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That small toggle on the thermostat marked "Auto" and "On" is easy to overlook, tucked below the temperature controls like an afterthought. But it controls a part of the HVAC system unrelated to heating or cooling: the blower fan. To find out what each setting actually does, I asked Dan DiClerico, director of the Good Housekeeping Institute's Home Improvement & Outdoor Lab, to break it down.
Many people assume "Auto" and "On" are just a matter of preference. In reality, they control whether the blower runs only during active heating and cooling cycles, or nonstop—a difference that shows up in both comfort and energy bills.
Below, DiClerico explains when to use each setting and the one mistake to avoid.
What is the best setting for most homes?
"Auto is the best setting most of the time," DiClerico says. Since the fan runs only when the system is actively conditioning the air, it uses less electricity and puts less wear on the equipment over time than running continuously does.
When should I switch the fan to "On"?
A couple of scenarios make the tradeoff worth it, according to DiClerico. During a heat wave, running the fan continuously lowers a home's relative humidity, which makes the air feel more comfortable and helps curb mold and mildew in damp spots like bathrooms and laundry rooms. It can also provide relief during allergy flare-ups, since a constantly running fan draws indoor air through the HVAC system's built-in filter.
The biggest mistake homeowners make is treating "On" as a default rather than an occasional fix. "It's a mistake to leave it running all the time," DiClerico says. Beyond the added electricity costs, constant operation puts extra wear and tear on the system, which can lead to more frequent repairs or premature failure.
What about the "Circulate" setting on some smart thermostats?
It's worth using, not just a marketing footnote. "The 'circulate' setting is a good middle ground between 'on' and 'auto,'" DiClerico says. Air moves through the system continuously, but at a lower speed than the "On" setting, so a home gets the comfort and air-quality benefits of regular circulation without the added energy costs and strain on the equipment.
The takeaway? Leave the fan on "Auto" for everyday use, switch to "On" during a heat wave or allergy flare-up, and use "Circulate" if available for the best of both worlds.
