Furnace Short Cycling: Why It Keeps Turning On and Off

A furnace is short cycling when it runs for only a minute or two, shuts off before the house is warm, and then starts again soon after. Besides leaving rooms cold, it wastes gas, wears out the igniter and blower motor, and can be a sign that the furnace is overheating. Most short cycling has a simple cause you can check yourself in under an hour, and the most common ones are worth ruling out before you book a service call.

A clogged furnace filter is the number-one cause: the furnace overheats, its high-limit safety switch trips, and the burners shut off. A dirty flame sensor makes the burners go out a few seconds after they light. On high-efficiency furnaces, the white PVC intake and exhaust pipes outside the house can be blocked by snow, ice or a bird's nest, and a clogged condensate drain can trip the pressure switch. A thermostat mounted near a heat register, a lamp, a sunny window or a drafty door gets a false reading. Closed or blocked vents and return grilles also make the furnace overheat. Less often, the furnace is simply too big for the house, which is common after a replacement that was not properly sized.

A flashlight, a replacement furnace filter (CAD $8–$18 for a basic 1-inch filter), fresh thermostat batteries, a soft brush or broom for clearing snow from the vent pipes, and the tools in our flame sensor guide if you get that far. No special tools are needed for the first checks.

1. Check the furnace filter and replace it if it is grey or you cannot see light through it.
2. Look at the thermostat: replace the batteries, make sure it is set to HEAT, and check that nothing near it (a register, lamp, TV or sunny window) is warming it.
3. Walk through the house and open every supply vent, and make sure furniture or rugs are not covering any return air grilles.
4. Go outside and find the two white PVC pipes where a high-efficiency furnace vents through the wall or roof. Gently clear any snow, ice or debris from both openings and keep the area around them clear all winter.
5. Look through the small window on the furnace front panel at the diagnostic light, count the flashes, and compare them with the code chart on the inside of the door. Write the code down.
6. On a high-efficiency furnace, check the clear condensate drain tube and trap for blockages or standing water, and make sure the drain line runs freely to the floor drain or condensate pump.
7. If the burners light and then go out within a few seconds, clean the flame sensor (see our guide to cleaning a furnace flame sensor).
8. Restart the furnace and watch a full cycle. It should run steadily until the thermostat setting is reached.

If you smell gas, leave the house right away and call your gas utility's emergency line from outside (Enbridge Gas in most of Ontario). Never tape over or disable a safety switch to keep the furnace running; a high-limit or pressure switch that keeps tripping is protecting you from overheating or exhaust problems. Make sure your carbon monoxide alarms work before and during heating season.

Call a licensed HVAC technician if short cycling continues after these checks, if the diagnostic light shows a high-limit, pressure switch or ignition code that keeps returning, if you notice a burning smell, or if your carbon monoxide alarm sounds. In Ontario, repairs to gas components must be done by a TSSA-registered gas technician. If the furnace is oversized, a technician can sometimes adjust its settings; otherwise sizing should be checked properly before the next replacement.