Auto vs. On: We Settle the Battle of the Thermostat Settings

Katy Willis
Written by Katy Willis
Updated July 7, 2021
Woman adjusting thermostat
Monty Rakusen/Cultura via Getty Images

Turns out there’s a big difference between setting your thermostat’s fan setting to “on” or “auto”

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Amidst the hustle of daily life, you probably don’t think too much about your thermostat unless your home becomes uncomfortable. However, there’s an important item you might be overlooking: the on vs. auto fan setting. Learn more about what your heating and cooling system’s fan does, and then see the pros and cons of each setting to determine which works best for your circumstances.

What Does the Fan Do?

Your heating and cooling system’s fan draws in air to be heated or cooled, then pushes the heated or cooled air through the vents in your home. When your system is running and you put your hand up to a vent, it’s the fan creating the airflow that you feel.

Virtually all modern air conditioners and heaters allow you to pick which setting you’d prefer for your fan. You can choose from “on” or “auto.”

Switching the Fan to "On"

What Happens

When you switch the fan’s setting to “on,” you’re sending the signal that the fan should run all the time, 24/7/365—or at least, until you change the setting again. This means that when you put your hand up to the vent, you’ll always feel airflow. That air will be cooler or warmer when the system goes through a cooling or heating cycle and is more neutral when it’s not.


  • More even distribution of heating and cooling: When your fan runs all the time, you’ll notice that the cool or warm air from a system cycle will travel farther from the vent. As it’s constantly circulating through your space, you’re less likely to encounter hot or cold zones.

  • Better air filtration: Leaving your fan running all the time also offers the benefit of better air filtration because the air in your home is constantly passing and repassing through the system’s air filter. This can help you breathe more easily if you live somewhere dusty or you suffer from allergies.

  • Cooler feeling in the summer: Your air conditioner’s fan isn’t too different from a ceiling fan in that they both promote better airflow. So during warmer months, having even a gentle breeze passing around you will help you feel cooler than if the air was stagnant. You’ll enjoy this effect even when the air conditioner isn’t currently going through a cooling cycle.


  • Costs more: One of the biggest drawbacks of leaving your fan running all the time is that it will use more energy, and in turn, result in increased energy costs. How much it costs to operate your fan will depend on the age and efficiency of your system, but it typically costs about $50 extra per month to take advantage of the “on” setting.

  • Goes through more air filters: Because the air in your home is constantly circulated through your air filter, the filter is going to trap more dirt. This is great news if there’s a lot of dust in your house, but this also means that you’ll need to buy and change out your filters more often—as much as every week.

  • Colder in the winter: Using the ceiling fan comparison again: You know how turning on the ceiling fan in cold weather makes you feel chilly, even when the room is warm and comfortable? This is caused by the wind chill effect, where the fan redistributes the air in the room so that the colder and warmer air mix. You’ll experience the same thing when you leave your heater’s fan running all the time.

Father playing with children on couch
Maskot/DigitalVision via Getty Images

Switching the Fan to "Auto"

What Happens

When you switch the fan’s setting to “auto,” you’re asking it only to run when your air conditioner or heater is cycling. This means the fan will kick on 30 to 60 seconds after a heating or cooling cycle starts and then deactivate within a minute of the cycle’s end. When you put your hand up to a vent, you’ll only feel airflow during a system cycle; all other times, there will be no air movement at all.


  • Uses less energy: Your fan actually won’t use any energy when it’s not in use. If you’re on a budget, using the “auto” setting is an easy way to keep energy costs down.

  • Air filters last longer: Because air isn’t constantly going through the air filter, your filters won’t clog nearly as often. This means you can probably maintain a monthly or every-other-month changing schedule.


  • No air circulation between cycles: There’s no way around it—the auto setting may rob you of a bit of comfort. When the fan isn’t running, nothing distributes air through your space, which can make the air feel stuffy and stagnant.

  • Fan wears out faster: The constant on and off that comes with your fan’s “auto” setting can cause premature wear, which means you might need to summon a local air conditioning or heating technician sooner than you would if it was constantly in motion.

  • Less air filtration: The fact that air isn’t constantly passing through your filter and causing clogs might seem like a benefit to some, but the truth is, dirt that’s not in your air filter is floating around your house. If dust causes your discomfort in the form of coughing, itchy eyes, and sneezing, less filtration is definitely not something you want.

Other Options

Depending on the age of your cooling and heating system, you may have some alternatives to simply “on” or “auto.”

Variable-Speed Air Handler

Newer air conditioners and heaters have variable speed motors that change the speed at which your fan runs to offer you better control over the temperature in your home. You can think of these newer air handlers giving you the benefit of the “auto” setting with much less energy usage than older systems.

Circulate Option with Smart Thermostats

Some newer thermostats offer a fan setting called “circulate.” This setting is a hybrid between “on” and “auto.” It runs the fan during system cycles and then again for a few minutes every hour. You’ll enjoy more and better air circulation, minus the power bill that comes with the “on” setting. Still stuck with an old thermostat? Smart thermostats are affordable, and they allow you to control your heating and cooling system’s settings from virtually anywhere.

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