301 0.378 0.047 0.0111,250 11.28 lb. 2.66 lb. 11.92 oz. 4.844 2.033 0.591 0.073 0.0171,500 16.25 lb. 3.83 lb. 1.07 lb. 6.975 2.927 0.851 0.106 0.0252,000 28.88 lb. 6.8 lb. 1.91 lb. 7.17 oz. 5.204 1.512 0.188 0.0442,500 45.13 lb. 10.63 lb. 2.98 lb. 11.2 oz. 4.70 oz. 2.363 0.294 0.0693,000 64.98 lb. 15.3 lb. 4.29 lb. 1.01 lb. 6.77 oz. 3.402 0.423 0.099

NOTE: Presusre drop data above not designated in pounds or ounces is expressed in inches water-column.

4-10

TABLE VI - STANDARD LENGTHS OF PIPE FITTINGS AND VALVES (IN FEET)NOMINAL PIPE SIZE, inches

TYPE OF FITTING OR VALVE 1/2 3/4 1 1-1/4 1-1/2 2 3 4Standard tee with entry discharge through side 3.4 4.5 5.5 7.5 9.0 12.0 17.0 22.0Standard elbow 1.7 2.2 2.7 3.7 4.3 5.5 8.0 12.0Medium sweep elbow 1.3 1.8 2.3 3.0 3.7 4.6 6.8 9.0Long sweep elbow or run of a standard tee or butterfly valve

1.0 1.3 1.7 2.3 2.7 3.5 5.3 7.0

45° elbow 0.8 1.0 1.2 1.6 2.0 2.5 3.7 5.0Close-return bend 3.7 5.1 6.2 8.5 10.0 13.0 19.0 24.0Gate valve, wide open, or slight bushing reduction 0.4 0.5 0.6 0.8 0.9 1.2 1.7 2.3

TABLE VII - GAS METER FLOW-TIME TABLE (CUBIC FEET PER HOUR)TEST METER DIAL

0.5 cu.ft. 1 cu. ft. 2 cu. ft. 5 cu. ft. 0.5 cu.ft. 1 cu. ft. 2 cu. ft. 5 cu. ft.

10 180 360 720 1800 47 38 77 153 38311 164 327 655 1636 48 37 75 150 37512 150 300 600 1500 49 37 73 147 36713 138 277 555 1385 50 36 72 144 36014 129 257 514 1286 51 35 71 141 35315 120 240 480 1200 52 35 69 138 34616 112 225 450 1125 53 34 68 136 34017 106 212 424 1059 54 33 67 133 33318 100 200 400 1000 55 33 65 131 32719 95 189 379 947 56 32 64 129 32120 90 180 360 900 57 32 63 126 31621 86 171 343 857 58 31 62 124 31022 82 164 327 818 59 30 61 122 30523 78 157 313 783 60 30 60 120 30024 75 150 300 750 62 29 58 116 29025 72 144 2878 720 64 29 56 112 28126 69 138 277 692 66 29 54 109 27327 67 133 267 667 68 28 53 106 26528 64 1269 257 643 70 26 51 103 25729 62 124 248 621 72 25 50 100 25030 60 120 240 600 74 24 48 97 24331 58 116 232 581 76 24 47 95 23732 56 113 225 563 78 23 46 92 23133 55 1098 218 545 80 22 45 90 22534 5 106 212 529 82 22 44 88 22035 51 103 206 514 84 21 43 86 21436 20 100 200 500 86 21 42 84 20937 49 97 195 486 88 20 41 82 20538 47 95 189 474 90 20 40 80 20039 46 92 185 462 94 19 38 76 19240 45 90 180 450 98 18 37 74 18441 44 88 176 440 100 18 36 72 18042 43 86 172 430 104 17 35 69 17343 42 84 167 420 108 17 33 67 16744 41 82 164 410 112 16 32 64 16145 40 80 160 400 116 15 31 62 15546 39 78 157 391 120 15 30 60 150

Reprinted - Courtesy of Gas Age, July 1968 NOTE: To convert to BTU per hour, multiply by the BTU heating value of the gas used.

Cubic Feet per HourSeconds for 1 revolution Cubic Feet per Hour

Seconds for 1 revolution

Determining Orifice Sizes

4-11

4 TECHNICAL GUIDE

TABLE VIII - UTILITY GASES (Cubic feet per hour at sea level)Specific Gravity = 0.60 For utility gases of another specific gravity, select factor from Table X.Orifice Coefficient = 0.9

Gas Pressure at Orifice - Inches Water Column

3 3.5 4 5 6 7 8 9 100.008 0.17 0.18 0.19 0.23 0.24 0.26 0.28 0.29 0.300.009 0.21 0.23 0.25 0.28 0.30 0.33 0.35 0.37 0.390.040 0.27 0.29 0.30 0.35 0.37 0.41 0.43 0.46 0.480.011 0.33 0.35 0.37 0.42 0.45 0.48 0.52 0.55 0.590.012 0.38 0.41 0.44 0.50 0.54 0.57 0.62 0.65 0.70

80.000 0.48 0.52 0.55 0.63 0.69 0.73 0.79 0.83 0.8879.000 0.55 0.59 0.64 0.72 0.80 0.84 0.90 0.97 1.0178.000 0.70 0.76 0.78 0.88 0.97 1.04 1.10 1.17 1.2477.000 0.88 0.95 0.99 1.11 1.23 1.31 1.38 1.47 1.5576.000 1.05 1.13 1.21 1.37 1.52 1.61 1.72 1.86 1.9275.000 1.16 1.25 1.34 1.52 0.16 1.79 1.91 2.04 2.1474.000 1.33 1.44 1.55 1.74 1.91 2.05 2.18 2.32 2.4473.000 1.51 1.63 1.76 1.99 2.17 2.32 2.48 2.64 2.7872.000 1.64 1.77 1.90 2.15 2.40 2.52 2.69 2.86 3.0071.000 1.82 1.97 2.06 2.33 2.54 2.73 2.91 3.11 3.2670.000 2.06 2.22 2.39 2.70 2.97 3.16 3.38 3.59 3.7869.000 2.25 2.43 2.61 2.96 3.23 3.47 3.68 3.94 4.1468.000 2.52 2.72 2.93 3.26 3.58 3.88 4.14 4.41 4.6467.000 2.69 2.91 3.12 3.52 3.87 4.13 4.41 4.69 4.9466.000 2.86 3.09 3.32 3.75 4.11 4.39 4.68 4.98 5.2465.000 3.14 3.39 3.72 4.29 4.62 4.84 5.16 5.50 5.7864.000 3.41 3.68 4.14 4.48 4.91 5.23 5.59 5.95 6.2663.000 3.63 3.92 4.19 4.75 5.19 5.55 5.92 6.30 6.6362.000 3.78 4.08 4.39 4.96 5.42 5.81 6.20 6.59 6.9461.000 4.02 4.34 4.66 5.27 5.77 6.15 6.57 7.00 7.3760.000 4.21 4.55 4.89 5.52 5.95 6.47 6.91 7.35 7.7459.000 4.41 4.76 5.11 5.78 6.35 6.78 7.25 7.71 8.1158.000 4.66 5.03 5.39 6.10 6.68 7.13 7.62 8.11 8.5357.000 4.84 5.23 5.63 6.36 6.96 7.44 7.94 8.46 8.9056.000 5.68 6.13 6.58 7.35 8.03 8.73 9.32 9.92 10.4455.000 7.11 7.68 8.22 9.30 10.18 10.85 11.59 12.34 12.9854.000 7.95 8.00 9.23 10.45 11.39 12.25 13.08 13.93 14.6553.000 9.30 10.04 10.85 12.20 13.32 14.29 15.27 16.25 17.0952.000 10.61 11.46 12.31 13.86 15.26 16.34 17.44 18.57 19.5351.000 11.82 12.77 13.69 15.47 16.97 18.16 19.40 20.64 21.7150.000 12.89 13.92 14.94 16.86 18.48 19.77 21.12 22.48 23.6549.000 14.07 15.20 16.28 18.37 20.20 21.60 23.06 24.56 25.8348.000 15.15 16.36 17.62 19.88 21.81 23.31 24.90 26.51 27.8947.000 16.22 17.52 18.80 21.27 23.21 24.93 26.62 28.34 29.8146.000 17.19 18.57 19.98 22.57 24.72 26.43 28.23 30.05 31.6145.000 17.73 19.15 20.52 23.10 25.36 27.18 29.03 30.90 32.5144.000 19.45 21.01 22.57 25.57 27.93 29.87 31.89 33.96 35.7243.000 20.73 22.39 24.18 27.29 29.87 32.02 34.19 36.41 38.3042.000 23.10 24.95 26.50 29.52 32.50 35.24 37.63 40.07 42.1441.000 24.06 25.98 28.15 31.69 34.81 37.17 39.70 42.27 44.4610.000 25.03 27.03 29.00 33.09 36.20 38.79 41.42 44.10 46.3839.000 26.11 28.20 29.23 34.05 37.38 39.97 42.68 45.44 47.8038.000 27.08 29.25 31.38 35.46 38.89 41.58 44.40 47.27 49.7337.000 28.36 30.63 32.99 37.07 40.83 43.62 46.59 49.60 52.17

Orifice Size Decimal or (DMS)

For altitudes above 2000 ft. the input must be reduced by 4% for each 1000 ft above sea level. Example: 25 cubic feet at sea level must be reduced by 20% at 5000 ft. elevation. 25x0.8 = 20 cubic feet. The orifice must be changed accordingly. (At 3.5 pressure, a number 42 orifice would be replaced by anumber 45 orifice.)

To select an orifice size for a desired gas flow rate:

1. Obtain the factor for the known specific gravity of the gas from Table X;

2. Divide the desired gas flow rate by this factor, which gives the equiva-lent flow rate with a 0.l60 specific-gravity gas;

3. Using the flow rate obtained in step 2, select the orifice size from TableVIII.

To estimate gas flow rate for a given orifice size and gas pressure when itis not possible to meter gas flow:

1. Use Table VIII to obtain equivalent gas flow rate with a 0.60 specific-gravity gas;

2. Obtain the factor from Table X for the specific gravity of the gas to beburned;

3. Multiply the rate obtained in step 1 by the factor from step 2 to estimategas flow rate in cubic feet. Then multiply cubic feet by BTU content(ft3) of gas in use.)

4-12TABLE IX -LP GASES (BTU PER HOUR AT SEA LEVEL)

Description Propane ButaneBtu per Cubic Foot = 2500 3175Specific Gravity = 1.53 2

11 11

Orifice Coefficient = 0.9 0.9Gas Input, BTU per Hour for:

Drill Size (Decimal or DMS) Propane Butane or Butane-

Propane Mixtures0.008 500 5540.009 641 7090.010 791 8750.011 951 1,0530.012 1,130 1,250

80 1,430 1,59079 1,655 1,83078 2,015 2,23077 1,545 2,81576 3,140 3,48075 3,465 3,84074 3,985 4,41073 4,525 5,01072 4,920 5,45071 5,320 5,90070 6,180 6,83069 6,710 7,43068 4,580 8,37067 8,040 8,91066 8,550 9,47065 9,630 10,67064 10,200 11,30063 10,800 11,90062 11,360 12,53061 11,930 132,85060 12,570 13,84059 13,220 14,63058 13,840 15,30057 14,550 16,09056 16,990 18,79055 21,200 23,51054 23,850 26,30053 27,790 30,83052 31,730 35,10051 35,330 39,40050 38,500 42,80049 41,850 45,35048 45,450 50,30047 48,400 53,55046 51,500 57,00045 52,900 58,500

Pressure at Orifice, Inches Water Column =

TABLE X - FACTORS FOR UTILITY GASES OF ANOTHER SPECIFIC GRAVITY

SPECIFIC GRAVITY FACTOR

SPECIFIC GRAVITY FACTOR

0.45 1.155 0.95 0.7950.50 1.095 1.00 0.7750.55 1.045 1.05 0.7560.60 1.000 1.10 0.7390.65 0.961 1.15 0.7220.70 0.926 1.20 0.7070.75 0.894 1.25 0.6930.80 0.866 1.30 0.6790.85 0.840 1.35 0.6670.90 0.817 1.40 0.655

5 GLOSSARY

Space Heating Terms

A

AIR-GAS RATIO - The ratio of combus-tion-air flow rate to the fuel-gas flow rate.

AIR SHUTTER - An adjustable panel onthe primary air opening of a burner for con-trolling the amount of combustion air en-tering the burner body.

ATMOSPHERIC BURNER - (SeeBurner.)

ATMOSPHERIC PRESSURE - The forceexerted on one square inch of surface bythe weight of the air above the earth. Atsea level atmospheric pressure is 14.7pounds per square inch.

ATOM - The smallest unit into which el-ementary matter can be divided and stillhave the same properties.

AUTOMATIC GAS PILOT - A gas pilotlight with a sensing device that shuts offthe gas supply to the burner when the pi-lot flame goes out.

AUTOMATIC GAS VALVE - An auto-matic or semi-automatic device consist-ing of a valve and an operator that con-trols the gas supply to the burner(s) dur-ing operation of a gas-fire appliance.

B

BIMETAL - A strip or coil made of twometals having different thermal expansionproperties laminated together so that tem-perature changes bend the strip or twistthe coil.

BRITISH THERMAL UNIT (BTU) - Thequantity of heat required to raise the tem-perature of one pound of pure water onedegree F.

BUNSEN-TYPE BURNER - A gas burnerin which: 1. gas leaving the gas orificedraws combustion air into the burner; 2.the air mixes with the gas within the burnerbody and the resulting gas-air mixtureburns at the burner port.

BURNER - A device for controlled burn-ing of gas, or a mixture of gas and air.

1. Injection Burner. A burner employingthe energy of a jet of gas to inject air forcombustion into the burner and mix itwith gas.

2. Atmospheric Burner. A burner in whichthe air injected into the burner by a jetof gas is supplied to the burner at atmo-spheric pressure.

3. Power Burner. A burner in which eithergas or air or both are supplied at pres-sures exceeding: gas line pressure of gasor atmospheric air pressure.

4. Yellow-Flame Burner. A burner that de-pends on secondary air only for the com-bustion of the gas.

BURNER HEAD - The part of a gas burnerthat contains the burner ports.

BURNER PORT - (See Port).

BURNING SPEED - (See Flame Velocity.)

BUTANE - A hydrocarbon gas (C5H10)heavier than methane, ethane, and pro-pane.

C

CASING LOSS - Heat lost through thewalls of a heater or furnace to the spacesurrounding the heater.

COMBUSTION - The rapid oxidation offuel accompanied by the release of heat orheat and light.

COMBUSTION AIR - Air supplied in aheater specifically for burning a fuel gas.

COMBUSTION CHAMBER - The partof a heater where combustion normally oc-curs.

COMBUSTION PRODUCTS - Constitu-ents in the exhaust stream resulting fromthe combustion of fuel gas in air, includ-ing the inert gases like nitrogen and argonbut excluding excess oxygen.

COMPOUND - A distinct substance con-sisting of two or more elements chemicallycombined in a definite proportion.

CONFINED SPACE - A heating equip-ment room that contains less than 50 cu-bic feet per 1000 BTUH of heater inputcapacity.

CONTROLS - Devices that regulate thegas, air or electricity used in a gas heateror furnace. A control device may bemanual, semiautomatic or automatic.

CONVECTION LOSS - Heat lost throughthe walls and roof of a building throughmovement of heated air.

CUBIC FOOT OF GAS (Standard Con-ditions) - The amount of gas that has avolume of 1 cubic foot at a temperature of60°F under a pressure of 30 inches of mer-cury.

D

DIAPHRAGM VALVE - A control valvewhich opens and closes according to gaspressure on a flexible diaphragm.

DILUTION AIR - Air deliberately mixedwith the flue gases to make the combus-tion products less concentrated.

DIRECT-SPARK IGNITION SYSTEM -A system in which two high-voltage elec-trodes create a continuous spark to ignitemain burner gas directly without an inter-mediate pilot flame.

DOWNDRAFT - Downward flow of gasesor air in a chimney or stack.

DRAFT HOOD (Draft Diverter) - A de-vice built into a heater or made part of thevent connector of a heater. It is designedto: ensure escape of combustion productsin case of no draft, backdraft, or blockagein the chimney or stack; prevent a backdraftfrom entering the heater.

DUCT LOSS - Heat lost through the wallsof ductwork to the space surrounding theducts.

5-1

E

ENERGY CUT OFF (ECO) DEVICE - Athermostatic element in the safety controlwhich shuts off gas supply when a mal-function occurs.

EXCESS AIR - Air that passes through aheater’s combustion chamber and flues inexcess of the amount required for com-plete combustion, usually stated as a per-centage of the air required for completecombustion.

EXTINCTION POP - (See Flashback.)

F

FAHRENHEIT - The method of tempera-ture measurement in the English systemof units in which the freezing point of wateris 32°F and the boiling point of water is212°F at standard pressure.

FAN AND LIMIT CONTROL - Usually acombination control and safety devicethat: turns a heater fan or blower on andoff according to preset temperatures; shutsoff main burner gas in case a malfunctioncauses combustion-chamber temperatureto rise above a preset limit. (See Controls.)

FLAME ROLLOUT - The spread of flameout of a combustion chamber as gas ig-nites at the burner.

FLAME VELOCITY - The speed atwhich flame travels through a fuel-air mix-ture.

FLAMMABILITY LIMIT - The percent-age of fuel in an sir-fuel mixture above orbelow which the fuel will not burn.

FLASHBACK - An undesirable conditionin which flames travel backward into aburner and combustion continues to oc-cur there.

FLASHBACK ARRESTOR - A metalgauze, grid or any other element in a burnerto prevent flashback.

FLOATING FLAMES - An undesirableburner operating condition in whichflames move up off the burner ports andquietly “float” above them. This condi-tion usually results from incomplete com-bustion.

FLUE GASES, FLUE PRODUCTS -Products of combustion and excess air influes or heat exchangers.

FLUE LOSS - Heat lost as a result of theexhausting of flue gases.

FLUE OUTLET - An opening for the es-cape of flue gases.

FUEL GAS - A combustible gas used tocreate useful heat.

H

HEAT ANTICIPATOR - A thermostatcompensating adjustment that preventstemperature overshoot or underheating.

HEAT EXCHANGER - A device for trans-ferring heat from one medium to another.

HEATING VALUE - The number of Brit-ish thermal units produced by the com-plete combustion of one cubic foot of gasat constant pressure. Heating value in-cludes the heat involved in cooling thecombustion products back to the initialtemperature of the gas-air mixture and con-densing the water vapor formed by com-bustion.

HOT-WIRE VALVE - A heat-actuatedvalve in which the valve is opened andclosed by expansion and contraction of ataut wire heated by an electric current pass-ing through it.

HYDROCARBONS - Chemical com-pounds composed of carbon and hydro-gen.

I

IGNITION - The starting of combustion.

IGNITION VELOCITY - (See Flame Ve-locity.)

INCHES OF MERCURY COLUMN -Pressure defined by the height of a col-umn of mercury measured in inches. Oneinch of mercury column equals a pressureof 0.491 pounds per square inch.

INCHES OF WATER COLUMN - Pres-sure defined by the height of a column ofwater measured in inches. One inch ofwater column equals a pressure of 0.578ounces per square inch (.036 psi). One inchmercury column equals about 13.6 incheswater column.

INCOMPLETE COMBUSTION - Com-bustion in which fuel is only partiallyburned.

INFRARED BURNER (Radiant Burner)- A burner that provides heat from a hot,glowing surface that radiates infrared en-ergy.

INPUT RATING - The gas-burning capac-ity of a heater, in BTUs per hour, specifiedby the manufacturer.

L

LIFTING - An unstable burner flame con-dition in which flames lift or blow off theburner port(s), often producing a blowingor roaring sound. Lifting usually resultsfrom too much combustion air. (See Float-ing Flames.)

LOCKOUT - A deliberately induced con-dition in which a control system shutsdown a heater and prevents further opera-tion until a malfunction has been correctedand the system has been reset manually.

LPG - Liquefied-petroleum gas.

5-2

5 GLOSSARY

Space Heating Terms

5-3 M

MANUFACTURED GAS - A “synthetic”fuel gas produced by passing steam overcoal at high temperature, consistingmainly of carbon monoxide and hydro-gen.

METHANE - A hydrocarbon gas with theformula CH4, the principal component ofnatural gases.

MIXED GAS - A gas in which the heatingvalue of manufactured gas is raised by mix-ing it with natural or liquefied-petroleumgas.

MIXER - The section of a Bunsen-typeburner where air and gas are mixed for de-livery to the burner ports.

MOLECULE - The smallest unit intowhich a chemical compound can be di-vided and still retain the identity and prop-erties of the compound.

N

NATURAL DRAFT - The motion of flueproducts through a heater and flue system,caused by the convection force of hot fluegases.

NATURAL GAS - Fuel gas found in theearth, as opposed to gases that are manu-factured.

O

ODORANT - A substance having a strongodor, added to odorless natural gas to warnof gas leakage and to aid in leak detec-tion.

ORIFICE - A small hole through whichgas is discharged, limited and controlled.(See also Universal Orifice.)

ORIFICE SPUD - A removable plug orcap containing an orifice which permitsadjustment of the gas flow by substitutionwith a spud having a different size orificeor by motion of an adjustable needle intoor out of the orifice. (Also known as anorifice cap or orifice hood.)

OVERRATING - Operation of a gas burneror heater at greater than its design capac-ity or manufacturer’s rating.

P

PILOT - A small flame for igniting the gasat the main burner.

PORT - Any opening in a burner headthrough which gas or an air-gas mixture isdischarged and burned.

PRESSURE REGULATOR - A device forcontrolling and maintaining a uniformoutlet gas pressure.

PRIMARY AIR - The combustion air in-troduced into a burner and mixed with thegas before it reaches the port. Usually ex-pressed as a percentage of the air requiredfor complete combustion.

PRIMARY AIR INLET - The opening oropenings through which primary air en-ters a burner.

PROPANE - A hydrocarbon gas (C3H8),heavier than methane but lighter than bu-tane. It is used: as a fuel gas alone; mixedwith air; and as a major constituent of liq-uefied petroleum gases.

R

RADIANT BURNER - (See InfraredBurner.)

S

SECONDARY AIR - Combustion air ex-ternally supplied to a burner flame at thepoint of combustion.

SENSOR - The component of a control ormeasuring system that detects or measuresthe level of a controlled variable.

SETPOINT - The temperature to which athermostat is set to result in a desiredheated-space temperature. This term canalso apply generally to other types of con-trol devices.

SINGLE-PORT BURNER - A burner inwhich the entire air-gas mixture issuesfrom just one port.

SOFT FLAME - A flame in which the com-bustion zone is extended and the innercone is ill-defined. A soft flame results frominsufficient primary air.

SOLENOID - A wire-wound coil that cre-ates a magnetic field when electricity flowsthrough it. The magnetic field will pull amovable iron core within the coil.

SOLENOID VALVE - A valve actuatedby a solenoid.

SOOT - A black substance, mostly con-sisting of small particles of carbon, whichcan result from incomplete combustionand may occur as smoke.

SPECIFIC GRAVITY - The ratio of theweight of a given volume of gas to theweight of the same volume of air, bothmeasured at standard conditions of tem-perature and pressure.

STANDARD CONDITIONS - Pressureand temperature conditions selected forexpressing properties of gases on a com-mon basis, normally 30 inches of mercuryand 70°F.

STATIC PRESSURE - The pressure ex-erted against the walls of an airway that iscreated by friction and impact of air as itmoves through the airway.

T

THERM - A unit of heat energy equal to100,000 BTUH.

THERMOCOUPLE - A device consist-ing of two wires or strips of dissimilar met-als joined together at one end called thehot junction. Heating the hot junction pro-duces a DC voltage across the other twoends.

THERMOSTAT - A temperature-sensitiveswitch for controlling the operation of aheater or furnace.

6-1

6 NOTES

THROAT - (See Venturi.)

TOTAL AIR - The total amount of air sup-plied to a burner, the sum of primary air,secondary air and excess air.

TRANSFORMER - An electrical devicefor changing AC electricity from one volt-age to another voltage.

TWO-STAGE PILOT - A pilot ignitionsystem in which a small standing pilotlights a larger pilot which is cycled by thethermostat so that gas flow to the mainburner is controlled by the automatic pi-lot valve.

U

UNIVERSAL ORIFICE - A combinationfixed and adjustable orifice designed forthe use of two different gases, like LPGand natural gas.

VENT - A pipe or duct for carrying flueproducts from a heater to the outdoors.This term also designates a small hole oropening for the escape of gas or air (as in agas control).

VENTURI - A section in a pipe or a burnerbody that narrows down and then flaresout again to increase gas velocity and cre-ate a negative pressure.

Y

YELLOW-FLAME BURNER - (SeeBurner.)

YELLOW TIPS (Yellow Tipping) - Yel-low flicks in the tip of an otherwise blueflame; a symptom of the need for addi-tional primary air.

For more information on Reznor HVAC Equipment,Contact your local Reznor Representative by calling

800-695-1901

or see our web site atwww.reznorOnLine.com

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