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It was first brought to my attention by a local HVAC Contracting company, that a programmable/setback thermostat would actually cause my home heating system to use more energy, and hence for me to have higher heating bills. Their explanation was that the energy required to increase from 60°F (16°C) to 69°F (21°C), was higher than that to simply maintain 69°F (21°C) over the same time period. According to them, this was especially the case when the outside temperature fell below 20°F (-7°C).

With a little further digging, I found this idea may be specific to heat-pump style systems, as described by The Capital Gazette. This may have led to the misconception that this applies to all heating system types.

The California Energy Commission describes the benefits of "setback thermostats" and mentions this misconception:

It's a common misconception that leaving your heating system on all day is more economical than setting your thermostat back. In truth, leaving the temperature inside your home constant will most likely cost you money.

[...]

Your savings can be impressive. Recent studies show that properly using your automatic thermostat should cut your heating costs from 20 to 75 percent. In summer, such devices may shave your cooling costs by 15 to 25 percent. (Your actual savings will depend on such factors as the climate in which you live, the amount of insulation in your house, what temperature you set the thermostat, and the rate structure of your utility company.)

I have looked elsewhere on the 'net and can find only contradicting unsubstantiated claims, most of which do not distinguish the type of heating system.

Is there any research to support the claims of either side? Does it really depend on what type of system is used?

  • I think this question is hard to have a single definitive answer for. Much of the details depend on the house itself: how well insulated it is determines how hard it is to keep it warm, for example. In Chicago, if you have R-40 or better insulation, 20°F would be not a problem if the house is left unheated for eight hours (though 0°F might be). In more temperate areas with lower insulation requirements it's more likely to be a problem. – Joe Nov 23 '15 at 15:30
  • What is the difference in energy costs during the time period while you are out? If power is cheaper during the day then it would make sense to keep the temperature constant during the day. But usually energy costs are higher during the day so this seems unlikely. – Moby Disk Nov 23 '15 at 20:43
  • Looking at the linked article it sounds like the issue is when it is suddenly called upon to raise your house considerably it turns on not only the heat pump but the backup electric elements also--and those are less efficient. – Loren Pechtel Nov 24 '15 at 5:22
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Here are the findings from a study from the Canada Mortgage and Housing Corporation (CMHC pdf)

They used a twin-house setup: two identical houses with different settings for the heating.

The findings:

The winter experiments demonstrated that as the setback temperature is decreased, energy savings increase. Higher savings are achieved (as a percentage) on colder days with longer furnace-on times. The greatest savings occurred on the coldest—cloudiest day (minimum -26.2°C [-15.16°F] to maximum -15.4°C [4.28°F] outdoor temperature) of the 16°C (60.8°F) night and day setback.

It's important to note that in this study, the energy efficiency of the heater was not changed by the rate at which heat was demanded. If that assumption does not hold, then you can get into odd edge cases where lowering the set-back temperature might conceivably increase energy consumption. That would happen if the proportional decrease in heater efficiency is greater than proportional decrease in heat-energy demand. As your Capital Gazette link says, this might happen if the lower setback temperature creates a requirement for a rapid increase in internal temperature when the setback period ends, and that rapid increase has to be met by a heater with efficiency less than or equal to 100% (boiler, furnace, resistance heater), rather than a heater with a much higher efficiency (a well-specified, properly installed heat pump).


footnote:

The physics behind the CMHC finding is fairly simple: the heat loss at any one moment, for a given property, is pretty much proportional to the difference between internal and external temperature. And to get the house back up to comfortable temperature, we're going to have to put back into the house, all the heat that it's lost. So if, for a period of time, that temperature difference is lower, then the heat loss will be lower, and that's less energy we have to put back in. The temperature difference is lower, when the setback temperature is lower (assuming it's colder outside than it is inside).

  • So, the best way to save energy is to turn the thermostat WAY up and leave it there? – user29285 Feb 9 '16 at 3:16
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    @nocomprende I'm not sure how you have construed this to be the exact opposite of what it says: which bit of what I wrote has led you to your erroneous inference? – EnergyNumbers Feb 9 '16 at 4:32
  • There are situations where a steam plant delivers steam to heat buildings, and turning down a thermostat causes cool water to counter the steam in the particular zone of the thermostat, and therefore energy is wasted. – DavePhD Jan 16 at 14:42
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    The first link is broken and the pdf hasn't been saved at archive.org. (Multiple hits at archive.org, but they're all 404 errors.) – David Hammen Jan 28 at 21:40
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    I don't understand your answer at all. Nothing makes any sense. Very unclear. – Martin F Feb 2 at 1:05
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According to The Effects of Thermostat Setting on Seasonal Energy Consumption at the CCHT Research Facility (2005):

An 18°C night and daytime setback reduced the length of time the furnace ran, resulting in furnace electrical savings of up to 6.4% and furnace gas consumption savings of up to 17% on the coldest day. A 16°C night and daytime setback 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 16°C day and night setback, and 10% with the 18°C day and night setback. Predicted furnace electrical savings were lower for the season: 2.3% and 1.9% savings for the 16°C and 18°C night and daytime setback respectively.

The setbacks are relative 22°C. So 18°C set back means reducing the thermostat setting by 4°C.

  • I searched the linked document and found zero mentions of heat pump or pump of any kind, and it is otherwise unclear what type of heating was used at the research facility. So while helpful, it does't really add anything new. – cobaltduck Jan 29 at 13:49
  • @cobaltduck They used a medium efficiency gas furnace with permanent split-capacitor motor. Output was 19.78 kW (67,500 Btu/h). I added this answer mainly because the link in the other answer isn't working. – DavePhD Jan 29 at 13:58

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