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Why Does My Furnace Keep Stopping and Starting So Much?

Gas Burner

I recommend to my readers that they keep a steady setting on their thermostats during the day. This is true in winter as well as summer—it helps the HVAC system avoid continual stopping and starting, which drains extra power. For a comparison, I like to sleep on a couch in the sunlight, and if I had to keep getting up and moving every ten minutes because the sunlight kept switching position, I’d use up a lot more energy than if I just could stay in one place. 

But what if you are keeping the thermostat steady, but notice that the furnace is turning on and off rapidly anyway? If your furnace isn’t running for at least 15 minutes at a time before it cycles down, something may be wrong. I’ll go over some possible problems to help you see if you need furnace repair in Knoxville, TN to solve the problem.

A clogged furnace filter

You’ve heard me talk about how much trouble a clogged filter can cause, and here’s another example. If the filter is too dirty, it won’t allow enough air into the furnace to remove the heat from the heat exchanger. If the heat exchanger gets too hot, a limit switch will shut the furnace off. Try putting in a clean filter to see if this helps the furnace run better.

A faulty flame sensor

The flame sensor is a safety device I talked about in a blog last year. The short version is that it’s designed to turn off the gas to the burners if the burners fail to ignite. If the flame sensor malfunctions, it could turn the gas off anyway even though the burners are lighting. Our technicians can clean or replace the flame sensor to fix this.

A misbehaving thermostat

The thermostat may be the culprit behind your furnace woes. If the thermostat is misreading indoor temperatures, it may end up shutting down the furnace too early, before it has a chance to warm the whole house. You’ll want this repaired—don’t try to overcome the problem by pushing the thermostat up farther. 

The furnace is oversized

Is this a new furnace? Then I’m afraid I’ve got some bad news: when a new furnace starts to turn on and off rapidly like this, it usually means that whoever installed it did not accurately size the furnace and put in one that’s too large. An oversized furnace will raise the indoor temperature so fast that it will incorrectly register that it’s already finished heating the house and shut down. This is why it’s so important to have thorough professionals like our team install a furnace: we take special care to precisely size a new heating system—no guestimates! You’ll need our pros to replace the incorrectly sized furnace. 

The furnace is just too old

The problem may be that your furnace has worn down so much from years of use that it can’t function correctly. If you have a gas furnace that’s more than 15 years old and it starts turning off early, it’s a major sign that you should look into getting a replacement. Our team of pros are glad to help you with making the decision: they’ll give an honest appraisal of your furnace and tell you whether it’s time to invest in a new one.