Why Won’t My Furnace Turn Off?

If your furnace won’t turn off or is running longer-than-usual cycles, it can lead to high energy bills, comfort issues, and potential damage to your system. HVAC.com explains the most common causes of a furnace that won’t turn off and offers solutions to help you troubleshoot the problem.

Common Causes of a Furnace That Won’t Shut Off

Wrong Thermostat Setting

The first place to stop when your furnace won’t turn off is the thermostat. Sometimes, simply adjusting the settings can solve the problem. It’s possible that someone accidentally raised the thermostat setting significantly above your desired temperature, leading to the impression that the system won’t turn off.

Solution: Lower the temperature setting below the current level and check if the system shuts down.

Malfunctioning Thermostat

A malfunctioning thermostat can stem from loose wiring, a broken switch, or a faulty sensor, resulting in the furnace not shutting off.

Solution: If you suspect a wiring issue, turn off the power to the furnace at the circuit breaker. You can remove the thermostat’s cover with a screwdriver and tighten any loose wiring connections. If you are uncomfortable doing this on your own, contact an HVAC technician. They can pinpoint the exact problem and repair or replace the thermostat. Thermostats typically last about 10 years.

Thermostat Is Set to ON, Not AUTO

The ON button on your thermostat keeps the HVAC fan running constantly, even when the furnace isn’t heating. While this helps with air circulation and filtration, it’s better to set the thermostat to AUTO. In AUTO mode, the fan only runs when the furnace is heating, which saves energy and improves efficiency.

Solution: To avoid unnecessary energy use and wear on your system, switch the setting back to AUTO.

Dirty Air Filter

A clogged air filter can greatly reduce airflow, making your HVAC system less efficient. When airflow is restricted, the system may take longer to reach the desired temperature, resulting in extended cycles which makes it seem like the furnace won’t turn off.

Solution: To maintain optimal performance, inspect your air filter monthly and replace it when it looks dirty, usually every 2-3 months. Regular maintenance can help prevent unnecessary strain on your system.

Air Duct Leaks

Air duct leaks can reduce airflow and heating efficiency. If hot air isn’t circulating properly, the furnace may run longer to compensate, creating the impression that it won’t turn off.

Solution: Inspect your ductwork for any leaks or obstructions. While a visual inspection can help, hiring a professional ensures a thorough evaluation. They can also use the appropriate tools to seal up any air leaks to ensure proper airflow.

Faulty Limit Switch

The limit switch is designed to turn off the furnace when it reaches a certain temperature. If it malfunctions, it may fail to recognize that the desired temperature has been met, leading to continuous operation.

Solution: Contact a professional HVAC technician to replace the faulty limit switch.

Stuck Relay

A relay is an electrical switch that controls the power supply to your furnace. If it becomes stuck in the closed position, it will keep sending power to the furnace, causing it to stay on indefinitely.

Solution: To address a stuck relay, you may need to reset the furnace by turning it off at the circuit breaker for a few minutes. If the issue persists, consult a technician for further evaluation and potential replacement of the relay.

Control Board Problems

The control board acts as the brain of your furnace. If it malfunctions, it may send incorrect signals, resulting in the furnace running longer than necessary.

Solution: Inspect the control board for visible signs of damage or wear. If you suspect an electrical issue, consult with a qualified technician for diagnosis and repair.