Boiler Facts

“How can I lower my fuel bill?” Getused to hearing that question! Home-owners everywhere are asking the samequestion. I think this is a goldenopportunity to talk to homeownersabout their heating system. With energycosts on the front page of everynewspaper and leading every nightlynews program, here is our chance! Oneof the easiest options you can offer anycustomer with a hot water heating systemis to install a “weather responsive”control. By simply turning down thetemperature of the water in the boiler,even to a minimum of 140°F willprovide—on average—10-15% fuelsavings.

Did you know that in a typical hotwater baseboard heating systemdesigned around 190°F water, if theflow rate were cut in half, the Btu/houtput would still be around 90%? Thisis because, of the three factors affectingheat transfer, flow rate has the leastimpact. The other two factors are thesquare footage of heat transfer surface(the radiation) and the temperaturedifference between the heating medium(water) and the air surrounding theheating element (room temperature).

In a typical system, once you installthe baseboard, the amount of heattransfer surface is not going to change.In addition, the difference in tempera-ture between the water in the baseboardand the air in the room is usuallysignificant (180°F-70°F = 110°F). Thethird factor, flow rate, has to bereduced considerably to cause asubstantial drop in the average watertemperature in the baseboard, whichwould reduce the heat output of thebaseboard. For example, if you look atany baseboard manufacturer’s literature,the Btu/h output per foot of board isbased on 1 gpm and 4 gpm. Thedifference in Btu output for each flowrate is minimal. This is the reason whymost hot water systems work adequate-

ly without extensive circulator sizingand/or flow balancing.

The design flow rate can be messedup 50% and still the system would get90% of the Btu/h capacity of thebaseboard. The reason is because thereduced velocity (flow rate) keeps thewater in “contact” with the baseboardfor a longer period of time, giving upmore of its temperature.

What is the significance of all this?Consider the way we size boilers for ourheating systems. We always pick theboiler based on the design conditions,the coldest day of the year! We aretrying to keep the customers warm bymaintaining 70°F inside their homeswhile the outside temperature may beas low as 0°F or even colder. However,what happens when the outdoortemperature is not at design conditions?Our boilers and radiation, in effect, areoversized. Did you know that the designconditions in the Northeast exist forless than 5% of the total heatingseason? In fact, for most of the heatingseason, the conditions are 50-60% ofour actual design conditions. So here weare sizing, selling and installing systemsthat are oversized for 95% of theseason. I think this is where “tempera-ture-responsive” controls provide anopportunity to sell a better job andprovide more comfort and cost savingsto the customer.

MATCH THE TEMPERATURE TO THE LOAD!Outdoor reset is when you increase ordecrease the water temperature goingout to the system according to theoutdoor temperature. The systemincorporates an outdoor sensor toinform the control of the outdoortemperature since this has the greatestimpact on the building’s heating load.When you reduce the supply watertemperature, you reduce the Btu/houtput of the heating terminal unit(baseboard). This is because you are

Why weather responsive controlsare a “No-Brainer”

changing the difference between the airtemperature surrounding the baseboardand the water temperature inside thebaseboard. By lowering the supply watertemperature, you can input the rightamount of heat, offsetting the heat lossof the building. A lower water tempera-ture will also create a more comfortableenvironment for the homeownerbecause the wide temperature swingsthat normally occur will be eliminated.A typical heating system uses athermostat, which is a switch that sendsa signal to the boiler and a circulatorsaying it is cold. The circulator turns onand sends 180°F water out to thebaseboard zone. And this happenswhether it is 10°F or 50°F. Most of thetime, the 180°F water heats up the zonequickly and the thermostat, sensing thistemperature rise, shuts off the circula-tor. However, remember the heat lossfrom the building has not stopped. Itcontinues as long as the outdoortemperature is below the desired indoortemperature. Therefore, the systemcontinues to cycle on and off, becomingtoo cold and then too warm.

With reset however, when you changethe temperature of the water to matchthe load, the circulators and/or zonevalves stay on for longer periods. Thiskeeps the radiation warm all the timeinstead of cycling on and off. This moreconstant supply of cooler, comfortablewater also eliminates the creaking andgroaning noises usually heard in systemsthat cycle 180°F water into a zone. Theroom temperature will not override dueto the excessive water temperatures.Another benefit of resetting a hydronicsystem is fuel savings. By lowering thewater temperature in the boiler andpiping system, the stand-by losses andstack losses are minimized.

The concept of changing the watertemperature to match the load of theheating system is very logical and hasbeen around for quite some time.

from the June 2008 issue of

Recently, however, control technology has advanced consider-ably. This has provided some reasonably priced, yet veryeffective residential and commercial hydronic controls.

BOILER RESET

Boiler reset is probably the most common method of resetbecause of its simplicity. The reset control uses an outdoorsensor, a supply water sensor and optionally an indoor sensor,and by combining all this feedback, the control calculates thenecessary water temperature by cycling the boiler on and off.

One major concern you should be aware of when usingboiler reset is flue gas condensation. If the temperature of theboiler water is allowed to operate below the dew point of theflue gases, they will condense back into liquid, possiblycorroding the boiler and its breaching and flue pipe. Theboiler will also experience plugged flue gas passagewaysbetween its sections. To prevent this problem from occurring,

most boiler-reset controls have a minimum supply tempera-ture setting feature. This setting is adjustable and can be setto satisfy any boiler manufacturer’s minimum water tempera-ture concerns. Unfortunately, the average minimum tempera-ture is around 135°F-140°F, which, during the shouldermonths (fall and spring), is too hot compared to the build-ing’s heat loss. To overcome this problem, it becomesnecessary to have some type of thermostat to stop thecirculator or close a zone valve when the zone starts tooverheat.

Even with these temperature limitations, installing a simpleboiler reset control on most hydronic systems results in fuelsavings in the range of 10-15%. Additionally, the system ismore comfortable, with fewer temperature swings and fewercreaking and expansion noises.

If you have any questions, please e-mail me atgcarey@fiainc.com or call 1-800-423-7187.

by George Carey