It's the middle of winter and you're ready to go on vacation. You pack your swimsuit, check your flight info, and get ready to turn down the thermostat. After all, you don't want to waste money or energy warming up a house that won't have anybody in it. But how low is too low? And will you really save that much money? Or does it take just as much energy as you saved to warm your house back up after turning it down?
Here's what we know:
Do You Really Save Energy by Turning the Thermostat Down?
It turns out, there really is a benefit to setting your thermostat lower, particularly over a long period of time like when you're gone on vacation.
The myth you've probably heard about having to use the same amount of energy saved to heat the house back up to temp is false. It’s a matter of temperature difference and time.
For example, if your house is at 70ºF and its only 5ºF outside, that’s a 65º difference to overcome. The greater the temperature difference, the more your home loses heat through the building envelope. So, if you reduce that difference by setting the thermostat back to 60ºF, you would reduce the differential by 10º — for a 55º differential.
It might help to think of the difference like it's a pressure difference. A lower "heat pressure differential" means less heat loss. And, of course, the longer the thermostat is set lower, the more energy you save.
Are there any situations where it doesn't make sense to set back the thermostat? A modulating gas boiler could fall into that category because the boiler efficiency is dependent on the low return water temperature. So, when you heat up the house at the end of your vacation, the water temp goes way up and makes the boiler much less efficient, thus negating the energy savings.
How Low is Too Low?
There's a limit to how much energy you can save, because the lowest allowable set point for most gas furnaces today is 60º F.
It may sound odd, but we're trying to avoid the furnace operating too efficiently.
Here's the potential problem: When a cooler return air temperature enters the furnace, it pulls more heat from the heat exchanger and cools down the flue gases (products of combustion) too much.
If that happens, there is less heat going up the chimney, or in the case of a high-efficiency condensing furnace, less heat going out through the PVC. That's great for efficiency, but it may be bad for the furnace.
Cold return air on a non-condensing (80% or less efficient) furnace could cause it to become a 90% condensing furnace. That's an issue because flue gases contain a lot of water vapor. If allowed to cool off too much, they condense and can cause corrosion problems inside the furnace's heat exchanger.
That can even be a problem on 90%+ furnaces where we want the flue gases to condense. If the heat exchanger is too cool, water forms in areas that are not meant to get wet, again causing corrosion problems.
Remember, residential gas furnaces are designed to do a specific function within specific parameters, and 60ºF is at the bottom of those parameters. That’s why we do not recommend using a residential furnace to heat a garage. Unit heaters are meant to operate in garage conditions. Furnaces are meant to make your home comfortable.