Four possible reasons your furnace keeps turning on and off

During the heating season, we typically receive some calls from our Edmond and Oklahoma City clients about their furnace turning on and off frequently. There are a number of reasons that may happen, and I’ll name four of the most common ones below.

Before deciding to contact your service provider and have a technician sent out to your home, there are two things you should consider.

First, it’s okay for your heater to cycle on and off when it’s operating as it should. Depending on how cold it is outside and how warm you want it to be inside, it’s acceptable and normal for your furnace to cycle on and off three to eight times per hour. If that’s the case with yours, you probably have nothing to worry about. But cycling faster than that could indicate an issue.

Second, think about the age of your furnace. If you have a newer furnace, it likely came with a long list of safety features to prevent unsafe operation or damage to system components. Your furnace may be shutting down because of one of those safety features. While that may still be annoying and uncomfortable, you can breathe easy knowing your furnace isn’t operating under dangerous conditions.

What you want to keep an eye out for is what we in the industry refer to as short-cycling. That means your system does start up and run, typically for 30-60 seconds, before something tells your system to shut down. That “something” will usually be one of two sensors in your furnace:

High limit switch

This switch (also called a limit switch) kills power to the furnace to prevent damage if your system is working too hard. Tripping this switch consistently can cause it to wear out and fail. Without an operational high limit switch, your furnace will simply not run.

Flue limit switch

This switch will turn off power to your system if air isn’t properly ventilating out of your home through the flue exhaust vent. Furnaces produce carbon monoxide when they burn their fuel source, which of course needs to be safely vented. Without a functioning flue switch, your system will not operate.

What’s behind the short-cycling?

Now, what could actually be causing your system to short-cycle? Here are four common causes.

1. Flue not venting correctly

Your flue exhaust may have something blocking airflow and causing it to short-cycle. The most frequently seen culprits are bird nests and other critters setting up shop in your vent.

2. Blower motor not working

It’s possible the blower itself is out, which means air isn’t circulating through your duct system. Your limit switch detects the heat buildup and shuts the system down.

3. Dirty air filters

If your air filters aren’t changed out at least every three months, that makes it difficult for your system to run properly. If your system can’t “breathe,” it can cause the limit switch to trip.

4. Faulty thermostat

If your thermostat isn’t functioning properly, it could be the cause of your short-cycling. You can test the thermostat by removing it from the equation and manually connecting the wires. There are plenty of tutorials for this online if you’re the DIY type.

Don’t forget, it’s okay for your furnace to be cycling three to eight times per hour—possibly more if it’s extremely cold outside. But a cycle of 30-60 seconds is something to be concerned about. If you believe your furnace is short-cycling and would like a professional’s opinion on the cause and solution, please give us a call. We’d be happy to help you resolve the issue.

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