Why Does My Furnace Keep Turning On and Off?

A normal furnace that’s properly sized for a home should turn off and on three to eight times per hour. Furnace cycling might occur more often if it’s cold outside or if a home needs more insulation. When a furnace frequently turns off and on, HVAC experts refer to the occurrence as “short cycling.” In general, short cycling is related to a furnace’s safety features—the furnace turns off because it thinks conditions are too unsafe to operate. Allowing the problem to persist could cause the vital safety features to fail, an increase in your power bills, and lead to a permanent break-down. Some basic furnace troubleshooting will help you determine if the issue is something you can fix or something that requires expert attention.

Why is My Furnace Turning On and Off: Common Problems and Solutions

Problem: Dirty Air Filter

Solution: Replace the air filter. 

Dirty air filters are a common cause of furnace cycling problems. Air filters trap dust, dirt and debris. With time, the filter becomes clogged and restricts airflow to the equipment’s heat exchanger. When this happens, the heat exchanger overheats, forcing the high-limit switch to trip and the furnace to cycle off. Replace the air filter every 30 days to prevent this problem in the future.

Problem: Air Supply Vents Closed

Solution: Open all the air supply vents in your home.

Closing the air vents in your home does not help save energy. Instead, it forces your furnace to work harder than necessary because it slows down the blower and reduces airflow to the unit.

Problem: Thermostat Issues

Solution: Fix or replace the thermostat or change its location.

Thermostats can start malfunctioning if the batteries are low. If changing the batteries doesn’t help and the thermostat is older, you may need to replace it.

A thermostat’s placement can also cause frequent furnace cycling if it’s in direct sunlight, by a heat register, or near another heat source. A licensed HVAC can advise on the best places to install a new thermostat.

Limit Switch Troubleshooting

In a gas furnace, a high-limit, switch turns the unit off if air doesn’t ventilate from the exhaust flue properly. When the switch is faulty, combusted gases and heat accumulate in the unit. This impacts the unit’s performance and can lead to a dangerous situation.

A high-limit switch will turn a furnace off if it becomes too hot or is obstructed. Changing the furnace’s air filter and cleaning the blower motor may help the switch perform better.

When to Call a HVAC Professional

• Furnace is too large for your home: Bigger isn’t always better. When a furnace is too large for your house, it will heat your home quickly, shut down, and repeat this cycle. The cycling causes unnecessary wear and wastes energy. The only thing you can do about an improperly-sized furnace is to hire a HVAC specialist to evaluate your home and install one that’s the correct size. If the equipment is newer, a labor warranty might cover these costs.
• Dirty sensor rod: Gas furnaces have a sensor rod that detects flames in the unit’s burners. If it doesn’t detect a flame, it shuts off the gas valve, causing the furnace to turn off. When a sensor rod is covered in soot, it might have trouble detecting the flame.
• Dirty blower wheel: Without proper maintenance, a blower wheel accumulates dust, which slows it down. When this occurs, it limits the amount of air that flows over a furnace’s heat exchanger.

DIY furnace troubleshooting can give you some good insight into your furnace’s cycling issues. If you aren’t able to resolve the problem on your own, call a HVAC expert soon to avoid the risk of your furnace shutting down during the coldest times of the year. Contact AAA Heating and Cooling today to schedule a free consultation.