Why Does My Gas Furnace Fan Continue to Run Even After I Turn Off the Thermostat?
One of the most common reasons for a gas furnace fan to continue running even after the thermostat is turned off is due to incorrect thermostat settings. If the thermostat fan setting is set to “ON” instead of “AUTO,” the fan will run continuously regardless of the heating cycle. The fan limit switch is another critical component that can cause the fan to run continuously. If the fan limit switch is set to “manual override” or is faulty, it can prevent the fan from turning off. Wiring issues can also cause the furnace fan to run continuously, such as short-circuits, crossed wires, or faulty connections between the thermostat and the furnace.
What Are the Common Reasons Why a Gas Furnace Fan Won’t Shut Off?
A malfunctioning fan limit switch is a common reason for a furnace fan to run continuously. This switch is responsible for turning the blower on and off at the appropriate times. If it is stuck in the “ON” or “manual override” position, or if it is faulty, the fan will not turn off. A stuck relay or fan switch can also cause the fan to run continuously. When the relay switch wears out, it may not automatically turn off the fan motor. Problems with the furnace’s control board can also lead to a continuously running fan, such as a constantly supplied 24-volt signal to the fan terminal (G terminal) without the thermostat calling for it.
How to Troubleshoot and Fix a Gas Furnace Fan That Won’t Turn Off?
-
Check Thermostat Settings: Ensure the thermostat fan setting is on “AUTO” and not “ON”. Adjust the thermostat temperature to a lower setting than the current room temperature and wait 3-4 minutes to see if the fan turns off.
-
Check the Fan Limit Switch: Locate the fan limit switch inside the furnace panel cover, usually near the upper right corner. Ensure the white button on the lower left corner of the switch is not pressed in, indicating manual override mode. If it is, reset it to “AUTO”. If the switch is faulty, it may need to be replaced by a professional.
-
Check for Wiring Issues: Inspect the wiring connections between the thermostat and the furnace for any signs of damage, short-circuits, or crossed wires. Ensure that no wires are touching where they shouldn’t, which could cause the system to short-circuit.
-
Check the Air Filter: A dirty or clogged air filter can cause the furnace to run continuously. Clean or replace the air filter as recommended by the manufacturer.
-
Check for Stuck Relay or Fan Switch: If the fan motor continues to run after the burners turn off, try gently rapping the fan switch to see if it will turn off the fan. If it does, the switch likely needs to be replaced.
Necessary Tools and Safety Precautions
- Tools: Screwdrivers, wire testers, and possibly a replacement fan limit switch or other parts.
- Safety Precautions: Always turn off the power supply to the furnace before attempting any repairs. Use caution when working with electrical components to avoid injury or further damage.
Potential Costs and Timeframes
- Costs: Replacement parts such as a new fan limit switch or control board can vary in cost but are generally affordable. Professional labor costs will depend on the technician’s rates and the complexity of the repair.
- Timeframes: Simple adjustments like changing thermostat settings or resetting the fan limit switch can be done quickly. However, more complex repairs involving wiring issues or replacing components may take several hours and require professional assistance.
