When your furnace short cycles, it turns on and backs off quickly. This can happen several times in an hour, putting a lot of stress on your furnace. Not only does this shorten the lifespan of your furnace, but it can also lead to costly repairs. 

If you notice your furnace is short cycling, you’ll want to know how to fix the problem. This can happen for several reasons, such as:

Blocked Flue Pipe

The flue pipe is located on the roof, or out the side of your home. It is responsible for carrying hot gasses away from the furnace. If the flue pipe becomes blocked, the hot gasses will build up inside the furnace and cause it to overheat. Newer furnaces have a safety feature that will cause it to shut down if these become blocked or obstructed.

Clogged Air Vents

If your furnace is overheating, there are a few potential causes. First, check to see if all of your vents are open. If any are closed or blocked, this can restrict airflow and cause your furnace to overheat. Make sure all registers are open to maintain system balance and performance.

Defective Blower Motor

If your furnace’s blower motor isn’t working, it can be a real problem. Not only will your furnace not be able to circulate the air properly, but it could also lead to other issues like your furnace overheating. Call in professionals to diagnose the problem and make the necessary repairs.

Dirty Air Filter

The HVAC filter is essential for keeping the furnace clean and functional. If the filter becomes clogged with dust and pollutants, it can reduce the airflow and cause the furnace to shut off. To avoid this, replace the filter every 30 to 90 days.

Dusty Blower Wheel

The furnace blower is responsible for moving air through the ductwork. If the blades on the blower are covered in dirt, the air won’t flow correctly, causing the furnace to overheat and switch on and off more frequently. 

To prevent the blower wheel from getting dirty, it’s crucial to replace the filter regularly. If the blades are already dirty, you’ll need to schedule a furnace maintenance appointment to have them cleaned.

Faulty Thermostat

Check your thermostat to see if it is in heating mode, and set it to the temperature you want. If the thermostat is in direct sunlight or near a heat source, it may give an incorrect temperature reading. Try changing the batteries in the thermostat. If you use a smart thermostat, visit the manufacturer’s website for troubleshooting instructions.

Oversized Furnace

If your home’s comfort equipment is too large, it will heat your home too quickly and create uncomfortable temperature changes. The only way to fix this problem is to install a new furnace.

Sooty Flame Sensor

The flame sensor is like a switch that turns off the gas valve when it doesn’t sense a flame. If the sensor gets dirty, it can’t feel the flame and the furnace will turn off. HVAC experts can help you clean the sensor and repair your furnace.


There are many reasons why your furnace may switch off unexpectedly. If your furnace keeps switching off, you should contact a qualified HVAC technician for help.

Do you need furnace maintenance in Lethbridge? Trust 4 Seasons Home Comfort. We specialize not only in installing and servicing heating, air conditioning, and ventilation systems but also offer stainless steel work and custom fabrication. Contact us!