My Furnace Keeps Turning On and Off

If your furnace turns on, runs for a minute or two, and then shuts off again, it’s short cycling. You’re probably wondering why it’s doing that and how you can fix it.

The most common issues that cause a furnace to short cycle are:

• A dirty air filter
• A blocked flue pipe
• A thermostat problem
• An oversized furnace

A short cycling furnace will struggle to heat your home, increase your energy bill, and lead to more frequent repairs due to wear and tear. So, it’s best to take care of the problem quickly. Let’s take a closer look at each of these issues to help you find a solution.

A dirty air filter

A dirty air filter reduces the amount of cool air your furnace pulls in, which can make the heat exchanger overheat. (The heat exchanger is the part of your furnace that warms the air.) The furnace will shut itself off as a safety measure when the heat exchanger starts getting too hot.

Once the internal temperature inside that area of your furnace goes back down to a safe level, it will kick back on again. If the filter remains clogged with dirt, your furnace will continually short cycle and suffer from unnecessary wear and tear.

An easy way to solve this problem is to replace your air filter if it’s dirty. Check your filter every 30 to 60 days and replace it as needed. However, if your furnace is still turning on and off frequently, you may be dealing with a different problem even after changing your filter. An HVAC technician can diagnose the issue and recommend a solution.

A blocked flue pipe

The furnace flue pipe (also called an exhaust vent or vent pipe) vents the exhaust from your furnace to the outside of your home. If debris is blocking the pipe, hot gases could build up with no way to escape, and your furnace could overheat, triggering the automatic shut-off feature.

Not sure if your flue pipe is blocked? Turn off your furnace before heading outside to inspect it. Most likely, you’ll find the flue pipe on the side of your house near your furnace or on your roof. When you locate it, take a peek inside to see if anything is blocking the airflow.

If you can’t find your flue pipe or you discover a blockage, contact a York County HVAC technician for help.

A thermostat problem

Your thermostat communicates directly with your furnace, telling it when to start and stop. However, if it’s sending the wrong messages, it might tell your furnace to shut off sooner than it should.

For example, here are a few common issues that might cause your thermostat to miscommunicate and result in short-cycling:

• The thermostat is poorly placed: If someone originally installed your thermostat near a heat source, like a supply vent, it could skew the temperature reading. For example, it might detect the heat from your supply vents when the furnace comes on and think that your home has reached the desired temperature. As a result, it might tell the furnace to turn off. However, once the hot air is gone, the thermostat may think your home is too cold and tell the furnace to turn back on again. The cycle could continue endlessly.
• The thermostat is malfunctioning: Your thermostat may have a faulty temperature sensor or wiring, causing it to send the wrong messages to your furnace.

If you’re having issues with your thermostat, it’s best to contact an HVAC technician who can come out to your home and inspect it. They may recommend relocating it, repairing it, or replacing it entirely, depending on their findings.

An oversized furnace

Your furnace may be too big for your home. If so, it will heat your house very quickly and then shut off once it reaches the temperature you’ve set on your thermostat. An oversized furnace will complete this cycle over and over again, frequently turning on and off to keep your home comfortably warm.

This frequent on-and-off pattern is bad news for your equipment. It overworks your furnace, shortening the system’s lifespan. A short cycling furnace also uses much more energy than necessary, which increases your heating bills.

The best way to solve this problem is to contact an HVAC professional. They can perform a load calculation to determine the optimal size furnace for your home.