4 Reasons Your HVAC Fan Won’t Stop Running and What to When This Happens

May 9, 2023
AC unit in Nacogdoches, TX

If you have your thermostat set correctly, the HVAC blower or fan should only run when your heating or AC is running and then shut off when the rest of the system does. When the AC is on, the fan should turn off just after the AC condenser outside shuts down. If your heat is running, the fan will typically run for a few minutes after the furnace shuts off. Although rare, there are cases when the fan may continue to run after the rest of the HVAC system shuts off. When this happens, the only way you can usually get the fan to shut off is to flip the circuit breaker so that the HVAC system no longer has power. If you ever find yourself in this situation, here are the possible reasons that your HVAC fan won’t shut off and what can be done to fix them.

1. Wrong Thermostat Setting

If your fan won’t shut off, the first thing you should do is check the thermostat to make sure the fan is set to AUTO. On this setting, the fan will only run when your furnace or AC unit is on. If you have the fan set to ON instead, it will run continuously. You should then try to turn the HVAC system completely off at the thermostat to see if the fan continues to run. If the fan still runs when the HVAC system is shut off, you should flip the circuit breaker to shut off the power and then call to have your system inspected.

2. Faulty Thermostat

A faulty thermostat or a damaged thermostat wire can sometimes lead to the fan not shutting off when it should. These issues will normally affect the entire HVAC system and prevent it from turning on or shutting off. However, there are rare cases when only the fan will be affected. If a technician does determine the thermostat is the issue, you will need to have the wiring fixed or a new thermostat installed.

If the thermostat is faulty, it may not be reading the temperature correctly. This means that your HVAC system may continue to run for longer than it should. To check the thermostat, try turning the thermostat up or down by 5 to 10 degrees to see if the system shuts off. You will want to turn the thermostat up if your AC is running and down if your furnace is on.

If the fan and the furnace or AC now shut off after adjusting the temperature, you can be fairly certain that your thermostat is the cause of the issue. It could be that the thermostat simply needs to be recalibrated or that it is faulty and needs to be replaced. The solution may also be to move the fan to a location where it can more accurately gauge your home’s temperature. If the furnace or AC shut off but the fan still runs, the problem is likely related to a damaged thermostat wire or some issue with the control board or fan relay switch.

3. Malfunctioning Control Board

The control board is what receives the signal from the thermostat and then tells the fan and furnace or AC to turn on and off. If the control board is faulty or malfunctions, it may not send the signal to the fan to tell it to shut off. The same can also happen if the wiring between the control board and the fan is damaged or there is a loose electrical connection. Damaged wires or loose connections are a fairly simple fix, but the only solution to a faulty control board is to have it replaced.

4. Fan Relay Switch Stuck Closed

When the control board signals the HVAC fan to run, the fan relay switch closes. This completes the circuit so that electricity now flows and the fan turns on. When the control board signals the fan to turn off, the relay switch then opens and breaks the circuit so that no more power flows to the fan. The relay switch can sometimes fail or get stuck in the closed position, and this will make it so that the fan won’t ever stop running unless you shut off the main HVAC circuit breaker.

A stuck fan relay switch is the most common reason that a fan won’t stop running, and the first thing a technician will do in this situation is reset the switch. If resetting the switch doesn’t fix the problem, the switch will need to be replaced.

If your HVAC fan won’t shut off or your heating or AC has any other issues, the expert technicians at Nacogdoches Sheet Metal, Plumbing & Air Conditioning, LTD. are always here to help. We service and repair all types of heating and cooling units, and we also offer professional HVAC installation, plumbing and sheet metal fabrication services for customers throughout the Nacogdoches area. For more information or to schedule a service appointment, contact us today.