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I have a propane-fired forced-air heating system

I have a propane-fired forced-air heating system with a Honeywell Chronotherm IV thermostat control system. The consistent recommendation I read is to reduce the setting for nighttime by a few degrees and return to comfort level for daytime in order to be more energy efficient.

However, I have noticed that the result is the system runs constantly for a long period of time before it ever gets back to comfort level. I would like to see research that shows this practice is more energy effective than leaving the setting at a constant level. All my other searches for this research have been futile.

Asked by Gene Byrd
Posted Jan 31, 2013 2:39 PM ET
Edited Jan 31, 2013 4:16 PM ET


3 Answers

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These articles will get your started:

If You Think Thermostat Setbacks Don't Save Energy, You're Wrong!

Does turning down the thermostat at night save energy?

Thermostat Setbacks - Do They Really Work?

The last article ("Thermostat Setbacks - Do They Really Work?") reports on Canadian research:

"The winter experiments demonstrated that setting back the thermostat during the day and night saved energy in the CCHT test house (see Tables 1 and 2). As the setback temperature decreased, savings increased. Also, higher savings (expressed as a percentage) were achieved on colder days, with longer furnace on-times.

"A night and daytime setback of 64°F (18°C) reduced the length of time the furnace ran, resulting in furnace fan electrical savings of up to 6.4% and furnace gas consumption savings of up to 17% on the coldest day. A night and daytime setback of 61°F (16°C) saved up to 8.1% and 21% in electrical and gas consumption respectively. On warm or sunny days, the heating demand is less, and so savings were reduced. Projecting these results to the entire heating season revealed furnace gas seasonal savings of 13% with the 61°F (16°C) day and night setback, and 10% with the 64°F (18°C) day and night setback. Predicted furnace fan electrical savings were lower for the season: 2.3% and 1.9% savings for the 61°F and 64°F night and daytime setbacks respectively."

Answered by Martin Holladay
Posted Jan 31, 2013 3:32 PM ET
Edited Jan 31, 2013 3:52 PM ET.


The exception to the rule only comes with modulating systems where part-load efficiencies can exceed the higher-modulated output rate of a recovery ramp. This description applies to modulating condensing boilers and inverter drive heat pumps, but not 1 or 2 stage propane furnaces.

If the furnace is sized appropriately for the 99% design-condition heat load recovery ramps to comfort levels SHOULD take a long time, and with simpler setback thermostats you have to start the recovery time earlier. Some Honeywell thermostats "learn" how long the recovery takes, and auto-adjusts the start of the recovery ramp based on how long it took on recent cycles to hit the comfort setpoint, so you can set it up to coincide with your waking time, etc. I thought the Chronotherm IV was one of those, but if it doesn't seem to be working that way I may be mistaken (or it isn't really working correctly.)

Answered by Dana Dorsett
Posted Jan 31, 2013 6:14 PM ET


Martin and Dana, thank you both for your responses. I read the research articles and especially appreciated the Canadian study. Dana, my Chronotherm IV thermostat does have the festure you describe where it "learns" how long it takes for recovery time. My conclusion is that we would be uncomfortable during the night with a setback to 64 degrees and higher than that would result in much less savings. And with temps down to -30 at the local airport we are stayin comfortable at 71 degrees. So it is highly individualistic.
Thanks again.

Answered by Gene Byrd
Posted Feb 1, 2013 11:48 AM ET

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