How Do You Fix a Furnace That Keeps Running?


Sinclaire Enterprises, Inc.

140 South St., Walpole, MA 02081
P 508.668.8200 Toll Free 877.668.8822
Master Plumber License #9530,
Sheet Metal Master-Unrestricted #5442

During the Massachusetts winter, you naturally want your furnace to be running. Nobody wants to be stuck with a broken furnace on a subfreezing day or when one of our infamous winter storms hits. That’s a recipe for discomfort and, in the worst cases, serious risks to the health and well-being of you and your family.

So it stands to reason that if your furnace is running, that’s a good thing, right? Most of the time, that’s certainly the case. But what happens if your furnace keeps running, even after your house reaches the temperature you set the thermostat to? That’s not such a good thing, and for a few reasons:

  • Your house may become uncomfortably hot. As a result, you might reduce the temperature or crack some windows open to cool the house off, which means you’ll just have to run the furnace more once things cool down a little too much.
  • Because the furnace is running more because of the above actions and because the furnace itself may not be running properly, your energy bills will go up. And depending on the system you have, it might be emitting higher levels of harmful gasses into the atmosphere.
  • It could be a sign that your furnace has a serious problem that might require expensive repairs or even a replacement to fix.

A broken furnace can increase bill cost

Before you call for a furnace repair in Bellingham or a furnace repair in Milford, though, you should know about some of the common reasons furnaces keep running longer than they’re supposed to. That’s because some of them have easy fixes that you can do yourself. So let’s look at these reasons and what the solutions are so that you know when and when not to seek professional HVAC services.

First, though, let’s address this question: How do you tell if the furnace is running too long or too much?

Obviously, if your furnace is constantly running, that’s a clear sign it’s running too much. If it’s not always on but you suspect it’s running too much, even if the house isn’t excessively warm, consider this: on average, a furnace goes through two or three cycles per hour, with each lasting 10 to 15 minutes. Bear in mind that in extremely cold weather, intervals and run times may increase.

Okay, so now let’s see about those causes:

1. Dirty Air Filter

The typical recommendation for replacing your air filters is to do so once every 1-3 months. It’s easy to forget to do this, but when that happens, problems can occur. Your filters collect dust and debris. When too much accumulates, airflow can be restricted or even blocked. As a result, the furnace has to work harder to get your home to the temperature you want, if it even can. In turn, this can cause the unit to overheat and malfunction. Dirty air filters are the top cause of excessively running furnaces and one of the easiest fixes. The only reasons you can’t do this yourself are lacking the means to reach the filters or having a condition that makes getting onto a stepladder inadvisable.

2. Thermostat Set to Wrong Temperature

If your thermostat is set too high, your furnace will run longer, and harder to get to that temperature. When you notice that your house feels too hot, a thermostat set too high is a likely cause. This fix is even easier than the air filter one; all you have to do is turn the temperature down.

3. The Fan Is On

This is another thermostat issue and another one that’s very easy to address. If you have your thermostat set to the “Auto” position, the furnace will run in cycles, but if you have the thermostat set to “On,” the furnace will run constantly. Sometimes using that setting is just a mistake, and other times it’s because you wanted to heat a cold home quickly (for example, when returning from a vacation during which you had the temperature set lower to save money) and you forgot to go back and change the setting. Either way, just switch to “Auto” and the problem is solved,

4. Pilot Light Is Malfunctioning

If you have a gas system, you have a pilot light that ignites the gas before it reaches the burner. A malfunctioning pilot light will make the furnace run constantly because the system isn’t receiving enough gas to warm the house to the set temperature. You can relight the pilot light with a match or lighter, or your system may have a switch that resets it.

5. Bad Limit Switch

The limit switch is what controls your furnace’s heat. It gets a signal from the thermostat that it’s time to turn the system on, and then it relays that command to the furnace. A faulty switch can result in crossed signals causing the furnace to constantly run. You’ll probably need to call a professional service for this and the remaining issues.

6. Issues with the Blower

If the heating system is on and warm air isn’t coming out, it’s probably an issue with the blower from corroded wires or a blown fuse.

7. The Furnace Isn’t Derated

If you haven’t heard the term derated before, you’re far from alone. It refers to the process that limits the amount of gas being sent to the burner, and it keeps the furnace from overheating. When derating isn’t occurring, a furnace will overheat and cause the blower to run constantly.

Call a Reliable Professional: Sinclaire Home Services

If you’ve done your troubleshooting for 1-4 above and haven’t found anything, or if you’re pretty sure it’s something from 5-7 or something else, it’s time to get professionals on the job. In the Boston area, Sinclair Home Services is the right company for the job.

Since 1948, our family-owned home services business has been helping families keep comfortable and safe in their homes. We’ve built a reputation for quality, professionalism, reliability, and integrity that’s unmatched.

For any furnace maintenance, installation, or repairs, Sinclaire Home Services has you covered. Get in touch today and stop worrying about your heating system!