Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Turn Down the Heat!!!

Learn more physics!

Most recent answer: 05/16/2013
How does turning down a thermostat save energy?
- Anonymous

Well, let’s say it’s really cold outside, so you’re running your heater. Maybe you normally set your thermostat to 68 degrees. If you turn the thermostat down to only 66 degrees, then the heater won’t have to run as much, for two reasons. It doesn’t have to get the house as hot in the first place, and it has to work less hard to replace all the heat energy which is lost to the outside. This second effect is the most important, particularly if the house is not insulated well. The rate at which heat is lost is approximately proportional to the difference between the temperature of the house and the temperature outside, and how much the heater runs is therefore also proportional to this difference. The less the heater runs, the less energy it uses.

On the other hand, let’s say it’s really hot outside, so you’re running your air conditioner. In this case, if you turn your thermostat from 78 degrees to 76, your air conditioner will have to run more in order to make your house that much cooler. Since the air conditioner is running more, it will use up even more energy.

So in sum, if you’re running the heater and you turn down the temperature on your thermostat, you will save energy. But if you do the same thing while running the air conditioner, you will not. If the temperature outside is near the same as the temperature you would like inside, then neither your heater nor your air conditioner needs to run.

For more information on how thermostats work, check out the answer to the question "How do furnaces work?".

-Tamara and Tom

(published on 10/22/2007)

Follow-Up #1: turning down thermostat

If you want to turn down the thermostat while you're gone (ex. in an office overnight up to 14 hours.), how many degrees saves money and how many just makes everything so cold it costs more to warm back up the next day?
Greensboro, NC, USA

That's a very important question. It turns out that the lower you set the thermostat (when it's cold out), the less energy you have to pay for. Over time, the total energy input to the house very nearly equals the energy output. Since the warmer your house is, the more energy flows out, turning the temperature down saves energy. Of course, you want to make sure that the pipes never get so cold that they freeze.

Mike W.

There is one interesting exception to this rule. If you have open-air heat-pump heaters (basically air-conditioners running in reverse) they run more efficiently when the outside air is warmer. So under some circumstances it's more efficient with them to overheat the house a bit during a warm day so that it can coast longer without running during a cold night, when the heat-pump is less efficient.

(published on 05/16/2013)

Follow-up on this answer.