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I’ve engaged in a conversation about using a smart thermostat, lowering temp at night and when away, and bringing it back up when home.

A number of people claim their HVAC guy says maintaining the same temperature 24/7 is more efficient. This seems wrong for so many reasons, but my simplest example - say the temperature swings from 45F to 75F each day. It’s very possible the house will just get chilly overnight and warm up during the day.

My own experience is that setting the first floor to drop to 58F at night and heat to 63F for when I wake up has dropped my utility bill. I’m just looking for some intelligent argument against the word of some random HVAC guy.

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It depends on a lot of factors. If it works for you, it works for you. Intelligent argument - your energy bills. Good luck with that, there's a lot more un-intelligent argument and refusal to accept facts than intelligent argument and willingness to change opinions based on facts than the intelligent kind, in my observation.

One is what type of heating you have. Mine is cold-climate mini-split heat pumps, and the problem seen with dropping the temperature at night and raising it in the morning (I tried it and looked at my energy use) is that morning is typically the coldest time of day, so asking for a large heat input at that point in time is relatively inefficient .vs. running steady-state. Gas, oil, or electric resistance heat generally does not have that issue. It's specific to the changes in efficiency of a heat pump working at various loads.

One is thermal mass or response time, which are somewhat related, though response time is also affected by how oversized your heating is .vs. the outside temperature you are at. If your house takes a long time to cool down and a long time to heat up it's more difficult to achieve actual comfort when adjusting the temperature up and down. Hot air and cold walls are not as comfortable as warm air and warm walls.

One (in actual cold conditions, not 45 °F to 75°F) is freezing your heating pipes (particularly - obviously only applicable to water-types of heating) which tend to run in the edges of the house where it's cold, when you setback the thermostat at night and the thermostat (generally located more centrally) does not cool off enough to call for heat while the heating pipes at the cold edges freeze solid, and then don't work when heat is finally called for. Can also happen with regular water pipes but generally less of an issue. Also happens when folks use their wood-stove for heat, but it doesn't heat the far edges of the house too well (both can be overcome by having a timed over-ride for the circulators to move water even if heat isn't being called for, when it has been too long since they last ran.) Not efficiency, strictly speaking, but certainly a huge inconvenience. When extreme cold is forecast, reducing or disabling night setback is wise if a boiler is your heating source.

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    Thanks. This confirms my belief that few things are absolute. Depends on system, insulation (I’d imagine) as well as high/low each day. Jan 22 at 23:30
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    Yeah, setbacks are best for fast responding systems that have excess capacity (forced-air furnaces and to a lesser extent electric resistance heat), while steady-state operation is better for slower responding systems (steam especially) or systems that don't have much excess capacity to play with (heat pumps) Jan 23 at 3:13

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