Why is my furnace running constantly?


This article was originally published in November 2015. It has been updated in November 2020 to reflect updated information.

If your furnace is always on and never cycles off, there’s a good chance it’s time to get some professional maintenance. A furnace that is constantly running is “short cycling.” There are four main problems that cause your furnace to short cycle.

4 Reasons Why Your Furnace is Constantly Running

1. The Furnace Filter is Dirty

A dirty furnace filter is the number one reason we’ve found that furnaces run constantly. The air filter captures dust, pet hair, and debris that could damage the furnace and keeps it from getting to the heat exchanger. If the filter is dirty, it restricts airflow across the heat exchanger, which causes it to overheat! The furnace goes into “cool down” mode and runs the fan constantly. Your furnace might even start blowing cold air if this is the problem.

The solution is simple: replace the furnace filter. You should do this before you start running your furnace in the fall, but it’s never too late to do it if you forgot. We suggest replacing your filter every three months. If you have pets, you should check it more often, about once a month. A clean furnace filter is pure white, so if it’s any shade of brown or gray, it’s time to change it.

Pro tip: New Mexico Air Filter is a great place to purchase your furnace filter! They are local and affordable. If you need a special sized filter, they can make it for you. All you have to do is take in your old filter so they can match it.

2. The Fan is Set to “On”

If your filter is fine, check your fan setting on your thermostat. If the fan is set to the “on” option, the blower will run constantly, no matter what the temperature is. Change this setting to “auto” so that the blower only runs when the furnace is actually on.

3. The Temperature is Set Too High

If the furnace filter is clean and the fan is already set to “auto,” try turning the temperature on the thermostat down. Give it a few moments to see if the furnace cycles off. Sometimes, the furnace is running constantly to keep up with the temperature, especially if it’s really cold outside.

4. The Furnace Isn’t Derated

When a furnace is installed, it needs to be derated. But what in the world is derating? Derating is when the HVAC installer adjusts the gas pressure to match the elevation. If it’s not set correctly, your furnace ends up overheating, kind of like we described above.

It goes something like this: the furnace runs, and with the gas pressure not properly adjusted, it gets too hot. The furnace shuts off, but the fan keeps blowing to cool it down. This leads to the furnace switching itself on and off constantly, with the fan never stopping! If you think this might be the problem, you’ll need to call an HVAC professional to come derate your furnace. Don’t try to do this yourself, it requires a pro.

None of the above?

If the issue continues after you’ve tried these suggestions, there is only one other thing it could be: ghosts! Of course, we’re joking! If your filter’s clean and the thermostat is set correctly, the issue is more serious and it’s time to call a heating professional. (You thought we were going to say Ghostbusters, didn’t you?)

The bottom line is that a lack of maintenance is almost always to blame for a furnace that runs constantly. By regularly changing your filter and getting an annual furnace inspection, you can avoid most emergency furnace repairs. TLC’s Preventive Maintenance Program makes maintenance easy and stress-free!

Need service? Call 505-761-9644 or schedule an appointment online.

Have questions? Ask a pro!