AC FAN Set To AUTO Or ON – What Is Best?

Should the AC FAN be set to “AUTO” or “ON” when running the central air conditioner? I am trying to set the thermostat to save money. The fan blows air into all rooms when I have the FAN set to “ON”. What is the best setting to save money and cool down my home at the same time? Please explain the difference between FAN “AUTO” vs FAN “ON” when setting a thermostat.


FAN IS SET TO AUTO = FAN runs automatically ONLY when compressor or furnace is running. (Recommended)

FAN IS SET TO ON = FAN runs AT ALL TIMES regardless if compressor or furnace is running. (Not Recommended)

Setting the FAN to “ON” is not needed to run the AC or HEAT. When the thermostat is set to “ON”, the FAN will run at all times. This is not needed to run the AC or HEAT. Keep the thermostat set to “AUTO” FAN so it is not constantly running. Save electricity and keep the FAN setting at “AUTO”.

Thermostat Set FAN To AUTO Or ONThermostatSet FAN To AUTO for energy savings

What does the FAN setting do?

The FAN button on the thermostat sets the blower fan. This FAN blows the air through the air ducts and into the rooms of your house. The AUTO setting sets the FAN to blow only when the unit is cooling or heating the air.

AUTO – In AUTO mode the fan runs ONLY when the heating or cooling system is on.
ON – If set to CIRCULATE or ON the fan runs continuously even if the heating or cooling system is off.

Setting the thermostat to FAN ON will simply move the air, not cool the air.

The AC unit does 2 things:
1. Moves the air – The blower FAN moves the air
2. Cools the air – The compressor cools the air

The BLOWER FAN has a switch on the thermostat that has 2 positions. AUTO and ON. If this is set to AUTO, the BLOWER FAN only operates when the furnace makes HEAT or the AC is cooling. If the thermostat is set to ON the BLOWER FAN runs continuously. When the thermostat is set to FAN ON and the unit is not cooling, you are simply MOVING the air, not cooling.

Here is why you should not use the FAN ON setting

1. Higher humidity
The AC can’t dehumidify the air as well when the FAN is ON.
(Your central air conditioner cools and dehumidifies your home. When you set the thermostat fan to “ON” you are making it more difficult for the AC to remove moisture from your home)

2. Electricity cost
The energy use will be higher when the FAN is constantly running.

3. Air duct leaks
Running the FAN constantly pulls out the air conditioned air.

What turns on when I run the AC?

  • AC compressor runs
  • Condenser fan runs
  • Blower FAN runs (Fan that is controlled with AUTO or ON)
  • Electric filtration system (Only in certain units)

What component circulates the air in my AC?

When the AC is running, the BLOWER FAN is what circulates the air from the AC through the ductwork to each room. This same BLOWER FAN is the one on the thermostat that says FAN ON or AUTO.

When to turn the FAN to “ON” on a thermostat?

You may have times where you want to run the FAN by setting the thermostat to FAN “ON” if trying to remove smells or need to circulate the air in your home. If you live in a very dry climate such as the southwest United States, running the FAN for 5 to 10 minutes once the compressor turns off can additionally cool your home.

Here is a common Programmable Honeywell Thermostat PDF Manual for reference.

Proper Use Guidelines for Programmable Thermostats from Energy Star (Gov).

Have some helpful information that you can share regarding air conditioner tips? Please leave your comment below to share with everyone.

1 thought on “AC FAN Set To AUTO Or ON – What Is Best?”

  1. Running your AC in the auto position is definitely the best way to go for the many reasons already mentioned.

    However, if you have floor vents you can make your AC work even better in the “auto” position by additionally doing a few simple things.

    (1) Close your basement vents and doors (which are usually cool anyway)
    (2) Use a new but very inexpensive product called the Ecoflap vent cover on your upstairs floor vents which helps keep AC air upstairs. The Ecoflap works by automatically closing the floor vent it’s attached to when the AC and/or furnace blower fan shuts off. This prevents AC air in the room from back flowing down through the floor vent and into the basement which is where it goes because cool AC air is heavier than ambient air.
    (3) Putting plastic seal strips on your basement door bottom(s) and perhaps other doors in the house will also help.
    (4) Running ceiling fans upstairs on the low setting and in the counterclockwise direction will also help and will use much less energy (about 25 watts) than the system blower fan which uses about 500 watts.

    And perhaps best of all is that if you do the foregoing your AC unit won’t kick on as often which will save you even more since the AC unit itself uses about 3500 watts.

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