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Sunset Air Blog

5 Reasons your furnace turns on and off frequently

furnace picture

Does your furnace’s cycle on and off frequently? That’s what we call “short cycling.”  It can wear down your furnace, possibly damaging it.

So, what’s causing it?

There are different possible causes. We’ll walk you through each of them and what you need to do to fix them.

Dirty air filter

A dirty furnace filter can cause your furnace to overheat and shut down.

How?

The dirty filter decreases airflow over your furnace’s heat exchanger (the part that actually heats the air). With little air to exchange heat to, the heat exchanger overheats, causing a limit switch to shut the gas valve. No gas = no flames.

Later, the furnace will try to turn on again, overheat, then shut down again.

Solution: Change the air filter if it is dirty.

Dirty flame sensor

The flame sensor’s job is to confirm that, when the gas valve is open, a flame is actually present in the furnace.

If a flame isn’t there, the flame sensor shuts the gas valve to prevent dangerous levels of gas from building up in your home.

However, if the flame sensor gets coated in carbon or rust, it won’t register the flame. So it shuts the gas valve.

Again, no gas = no flames.

Later, the furnace tries to turn on again and quickly shuts down because of the flame sensor.

Solution: A professional needs to clean the flame sensor.

Improperly placed thermostat

Is your thermostat placed near an air vent or a window that has sunshine coming through?

If so, those heat sources can trick your thermostat into thinking that your home’s air is warmer than it actually is, shutting off the furnace prematurely.

Solution: Make sure your thermostat is located on an interior wall away from windows and vents.

Other issues

A furnace that starts and stops often may be due to:

  • Undersized air ducts (which can cause the furnace to overheat)
  • Oversized furnace (which can heat your home too quickly)

Both of these issues can cause the furnace to start and stop frequently. Both issues need a professional’s touch to fix.

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