Why is My Furnace Overheating? A Pittsburgh Tech Answers

| Posted in:  Help Guides/Heating
Tech Investigating Furnace Overheating

If you’ve noticed that your furnace is...

- Frequently cycling on and off 

- Making a humming noise

- Producing a burning smell

...it may be overheating. 

Luckily, your furnace has a built-in limit switch, which shuts your furnace off when it gets too hot, preventing any serious safety issues or long-term damage to your system. 

However, even though your furnace is built to shut off when it overheats, consistent overheating will cause damage to your furnace...and the longer you wait to correct the issue, the more costly those damages can be.  

Below, we’ll look at a few common reasons your furnace may be overheating, like:

  • Restricted airflow
  • Old age
  • Mechanical failure
  • Broken/malfunctioning gas valve

Prefer to have a professional determine why your furnace is overheating?  That’s no problem. We work with all makes and models of furnaces, so no matter what kind of furnace you have, we can determine the issue and make the repair. Learn more about the furnace repair services we offer or...

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My furnace stops running as soon as it starts overheating, should I still be concerned?

As we mentioned above, if your furnace overheats, it will usually trigger the limit switch to turn your system off. However, your thermostat is still telling your system that it needs to produce heat. So shortly after being turned off, your furnace is going to turn back on again. Once the limit switch senses that your system is overheating, it turns your furnace back off again.

This frequent cycling on and off is an issue called short cycling and can result in issues for your furnace like:

  • Reduced lifespan
  • Increased energy bills
  • Hot and cold spots around your home
  • Increased wear and tear on system components

So, if you notice the signs that your furnace is overheating (turning on and off frequently, smell burning, hear a humming noise), then you should reach out to a technician as soon as you can before a small issue turns into a larger one.  

Overheating Cause #1: Restricted Airflow

The most common reason your furnace will overheat is due to restricted airflow. 

Restricted airflow is usually the result of:

  • A dirty air filter- Your air filter is put in place to block dust and debris from entering your furnace and eventually, your home’s air supply. However, if your filter is dirty or clogged, it will restrict the amount of air that enters your heating system as well. If there is not enough air making its way to your system, your furnace can overheat. To see if this is your issue, check your air filter. Does it look like the filter on the right (see below)? If so, it’s time to replace your air filter. If you change your air filter and your system stops turning on and off, a clogged filter was likely your issue
Clogged Air Filter Furnace Loud Noise
  • Blocked return vents- It’s important that you ensure all of your return vents are opened and unobscured. Return vents are sometimes blocked by household items like furniture, dog crates, trashcans, etc. Just like with a clogged air filter, if there isn’t enough air circulating through your system, it can cause your system to overheat and shut off. Ensure that all of your return vents are open and unobstructed. If your furnace stops cycling on and off after you’ve opened all vents, this was likely your issue.
Return Vent

Example of return vent

  • A dirty blower wheel- The blower wheel is part of the blower motor, the component that’s responsible for pushing air over your heat exchanger. If you haven’t properly maintained your blower wheel, over time dust and debris can gather on and around it, making it work harder to turn. This will reduce the amount of air that is blown over your heat exchanger...and if there isn’t enough air making its way to your heat exchanger, the furnace can easily overheat. If you think this may be the reason that your furnace is overheating, you should reach out to a technician. The blower is something that should be maintained by a professional, and a technician can easily clean this and other parts of your furnace to ensure it’s working well. 

Overheating Cause #2: Old Age

Your furnace is built to last about 15-20 years. As your furnace approaches this age limit, parts start to wear out and stop functioning the way they should. Some of those parts include the safety features of your furnace, like the limit switch. 

If the limit switch isn’t able to recognize that your system is about to overheat, your furnace will keep working and will (you guessed it) overheat. 

While a technician is able to repair/replace a worn-out limit switch, when your furnace gets to this age, the best long-term financial decision is usually just to replace your system as a whole. Otherwise, you’ll probably invest quite a bit in repairs, only to have to replace your entire system a few years later.

Overheating Cause #3: Mechanical Failure

There is always a chance that your furnace is overheating due to a mechanical failure. Sometimes, parts of your furnace can just break down or malfunction, and if they are part of the system that keeps your furnace from overheating (motor, fan blower, types of wiring, etc.), then your system can obviously...overheat. 

If you think your furnace could be overheating due to a mechanical failure, you’ll need to contact a technician to come and take a look at your furnace. 

Going forward, you can prevent a lot of mechanical issues by having a professional inspect and tune-up your furnace on a yearly basis. Typically, mechanical failures don’t happen all-of-a-sudden. A technician will usually be able to spot the signs of a faulty or damaged part before it fails, saving you money and time in the long-run. 

Overheating Cause #4: Broken Gas Valve

In a gas furnace, heat is created when the burners ignite gas. There’s a part of your furnace called a gas regulator, that determines how much gas should enter your furnace to keep it going at the correct temperature. 

If your gas regulator is broken, it won’t be able to properly regulate the amount of gas that is flowing into your furnace.

If there is too much gas in your furnace, the flames will increase in size, providing more heat. Depending on how much extra heat there is, it can cause your heat exchanger (the part of your furnace that actually heats your home’s air) to overheat.

Still having issues with your furnace? Contact Pittsburgh’s most-experienced pros: Calfo

Whether you’re experiencing one of the issues listed above or you think you have a completely different furnace issue, reach out to us. We’ve been helping Pittsburgh homeowners with their heating systems for years, so no matter what kind of repair you need, we can handle it.

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