4 Reasons Why Your AC Fan Won’t Turn Off

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Air conditioning (AC) units are an essential part of keeping a home comfortable and well ventilated. Not only do they help keep the interior of a home at the desired temperature, but they help prevent the air from becoming too stagnant or stuffy. The AC fan keeps the air moving and circulating throughout the home.

As great as AC units are, they consume a lot of energy to run, and they often make up a large portion of a household’s utility bill, especially during hotter months. 

We don’t want the AC to be running all the time because it can get costly very quickly, make the air too cold, or even do long-term damage to the AC unit. If you find that your AC fan is running continuously and won’t turn off, something is most likely wrong. 

Before you determine the problem and any potential solutions, it’s helpful to understand how an AC unit functions and where the AC fan fits into the picture.

What Does the AC Fan Do?

To control the temperature and air quality in a home, an AC unit must constantly move large volumes of air.

Air is first drawn from the home’s interior into the AC unit through a large vent, passed through a filter to collect dust and particles, and is cooled by refrigerated coils. The cooled and conditioned air is then sent back into the home through the ventilation system out of smaller vents in each room.

A strong AC fan, also called the blower, is required to keep the air circulating through all the stages of this process, and its function is coordinated with the rest of the AC unit.

How the AC Fan Works

The AC fan must be blowing while the AC unit is running, or air will not circulate. The unit’s electronic controls will turn on the fan at the appropriate time based on the settings and ambient temperature.

When the thermostat reaches the user-set temperature in the AC settings, the refrigeration unit (including the condenser outside the home) activates. The control system sends a signal to a relay switch that turns on the AC fan. The relay switch is what allows power to reach the fan.

After cooling the air in the home, once the thermostat reaches the user’s maximum temperature, the AC unit will stop, and a signal will be sent once again to the AC fan relay to deactivate the fan.

However, in some cases, the user may wish to circulate air in the home without the cooling function. In this case, there is often a “fan” or “fan-only” setting on the AC controls.

Why does the Fan Keep Running On My Air Conditioner?

It’s usually easy to tell when an AC unit is running – air can be felt coming from the vents, and the sound of the AC fan moving air through the ventilation system is distinctive. 

If you hear the fan running when you want or expect the AC unit to be turned off, there are a few different reasons why this could be the case.

1. Fan Only Switch

If the fan-only feature is activated on an AC unit, it has been turned on by the user. Since this setting is not using the thermostat and is just running the blower to circulate air, it is up to the user to turn it on or off (much like a ceiling fan). 

If you can hear the fan running and you can feel air coming out of the vent, but the air is not cooled, that could mean that the fan-only switch is on. It is common for users to accidentally turn on that feature to realize that it must be turned off manually.

2. Faulty Fan Relay

If the fan continues to run when the AC unit is in “automatic” mode (not fan-only mode), then there may be a problem with the relay switch that controls power to the blower.

This could be due to a problem with the wiring between the thermostat and relay or a problem with the relay switch itself. Some relays can get stuck in the position because of humidity or corrosion.

3. Thermostat Settings

Once the temperature drops below the user’s target, the AC unit and fan should both stop automatically.

If you hear the AC fan running continuously, it could be that the home is unable to reach the desired temperature. If the user sets the thermostat temperature very low on a very hot day, for example, it could take the home many hours (if ever) to cool – especially in larger homes.

Also, while some thermostat controls are wired into house power, others run on batteries. If the control batteries fail while the AC is in use, the unit may continue running.

4. Air Leakage and Insulation

One often-overlooked factor that can affect AC units’ performance is the proper insulation and the sealing of windows and doors. If the AC unit is constantly working against the outside temperature, it can have difficulty reaching the user’s turn-off temperature.

Well-insulated homes with good windows and proper weather stripping on doors help AC units run efficiently. When warm air from the outside leaks in (or cool air leaks out), the unit and fan will continue running longer. This is also why doors and windows should be closed when an AC is operating.

What Do you Do if your AC Fan Won’t Turn Off?

If the AC fan is running continuously, look for the easiest answers first, then work your way up to the harder ones.

  • If the fan is running without cooling, check the mode setting on the AC thermostat control to see if it’s set to “fan” or “fan-only.” Switching off the unit or setting it back to automatic mode should solve the problem. Note that if you’re switching to auto, you may have to wait until the thermostat temperature is reached before the unit turns off.
  • If the fan continues to run alone, there could be a problem with the relay switch or relay wiring. Solutions include rewiring, cleaning the relay contacts, or replacing the relay switch. Wiring and relay replacement involves electrical work that requires great care and should be done by experienced people only.
  • If the AC unit is on automatic mode and constantly running, check the thermostat’s batteries if it uses them. If that’s not the problem, check the thermostat to ensure that the temperature isn’t set too low and that air isn’t escaping the home through doors, windows, or other openings.

AC Fan Maintenance Tips

General AC maintenance like the periodic cleaning of vents, filter replacement, and checking the thermostat batteries is recommended to keep everything running well. 

To prevent long-term problems with the fan relay switch, it may be necessary to occasionally clean the relay contacts, which requires experience with electrical work.

Summary

A continuously running AC unit or AC fan can quickly become an expensive maintenance headache if not addressed right away. Many solutions to this problem involve simple settings changes or battery replacement. More serious issues may be due to faulty equipment that could require professional help to resolve.