Post Author's Photographer By Paul Cutting July 13, 2026

Take a look at your AC thermostat and you’ll notice a toggle switch labeled “Fan” with two choices: On or Auto. Which do you choose to keep cool?

It’s a commonly debated setting with potentially big impact on how comfortable your home feels during the hot, humid summer.

Air movement is good… right?

Some prefer to leave the setting to On, allowing the fan to run continuously in the background, filtering the air and reducing temperature stratification, while others like to let the fan cycle in the background with the Auto setting. While it seems like a minor preference, this setting has a real impact on your indoor humidity levels and overall summer comfort.

Setting your thermostat fan to On means the indoor blower motor runs 24×7, completely independent of whether the air conditioner is actively cooling the house or not. Proponents of this method point to continuous air circulation, which can eliminate stagnant air and ensure air is continually passing across the system’s filtration. And if your system has a modern, variable-speed blower motor, doing this uses very little electricity and adds a nominal amount to your monthly electric bill. 

The hidden drawback to the On setting isn’t the electric bill, but how it handles moisture.

When your air conditioner is actively cooling, hot and humid indoor air runs across the indoor coil, causing water vapor to condense on the metal coils. From here that water drips down to a pan below the coil and is either allowed to flow to a basement drain or pumped out using a condensate pump. But if the fan stays running after the cooling cycle ends, warm, indoor air is blown across the wet coils, causing that collected moisture to evaporate right back into your living space, making things feel damp and clammy.

Auto wins moisture control

Making sure the fan switch is toggled to Auto instructs the fan to run only when the system is actively cooling. Once the inside reaches the target temperature, both the outdoor condensing unit and indoor fan shut off. This pause gives the moisture collected on the indoor coil the opportunity to drain out, rather than having that moisture be evaporated back into the living space. 

Keeping the thermostat set to Auto during the summer cooling season is the clear winner for moisture control. If you find yourself wanting air movement on the hottest of days, the better approach is to leave the fan setting to Auto and use ceiling fans to circulate air.

Reverse logic in winter

Interestingly, the rule flips when the weather turns cold. Leaving the fan set to On during the winter heating season can be a great way to reduce temperature stratification caused by hot conditioned air rising to the higher reaches of your ceiling, and can be effective in making your floors and feel warmer.

Questions about home comfort or home energy? The Winneshiek Energy District offers personalized energy coaching to Winneshiek County residents – a free consultation thanks to support from Winneshiek Energy District supporters! Learn more & sign up for energy coaching.


Winneshiek Energy District Energy Planner Paul joins Winneshiek Energy District with a background in teaching and carpentry. For the last seven years Paul was a high school industrial tech teacher in Lanesboro, MN. There he taught carpentry, cabinetmaking, electricity, engineering design, graphic design, CAD, entrepreneurship, and more generally, enjoyed helping youngsters grow into capable and confident young adults. Prior to teaching, Paul spent several years as a carpenter disassembling and reconstructing 19th century Norwegian-built log buildings for clients. He also worked several seasons for HistoriCorps, a non-profit that pairs volunteers with hands-on historic preservation projects throughout the mountain west. In his free time Paul enjoys making furniture, trail running with his dog Bobo, scheming backpacking trips out west, camping in his homemade teardrop camper, and rehabbing his 170 year old brick house.

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