Geothermal Heat Pump Thermostats: Keep it Consistent!


It is always best not to turn your geothermal heat pump's thermostat down and then back up to save energy; you will not save and probably it will cost you a lot of money turning the thermostat temperature up and down. You have probably heard otherwise, that this is a good way to save money on your heating bills, so why should we say this? The simple answer: it works well for fossil fuel furnaces, but it backfires when used with geothermal heat pumps.

A few years back, we had a customer buy a new thermostat and install it with a geothermal heat pump system we installed. The first electric bill they got, after they installed their new thermostat, was $425.00 higher then it should have been.

The new thermostat that they replaced our original thermostat with was a programmable one. Someone else told them they could save a lot more money if they installed a programmable thermostat and programmed it to set back to 62 degrees during the day when they were at work, and back up to 70 at 5:00 in the evening, and down again at 11:00 at night, and then up again so their home would be warm in the morning.

This works well with fossil fuel furnaces, because fossil fuel furnaces are sized way bigger than they need to be to get the job done. They can play "catch up" with temperatures in the house. They can do this without becoming very much more inefficient than they already are. Geothermal heat pumps are extremely efficient when sized correctly. This means, however, that they don't "catch up" as fast as fossil fuel furnaces. This isn't a problem for normal heating or cooling of a home, since we want to live in a pretty steady temperature anyway, but it does mean setting the thermostat back to save money won't work. If you let the temperature drop say 10 degrees, and then want it to come back up those 10 degrees in, say, an hour, the back-up heat (electric resistant or natural gas or fuel oil) has to come on to help the geothermal do it. Backup heat is more expensive than geothermal heat, and the few pennies of savings from turning the geothermal heat down are wiped out with the extra dollars it costs to quickly heat it back up with backup heat.

Our customers ended up heating their home mostly with electric resistant heat, and that cost them $425.00 more than the $150.00 it should have cost to heat their home for that month, a February in Akron, Ohio.

Setting a furnace back is what people with expensive, inefficient fossil fuel furnaces had to resort to, so they could save money somehow. Geothermal heat pumps are already money saving and efficient, and because of their design, don't benefit from the technique of setting temperatures back.

We supply a programmable thermostat with the geothermal heat pump systems we sell, in case a customer likes the temperature in their home to be a few degrees cooler or warmer at certain times of the day or night for their own comfort. They will have the convenience of having the thermostat do the work so they don't have to be running to turn the thermostat up or down whenever they want a little change in temperature. This is not done to save money, only for personal comfort preferences. Even a couple of degrees to catch up will most likely require the use of auxiliary heaters, if it is very cold outside.

The moral of the story is: set the thermostat where you want it, and keep it there.