When you turn on your furnace, you expect it to run for a full cycle and then turn off for at least the same amount of time it took to run the heat cycle.

If your furnace keeps turning off and back on quickly, you are experiencing something called short-cycling, which occurs when your thermostat turns off before it actually heats your home up and then turns back on shortly after. Short-cycling can be caused by various issues with your furnace.

Misfiring Flame Rod

One of the most common reasons that your furnace may be short-cycling is because of a bad flame sensor, also referred to as the flame rod. The job of the flame sensor is to determine if the gas valve in your pump has fired and is being lit up.

The flame sensor is designed to shut down your furnace if it determines that the flame isn't on and that gas is just being pumped into your home. It is an important safety feature.

When the flame rod starts to go out, it may work sometimes, resulting in your heating running properly, then short-cycling for a while, then running properly. Generally, that will then progress to your furnace just cycling on and off without producing any hot air.

A heating repair technician can change out the sensor rod and get your furnace working correctly again.

An Exhaust Vent That Is Blocked

Another reason your furnace may be short-cycling is that you have an exhaust vent that is blocked. Your furnace has a vent that exhausts outwards and is necessary for your furnace to properly function.

Most furnaces have a flue that allows the air to exhaust through the roof, although some furnaces have a flue that exhausts through the wall.

A variety of things can clog up your flue, such as a bird making a next on top of it or snow building up on the flue. You can try and find the exhaust vent on your own and see if it is clogged. Or a trained heating professional can find your exhaust vent and see if it is clogged and what action needs to be taken next.

If there are animals or insects are clogging your vent, your heating specialist may call in a pest control specialist to deal with the animals or insects if they don't feel like they can deal with the issue on their own.

If your furnace has suddenly started to short-cycle, you may have a misfiring flame rod or a clogged furnace exhaust vent. You can investigate the issue on your own, or you can just call your local heating technician and have them come in, figure out the cause of the issue, and take care of the repair for you