Programmable thermostat temperature spread

Which is better, let temperature rise and fall or keep constant?

What is the best programmable thermostat temperature spread? Is it better to let the temperature of your house differ widely between the comfort temperature and energy-saving temperature - say, allowing it to go from 68F when you're up and about to 61F at night - or better to keep a more narrow range between the two settings? Which saves more energy?

Answer from Green Energy Efficient Homes

You should set your programmable thermostat temperature spread as wide as you can handle it - in other words, set your night-time and out-of-home temperature as low as possible in winter, and as high as possible when the AC is running. The bigger the spread, the more money you save.

Heat is constantly escaping from your home. The bigger the difference in temperature between inside and outside, the more heat escapes. So you'll lose less heat when the house is cool (e.g. down to 64 or 61 or whatever) than when it's warm (68). It will take a fair bit of energy to warm your house back up to 68, but remember that you have saved energy by letting the heat coast down to 64, and the savings outweigh the energy used to bring it back up to temperature. Where people tend to get confused is that, while it's true the furnace has to work longer to bring your house back up to comfort temperature if your night-time temperature is lower, the furnace doesn't work longer overall - it got a nice break for most of the night!

The notion that you'll use more energy overall by letting the temperature fluctuate between comfort and night temperatures is a violation of the first law of thermodynamics. You can't have less energy escaping when the house gets cool, and yet have more energy consumed overall.

The bigger the gap between your comfort and night-time settings on your programmable thermostat, the more you'll save. And of course the lower BOTH of them are the more you'll save. I've gotten used to a daytime temperature of 67 instead of 68 (sounds like a trivial difference but it does help), and a night- time temperature of 61. And when we're all away at school or work we set it down to 60.

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