hvac fundamentals

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HEATING, VENTILATING, &

AIR CONDITIONING FUNDAMENTALS

HVAC FUNDAMENTALS

INTRODUCTION

•HVAC = Heating , Ventilating and Air Conditioning

•Heating and cooling load calculations

• Energy and Heat Transfer

• Material Properties

• Climate/location

• Building Construction & Envelope

•Ventilation requirements

• Natural vs Mechanical

•System and Equipment types

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THERMAL PROCESSES

DEFINITIONS

• Temperature – measurement of heat

• Specific Heat – storage capacity of a material

• Latent – change of state energy; related to moisture

• Sensible – “what you feel”; temperature changes

• Conduction – heat transfer through contact

• Convection – warmer air rises, cooler air falls

• Radiation – heat transfer without a medium



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DEFINITIONS

• R = thermal resistance (of a material and specified thickness)

• K = conductivity (material property)

• U‐value = reciprocal of R

• x/k = R (where x is thickness of material)

• R = 1/U

• U = 1/ (R1 + R2 + R3 + R4)

• Where each R is different section of material in any given construction assembly

• Ex: Chart of building materials in construction

R & U VALUES



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FORMULA

• Q = UA (T2‐T1)

• (heat transfer coefficient) x (area) x (temperature difference)

• Degree Days = mean temperature of a day is 1 degree different from 65oF. (i.e. mean temp of 63oF is two heating degree days)

• Infiltration

• Air changes

• Crack method

HEATING LOAD CALCULATIONS



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ELECTRIC HEAT

• More efficient

• More flexible sizing choices

• One power (utility) source

• No flues / piping on room

HVAC FUNDAMENTALS

GAS HEAT

• Perception ‐ warmer

• Direct and indirect

• Less flexible for unit selection and staging

• Duct furnaces



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VENTILATION

‐ Design requirements and focus:

• Code requirements

• Odor control

• Smoke control

• Energy recovery

HVAC FUNDAMENTALS



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FACTORS

• Exterior elements (envelope loads)

• Roof (actual temp over 120oF)

• Walls (heavy walls, heat moves slower)

• Windows (warm surface)

• Windows (solar)

• Outdoor air for ventilation (people)

• Outdoor air due to infiltration (leaks)

• Interior elements

• People

• Lights (100% conversion to heat)

• Equipment

COOLING LOAD CALCULATIONS



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COOLING LOAD CALCULATIONS



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PSYCHOMETRICS

• Dry Bulb

• Wet Bulb

• State Point

• Relative Humidity

• Dewpoint



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PSYCHOMETRICS



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DEFINITIONS

• Absolute Humidity ‐ actual mass of water vapor present in the air water vapor mixture. The absolute humidity may be expressed in pounds of water vapor (lb).

• Specific Humidity Ratio or Humidity Ratio ‐ ratio between the actual mass of water vapor present in moist air ‐ to the mass of the dry air.

• Humidity Ratio ‐ normally expressed in pounds of water vapor per pound of dry air or in grains of moisture per pound of dry air. There are approximately 7,000 grains in a pound.

• Relative Humidity ‐ The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

• Dewpoint temperature ‐ temperature below which moisture will condense out of air. Air at a given humidity ratio has a constant dewpoint. If air is cooled below this point, moisture condenses out, thus changing its humidity ratio.

PSYCHOMETRICS



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PSYCHOMETRICS



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COMMON FORMULAS

• Sensible Btuh = Cfm * ΔT * 1.08

• Derive the 1.08 factor as follows:

• 1 CFM x 60 = 60 CFH

• 60 CFH x .075 lbs of air/cu ft = 4.5 lbs of air/hr

• 4.5 x 0.24 Btu/lb ‐°F (specific heat of air) = 1.08 Btu/hr °F ‐ CFM

• Total heat = Cfm * ΔH * 4.5

• Derive this one on your own?

• Mixed Air ºF = (Cfm RA * ºF) + (Cfm OA* ºF)

Cfm SA (Total Cfm)

PSYCHOMETRICS



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BASICS

• Human Body generates 450 – 2500 BTU/Hr

• Psychometric Chart

• Sling Psychometer ‐measures wet bulb temperature (grains of moisture per pound of air or pounds of moisture per pound of air)

• Ranges for comfort:

• 65 – 78 deg F (dry bulb)

• 20% ‐ 50% RH

HUMAN COMFORT



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HYDRONIC SYSTEMS

• Steam, Heating Hot water, Chilled Water, etc.

• Often Radiant Design (radiators)

• Single Pipe

• Typically Boiler system (heating)

• Common supply/return pipe

• Two Pipe

• Either Boiler or Chiller (heating or cooling)

• Dedicated supply and dedicated return pipes

• Three Pipe and Four Pipe Systems

• Heating and Cooling systems

• Separate coils or mixing valves and single coils

HVAC SYSTEMS



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HVAC SYSTEMS



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FORCED AIR SYSTEMS

• Single Duct vs Dual Duct Supply

• Constant Volume vs Variable Volume

BUILDING PRESSURE

• Positively Pressurize Building

• Limits infiltration

HVAC SYSTEMS



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FORCED AIR SYSTEMS

HVAC SYSTEMS



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What is a “ton” of AC?

HVAC SYSTEMS



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PLANT SIZING

• Capacity of all loads on a ‘design day’ based on the system (peak vs. block load)

• Heating: MBH or kBTU’s (1,000 BTU/HR)

• Cooling: Ton of cooling (12,000 BTU/HR)

• 1 ton of ice – change of state/melting into water in 24 hours

HVAC SYSTEMS



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SYSTEM SIZING

• Air systems

• CFM = Q(BTU)/1.08 * (T2‐T1)

• CFM = Cubic Feet of air per minute (Airflow)

• Ductwork

• A = 144 * CFM / V

• V = velocity in feet per minute

• Common: 300 to 2,000 fpm

• A = duct area in square inches

• Fans

• Static Pressure: created by fan on duct system to overcome friction loss and throw at diffuser or grille.

• Friction loss as H2O = Inches of water column

HVAC SYSTEMS



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HVAC SYSTEMS



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BASICS SUMMARY

• Heat flows from object at higher temp to object at lower temp

• Air Conditioning (AC) does not add cool, it extracts heat

• Refrigerant (aka “Freon”)

• Common characteristics

• Changes states

• Transfers energy

• Highly efficient

HVAC FUNDAMENTALS



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REFRIGERATION CYCLE (VAPOR COMPRESSION)

HVAC FUNDAMENTALS

Hot, high pressure gas is circulated through a heat rejection coil outside and changing refrigerant state to liquid.

Compressor squeezes low pressure refrigerant gas into hot high pressure refrigerant gas.

Refrigerant expands into cold, low pressure liquid.

Cold liquid/gas runs through a coil while air passes over. The cold gas absorbs heat from the air.



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REFRIGERATION CYCLE FOR HEAT PUMPS

HVAC FUNDAMENTALS

Reverses on a Heat Pump



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EQUIPMENT PROS AND CONS

• Package

• Splits

• Rooftop Units

• Central Water Source Heat Pumps

• Central Rooftop VAV

• Central Chilled Water

• Distributed Air Handlers

• Central Large Air Handlers

• Central… District Chilled Water?

HVAC FUNDAMENTALS



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WINDOW UNIT

HVAC FUNDAMENTALS



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WINDOW AC OR PTAC UNIT

HVAC FUNDAMENTALS

• Pros

• Low first cost

• Easy to install

• Easy to replace / maintain

• Cons

• Low efficiency

• Appearance

• Requires outside

• Short life cycle

• Noise



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SPLIT SYSTEM

HVAC FUNDAMENTALS



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SPLIT SYSTEMS – FURNACES OR AHU’s & CU’s

HVAC FUNDAMENTALS

• Pros

• More options for location

• Larger capacity

• Condenser locations

• Sound

• Cons

• Efficiency

• O.A. needs to be ducted

• Equipment access –usually located in ceilings

• Condensate piping

• Replacement costs

• Zone control challenges



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ROOFTOP UNIT

HVAC FUNDAMENTALS



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ROOFTOP UNITS

HVAC FUNDAMENTALS

• Pros

• Higher efficiencies

• Larger capacity

• Ease of installation

• Ease of service and maintenance

• No internal O.A. ductwork

• Cons

• Zoning issues for larger systems

• Need to be concealed

• Zone Control (non‐VAV)

• Structural requirements

• Limit to number of floors

• Shaft requirements



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ROOFTOP VAV

HVAC FUNDAMENTALS

VAV boxes vary the amount of air to meet cooling needs in individual spaces.



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ROOFTOP VAV

HVAC FUNDAMENTALS

• Pros

• Higher efficiencies

• Larger capacity

• Zone control

• No internal O.A. ductwork

• Cons

• Structural requirements


• Initial equipment costs

• Concealment

• Limit to number of floors

• Shaft requirements



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CHILLED WATER

HVAC FUNDAMENTALS



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CENTRAL CHILLED WATER AIR HANDLERS

HVAC FUNDAMENTALS

• Pros

• More options for locations

• Maintenance is centralized

• Sound

• Zone control

• Opportunities for LEED points

• No limit to number of stories

• Cons



• Experienced HVAC on‐site staff

• First cost of equipment and distribution



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• Water Source Heat Pumps

• Closed Loop Cooling Towers

• Evaporative

• Fluid Coolers

HVAC FUNDAMENTALS



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WATER SOURCE HEAT PUMPS

HVAC FUNDAMENTALS

• Pros

• High efficiencies / energy savings

• Zone control

• Location options

• Cons



• Equipment access –usually located in ceilings


• Sound



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RULES OF THUMB

• 1 kilowatt = 3.413 btu (British thermal units)

• 1 btu = heat needed to raise 1lb water 1°F

• One ton = 12,000 btu/hour

• Air leaves AC unit at 55°F

• Airflow required to space is approx 1 cfm/ft2

• AC units has nominal capacity of 400 cfm/ton

HVAC FUNDAMENTALS



HVAC FUNDAMENTALS

THANK YOU

QUESTIONS / DISCUSSION ?

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