Why Does My Furnace Keep Shutting Off?

Does this sound familiar? Your furnace turns on, runs for a few minutes and then shuts off. Soon after, it turns back on and the process repeats.

What’s the problem?

Typically, a furnace keeps turning off and on (or “short cycling”) like this because of one of these 


  • Heat exchanger is overheating due to low airflow 
  • Dirty flame sensor rod
  • Oversized furnace
  • Change the furnace filter if it’s dirty
  • Open all your air supply vents (yes, even in rooms you don’t use)
Clean air filter (left) and dirty air filter (right)
An open air supply vent

If these DIY fixes didn’t solve the problem, you most likely need to call a professional HVAC tech for help.
Now, if you want to learn more about these issues and why they cause the furnace to short cycle, read on.
Problem #1: Heat exchanger is overheating
A common reason a furnace short cycles is because the heat exchanger, the part that heats your air, is getting too hot due to a lack of airflow
Once the heat exchanger overheats, it trips the high limit switch, which shuts the furnace down to protect it.
If the heat exchanger (highlighted above) gets too hot, the furnace shuts down. If the high limit switch goes bad, the furnace won’t shut down and the heat exchanger will crack.
3 common causes of low airflow
  1. Dirty air filter: If the filter gets too dirty, the blower will struggle to pull in enough air over the heat exchanger.
  2. Closing air supply vents: Closing too many air vents can slow down certain blower types and therefore reduce airflow.
  3. Dirty blower wheel: Over time, the blower wheel collects dirt, slowing it down and therefore limiting how much air it can deliver over the heat exchanger. Contact a technician to clean the blower if needed.
With this in mind, you can see why our initial DIY fixes include changing the air filter and opening the supply vents
Problem #2: Dirty flame sensor rod
If the furnace tries to turn on multiple times before stopping for awhile, the problem is probably a soot-covered flame sensor rod.
 So what is the flame sensor rod? And why does it being covered in soot cause short cycling? 
Well, the flame sensor rod is a safety device designed to detect a flame in your furnace burners. If it does not detect a flame, it shuts the gas valve.
Now, normally, this is a good thing for you because if it didn’t shut the gas valve, your furnace could fill your home with gas. And one well-lit match could cause an explosion. 
But if the flame rod gets covered in soot, it can’t accurately tell if there’s a flame or not, so it shuts the gas valve, even if there is a flame. And no gas means no flame, which means no heat for you.
Solution: Clean the flame sensor rod. Since this isn’t an easy DIY job, we suggest calling a professional.
Note: If you have an older furnace with a pilot light, the problem is similar, except the flame sensor is over the pilot light, not the furnace burners.
Problem #3: Oversized furnace
Has this short cycling problem been ailing you for years? Or did you just get a new furnace? If you said “Yes” to either of the above, the problem may be that the furnace isn’t sized properly for your home.
You see, furnaces are like shoes: They have to be sized just right or you’ll have problems. For example, an oversized furnace will heat the home too quickly, causing it to turn off and on too often. 
If this is your problem, you have a few options depending on how old your furnace is:
  • If it’s brand new: Contact your installer about this issue so they can fix it. Hopefully you hired a contractor who gave you a labor warranty on their work. 
  • If it’s old (15+ years): Consider getting a new furnace. Make sure that your contractor performs a Manual J heat Load Calculation as part of their free estimate. This calculation helps the contractor find the furnace size you need.

Need more furnace advice? Talk to an expert
Contact the HVAC experts at Comfort Zone Service

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