Short answer
Replace a clogged filter first. It is the most common cause: the furnace overheats and a safety switch shuts it off. Open every vent and move furniture away from vents and return grilles. Check that the thermostat is not near a vent, a lamp or a sunny window, and give it fresh batteries. If it still runs only a few minutes at a time, call a technician.
Safety first
If you smell gas, do not touch switches: get out and call 911 or your gas company from outside. Do not open the burner compartment or clean parts inside the furnace yourself.
How to fix a furnace that short cycles
1On, off, on again every few minutes
The furnace lights, runs a few minutes and stops, over and over. The house never gets quite warm and the parts wear out faster.
Read all the steps
- On, off, on again every few minutes. The furnace lights, runs a few minutes and stops, over and over. The house never gets quite warm and the parts wear out faster.
- Replace a clogged filter. Without enough air, the furnace overheats and a safety switch shuts the burners off. A new filter of the same size fixes many cases.
- Open every vent all the way. Open the vents in every room. Move furniture, rugs and laundry away from vents and return grilles so air flows freely.
- Check the thermostat spot and batteries. A vent, a lamp or a sunny window next to the thermostat fools it into shutting off early. Put in fresh batteries while you are there.
- Longer, steadier cycles. It should run longer and shut off once the room is warm. Still on and off every few minutes? Call a technician.
- Still not fixed? Call an HVAC contractor Especially if: it still runs only a few minutes at a time with a new filter and open vents.
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Why it happens
Too little airflow is the most common cause. Next comes a dirty flame sensor: the burners light, then shut off within seconds. A blocked vent pipe or condensate drain on a high-efficiency furnace, or a furnace too big for the house, can also do it. A technician checks and cleans those parts.
What not to do
- Do not close vents in rooms you do not use. It cuts the airflow and makes short cycling worse.
- Do not clean the flame sensor or open the burner compartment yourself.
- Do not keep resetting the furnace if it shuts down with a blinking code.
When to call an HVAC technician
- It still runs only a few minutes at a time with a new filter and open vents.
- The burners light and go out within seconds.
- The status light blinks a code, or it smells hot or burnt.
- The furnace is new and has short cycled since day one.
Frequently asked questions
Why does my furnace turn on and off every few minutes?
Most often a clogged filter makes it overheat and shut off. Closed vents, a thermostat in a warm spot, a dirty flame sensor or a blocked vent pipe come next.
How long should a furnace run per cycle?
It depends on the weather and the house. In cold weather, a furnace that runs just a few minutes, shuts off and starts again soon after is short cycling.
Can a dirty filter cause short cycling?
Yes, it is the most common cause. The furnace overheats, a safety switch turns the burners off, and it starts again once it cools down.
Is furnace short cycling dangerous?
It is usually not an emergency, but it wears out parts and wastes fuel. If it comes with a burning smell or a blinking code, turn the furnace off and call a technician.
Can a thermostat cause short cycling?
Yes. A thermostat near a vent, a lamp or a sunny window warms up fast and shuts the furnace off early. Weak batteries can do it too.