4 Possible Reasons Why Your HVAC Fan Won’t Shut Off
The HVAC blower, more commonly known as the HVAC fan, is what draws air into your furnace and AC system and circulates the hot and cold air from these units throughout your home. If everything is working correctly, the fan should only ever run when your AC or furnace is running. However, there are some situations where the fan may fail to shut off and continue to run even after the AC or furnace does. Let’s discuss the reasons why an HVAC fan won’t shut off and what can be done to fix this issue.
1. Incorrect Thermostat Setting
The most common reason that an HVAC fan will continue to run after your furnace or AC shuts off is because the thermostat isn’t set correctly. The fan setting on your thermostat should generally always be set to “Auto” and not “On,” and this is the very first thing to check if you ever find that your fan doesn’t shut off.
If the thermostat is set to “On,” it means that the fan will run constantly. If you leave the fan set to “On” during the summer, it will quickly start to blow hot air out of your vents once your AC shuts down. The opposite is true during the winter as the fan will begin blowing cold air out of your vents after your furnace shuts off.
2. Faulty Thermostat or Wiring
If the thermostat is set correctly, your fan should only ever run when the thermostat calls for heat or AC. This happens whenever the thermostat registers that the indoor temperature is above or below what the thermostat is set to.
If the fan (or your heating or AC) ever runs when the thermostat shouldn’t be calling for heat or AC, it could be that the thermostat is faulty and signaling the system to run when it isn’t needed. This can also happen due to a short caused by loose or damaged wiring between the thermostat and the rest of the HVAC system. In this case, the solution is to have the thermostat tested and either replaced or repaired if it’s faulty.
3. Malfunctioning Fan Relay Switch
The fan relay switch is what signals the fan to turn on and shut off. The relay switch can sometimes fail or malfunction, and this can prevent the fan from receiving the signal to shut off. If this happens, the only way to shut the fan down is to flip your HVAC circuit breaker to shut the electricity off to the system. You will then need to have a technician check the fan relay switch and replace it if it’s broken.
4. A Triggered Safety Switch Causing the Furnace to Shut Down
Any of the above issues can occur when either your AC or heater is running, but sometimes the issue is related to the furnace itself. All modern furnaces have a number of different safety switches that are designed to automatically shut down the unit in certain situations to prevent both damage to the furnace and potential safety hazards.
For instance, the furnace’s pressure switch detects if the inducer fan is running to ensure that carbon monoxide and other combustion fumes are being properly vented out of the unit. There is also a limit switch that senses the temperature inside the furnace and will trigger if the unit starts overheating. Many furnaces also have a flame rollout switch that will trigger if the flames start to escape or “roll out” of the combustion chamber.
Whenever the furnace is running, its control board sends an electrical current through the unit’s safety circuit. When everything is working properly, all of these safety switches remain closed so that the current travels through the circuit and back to the control board. When any of the safety switches is triggered, it will open so that the circuit is broken and the current can’t travel back to the control board. As soon as this happens, the control board will then signal the furnace to immediately shut down.
If any of the safety switches trigger, the blower fan may still continue to run even though the furnace shuts off, and it may continue to do so until the switch is reset. Depending on which switch was triggered, it might reset automatically after some time, or it might need to be reset manually. If your fan continues to run and your furnace doesn’t come back on within an hour or so, you will need to have it inspected to determine if it needs to be reset or if there is some other issue causing the fan to fail to shut off.
If your HVAC fan won’t shut off or you’re having any other issues with your heating or cooling equipment, Totally Cool Heating & Air is here to help. We specialize in all types of residential and commercial HVAC repairs as well as maintenance and installation, and we also offer duct sealing, attic insulation, and indoor air quality services. Give us a call today if you need to schedule any HVAC service in the Austin area.