Let’s talk thermostats today. This ubiquitous device in homes is the user interface that allows people to control their heating-and-cooling systems. It might seem simple. Turn it up when you need more heating or cooling, and down when you want less. But there’s a lot more to it than that. Should you do setbacks to save energy? Should you put the fan in the “on” position or leave it in “auto”? Should you use the “emergency heat” setting? Let’s sort all this out.

Deep setbacks with right-sized equipment

Many homes have a heating-and-cooling system that has two or three times the capacity it needs. You can tell if that’s the case by monitoring how long it runs on a hot afternoon or a cold night. If it runs only 30 minutes an hour when it’s hot or cold, your system is about twice as big as it needs to be. (See my article on design temperatures for the definition of hot and cold for HVAC design purposes.)

This is the main thing you need to know if you’re thinking about using setbacks to save energy. Yes, setbacks do save energy, but that’s mostly true only in leaky, underinsulated houses with oversized heating-and-cooling systems. If you have a good building enclosure and right-sized (or in my case, undersized) systems, you need to be very conservative with setbacks.

The reason for that is that a right-sized system doesn’t have the capacity to recover quickly. If it’s a design day, setting your thermostat back may make it difficult to get the house back to the temperature you want.

Or maybe you have a heat pump that has auxiliary heat. That just exchanges one problem for another.

Deep setbacks with electric resistance auxiliary heat

A different problem with setbacks occurs if you have a heat pump with electric resistance auxiliary heat. Say you do a deep setback at night and then turn the thermostat up several degrees in the morning. What happens is that the heat pump will turn on the resistance heat to catch up more quickly. And that can jack up your heating bill significantly.

The heat you get from the heat pump itself is about three times more efficient than the electric resistance heat. So you want the heat pump to provide most or all of your heat. The majority of the hours you heat your home in winter—at least in the places where most people live—are prime heat-pump conditions. It’s not too cold for your heat pump to work at high efficiency, so you should be letting your heat pump do most of the work then.

Setting it to “emergency heat” because it’s “too cold”

This mistake is way too common for homes with heat pumps. Worse, it’s promoted by some in the HVAC industry who should know better. Installers and technicians sometimes tell homeowners that heat pumps “don’t work” when it’s too cold outside. And their definition of “too cold” varies. Occasionally it’s as high as 45°F.

And that’s wrong, wrong, wrong! Even heat pumps made for milder climates keep pumping heat as the temperature drops into the teens and single digits or below zero. They just pump less and less as it gets colder. (See my article on the balance point temperature.)

So keep the thermostat set to “heat.” When the heat pump can’t provide enough heat for the house, it will kick on the auxiliary heat to supplement the heat pump. Or if it’s a dual-fuel setup, it will switch over to the furnace.

Also see the article I wrote about the “emergency heat” setting a while back.

Setting fan to “on” for cooling in a humid climate

In a mixed-humid or hot-humid climate, you need to be really careful with the fan setting. When you’re cooling the house, there’s a dangerous setting that can make your house both less comfortable and less healthy.

In short, the problem is that that setting keeps the fan blowing air over the coil whether the AC is cooling or not. In a humid climate, the coil gets cold and does two jobs: It cools the air passing over it, and it removes humidity from the air passing over it. That second part happens by water vapor condensing on the coil.

By keeping the fan on when the AC goes off, the now liquid water sitting on the coil evaporates back into the air and gets blown back into the house. And that raises your indoor humidity. The graph below shows what happened when I did this in my home.

That setting is to keep the fan in the “on” position rather than “auto.” (See my full article on this for all the details and data from a little experiment I did in my house.)

Turning the thermostat down when it’s too humid indoors

Southerners who go north in winter find it to be too cold. Northerners who go south in summer also find it to be too cold. So many buildings in the South are kept way too cold.

Why? Often it’s because the building lacks adequate humidity control. If it’s too humid indoors, you feel uncomfortable. It feels “sticky.” So people head to the thermostat and knock it down a few degrees.

This is a mistake for several reasons. First of all, it doesn’t solve the comfort problem. Yes, a lower setpoint causes the AC to run more, which makes it dehumidify more. But you still may end up with a higher relative humidity because the temperature is lower. So you end up feeling cold and clammy.

Another reason it’s a mistake is that you can cause serious damage to a house by keeping it really cold in a humid climate. You can read about that and more in my article 10 Consequences of Keeping Your House Really Cold in Summer.

And that’s not all

The mistakes above are things homeowners do wrong. There’s another set of mistakes that installers and technicians make with the settings that aren’t easily accessible to homeowners. I’ll cover those another time.

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Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.