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Furnace Keeps Turning On and Off? Here’s Why

August 04, 2016

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It’s a little scary. Your furnace is turning on and off frequently, and you’re wondering what the heck is wrong.

Well, you should be concerned! This ‘short cycling’ is a big problem since it will run up your energy bill and eventually cause your furnace to break down.

Not good.

So what’s causing it?

There are 5 common causes:

  • Dirty air filter
  • Blocked exhaust vent
  • Closed supply vents
  • Oversized furnace
  • Misplaced thermostat

Let’s cover each of these issues in detail and show you how to get your furnace back to working order.

Dirty air filter

A dirty filter can overheat the furnace’s heat exchanger (the part that heats the air), causing the furnace to do an emergency shutdown so it can cool off. Then it turns back on, allowing the cycle to happen all over again: 

  1. Furnace overheats 
  2. Shuts down
  3. Cools down 
  4. Turns back on
  5. Rinse/repeat

Why does a dirty air filter cause the heat exchanger to overheat?

A dirty filter prevents enough air from flowing over the heat exchanger, causing heat to build up until—BAM—the furnace shuts down. This is done to keep the heat exchanger from cracking (think of replacing a cracked heat exchanger like replacing a busted car engine—EXPENSIVE!) 

Do this: Replace the filter if it looks like the one on the right in this picture.

Blocked exhaust vent

A blocked exhaust vent can also cause the furnace to overheat. Here’s why. As your furnace runs it has to exhaust certain gases out the exhaust pipe called a flue.

However, if that flue gets blocked by a bird’s nest, leaves, or other debris, those hot gases back up into the furnace, causing it to overheat. 

That’s not even the worst part. Carbon monoxide—an invisible, poisonous gas—will back up into your home and can cause sudden illness or death. Yikes!  

Do this: Check your furnace’s exhaust (if you know where it is) and make sure it’s clear. It may be on the roof, so watch your step.

Shut/blocked air vents

You know those metal grates that blow air into your home? Yeah, don’t shut those (even in rooms you don’t use) because doing so can cause the furnace to overheat (noticing a pattern here?).

Here’s what happens when you shut the vents:

  1. You close the vent.
  2. Pressure in the ductwork increases.
  3. The blower, which is designed to deliver air against a certain pressure, slows down and delivers less air.
  4. Low airflow over the heat exchanger causes too much heat to build up and, as a result, overheat.
  5. Furnace shuts down.
  6. Furnace cools down, then turns back on.

Do this: Keep your air vents open. Also make sure you don’t have any furniture or drapes blocking the vents, either

Oversized furnace

Did you get a new furnace installed recently? If it’s short cycling, the furnace may be too large for your home, and therefore heating it too quickly. 

Learn more about furnace sizes here: “What Furnace Size Do I Need?” [FAQ]

Solution: Contact the furnace installer and tell them your issue since they are the ones who are responsible for choosing the correct furnace size.

Misplaced thermostat

If your thermostat is placed near a heat source, it may think the room is hotter than it actually is and shut down the furnace prematurely. 

For example, if the thermostat is near a supply vent, the thermostat is like “Oh, look at all that heat. The home must be hot enough. Better turn the furnace off.” Then, once the furnace turns off, the thermostat is all like, “Oh, where’d all the heat go? Better turn the furnace on.” So that may be one reason why your furnace is turning on and off like that.

Do this: Make sure your thermostat is placed on an interior wall away from heat sources like a supply vent or a window.

Need an Kansas City-area furnace repair technician?

If you’re not sure why your furnace is short cycling, contact Santa Fe for a furnace repair.

Santa Fe Air Conditioning and Heating serves Olathe, Overland Park, Lenexa, Shawnee, Leawood and the Kansas City area. For more information, contact us online