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Is “On” or “Auto” the Better Thermostat Setting?

Posted: November 14, 2016

touch screen thermostat

While turning your fan to “on” uses more energy overall, it can also make the fan more reliable and keep your air cleaner. (Photo by Summer Galyan)

Which thermostat setting costs more? An HVAC pro weighs in on the pros and cons.

Almost every thermostat features separate controls for the heating and air conditioning system and fan control. When you want the system to heat, you set it to heat. When you want it to cool, you set it to cool. For the few months a year you need neither, you set it to off.

But what about the fan? Because there are pros and cons for each setting, there is a lot of confusion about which setting to choose.

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Setting the fan to “on”

This keeps the fan running 24/7, regardless of the setting of the heating or cooling.


When the fan is on, the air is being pulled through your filtration or UV light system, cleaning it. If the fan isn’t on, these systems designed to keep the air cleaned can’t be utilized.

Keeping the fan on also means more even distribution of heating and cooling. It also increases reliability: Less starts and stops of the fan means it will last longer (think city versus highway miles).


Expensive! Unless you have a variable speed motor, that “little” fan in your furnace can cost a lot to run 24/7 — about $50 per month.

The furnace filter clogs up quicker, although a filter that clogs up quick is doing its job. You will have to adjust your filter change interval accordingly.

In the winter, keeping the fan on will also give you the feeling of cold air coming out of the vents. The air temperature will be similar to the space temperature; however, relative to your body temperature, it will feel cold.

Setting the fan on “auto”

The fan only runs when the furnace tells it to. During heating, the fan is controlled by either a timer circuit or a limit circuit in the unit. The fan will start about 40 seconds after the burners light and go off about one minute after the temperature is satisfied. On cooling, the fan will run only when the air conditioner is running.


The “auto” setting means less energy usage, because it keeps the fan running the least amount of time at the slowest speed needed for heatingand cooling.

Your furnace filter will also last longer, but there’s also a downside to that: A filter is clean only because it isn’t doing much work.


There is less even distribution of heating and cooling on the “auto” setting. Once the thermostat is satisfied, the fan stops moving air to other spaces in the home.

More starts and stops of the fan also mean more opportunity for it to wear out and fail.

Other options

Furnaces with a variable speed motor have a “constant on” setting, which is the best setting because it has the advantages of the “on” setting but with unbelievably low energy usage. However, you don’t have to invest in a new furnace to get benefits of both settings.

Most new thermostats have a “circulate” option that allows you to set the fan to run a certain amount of time per hour. This evens out hot and cold spots, uses your high-efficiency filtration or UV system more effectively, and keeps energy usage from going through the roof.

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