Millions leave this thermostat setting on by mistake; HVAC experts say it could be raising your power bill

Most of us set our thermostat's temperature and never think twice about the little "Fan" switch next to it. But according to the U.S. Department of Energy, which specifically recommends running your system's fan on "Auto" rather than "On," that overlooked switch could be adding unnecessary dollars to your monthly power bill.

It's an easy setting to overlook
Your thermostat actually runs two separate jobs. One controls the temperature you want. The other, the fan setting, controls when air physically moves through your vents. "On" keeps the blower running nonstop, day and night, whether your AC or heater is doing anything or not. "Auto" only turns the fan on when the system is actively heating or cooling.

The setting is really easy to get wrong because when you see "On," you think it is going to make everything work better. That is not what it does. What it really means is that the fan motor is using electricity all the time, every day, even when your home is already at the temperature you want it to be. The fan motor keeps using electricity, which is what happens when you have the setting on "On."

Why it quietly adds up
The Department of Energy says that running the fan all the time can actually make your system work efficiently. This is true unless your system has a high-efficiency motor that is made to run all the time. Most home systems might not have this kind of motor. So running the fan all the time just uses energy and does not help. The Department of Energy notes that the fan is just burning energy for no real reason. Your system's efficiency is important, and running the fan nonstop can work against it.

There's also a humidity angle. According to the Environmental Protection Agency (EPA), keeping indoor humidity ideally between 30 and 50 percent is key to avoiding mold and moisture issues at home. When your AC cools your home, moisture collects on the evaporator coil and is meant to drain away once the cycle ends. On "Auto," the fan shuts off along with the system, giving that coil time to drain. When you turn it to "On," the fan keeps blowing straight across the still-wet coil, pushing some of that moisture back into your living room. Your AC then has to work harder to remove the humidity again, which can use more energy and raise your bill.

What "Auto" actually gets you
For most households, "Auto" is the more practical everyday choice. It only runs the fan when it's actually needed, which cuts down on unnecessary electricity use, and it gives your system a real chance to dehumidify properly between cycles. It also tends to be gentler on your filter, since air isn't constantly being pushed through it.

Since heating and cooling typically make up close to half of a home's total energy bill, according to ENERGY STAR, small habits like this one can genuinely move the needle over a full season.

When "On" still makes sense
That said, the "On" setting is not always a bad choice. The "On" setting can actually be helpful in some cases. If some rooms in your home are warmer or cooler than others, running the fan continuously can help even out temperatures.

This way, air keeps moving through your filter, which can improve filtration. That can help people with allergies by keeping air moving through the home, and the fan setting helps with air circulation.

The thermostat's "On" fan setting keeps air circulating continuously, providing more consistent filtration and temperature throughout the home. Image Credit: Wikimedia Commons

So it's not really about finding the one answer; it's about what you want to achieve. If you want to save on your bill and have control over humidity, then "Auto" is the way to go. But, if you want air to keep circulating all the time, then "On" has its uses; be prepared to pay a bit more.

The simple solution
If you have not checked your fan setting before, it is worth a look. Many people move into a home and never change the fan switch, so it may have been running continuously without anyone noticing.

Switching to "Auto" costs nothing, but this change could help lower your electricity bill, especially during the hottest summer months.