Why Is My Furnace Overheating?

Does your furnace turn off and on frequently?

That’s called short cycling, and it’s happening because your furnace is overheating.

Besides being dangerous, an overheated furnace is also highly inefficient, costing you more in energy bills.

But what causes a furnace to overheat? It can be due to a combination of one of these 5 reasons:

  • Dirty air filters
  • Blocked vents
  • Filthy blower wheel
  • Clogged evaporator coil
  • A broken gas valve

Before we get into why these cause a furnace to overheat, let’s talk about what’s happening inside a furnace when it overheats:

What happens when a furnace overheats

A furnace, just like a car or other motorized machine, can overheat when its system parts are malfunctioning.

Luckily, your furnace has a built-in limit switch, which shuts off your furnace when it gets too hot. Since the limit switch turns off your furnace prematurely, the furnace will turn on and off frequently—which is called short cycling. Short cycling isn’t good for your furnace, and it will raise your monthly energy bill.

Overheating is usually caused by these 2 problems:

  • Too little airflow over the heat exchanger: This is the most common overheating problem, which is caused by reasons #1–4.  
  • Overly high gas pressure: This is usually due to a problem with your furnace gas valve (see reason #5). You should call an expert to help adjust or replace this.

Now let’s look at 5 specific reasons that cause those overheating problems and how you can fix them:

#1: Dirty air filters

Dirty air filters are the #1 cause of an overheated furnace—but they’re also the easiest to fix.

Dirty filters are dangerous because they block airflow to the heat exchanger, which causes it to overheat.

If your filter looks like the one on the right, you’ll want to change it ASAP.

To prevent overheating, simply check your air filters every month, and change them every 3 months (at the latest). It’s an easy fix that can save you a service call.

#2: Blocked vents

If you block or close more than 60% of your home’s vents, your furnace will start to overheat.

To prevent overheating, keep your vents open and clear of furniture.

When you close or block your indoor air vents, you increase pressure in the air ducts. This makes some furnace blowers slow down, which reduces airflow over the heat exchanger and causes your furnace to overheat.

#3: Filthy blower wheel

If your blower wheel is filthy, then it’s working extra hard to move air through your furnace. This causes overheating and severely reduces the efficiency of your furnace.

To know if there’s an issue with your blower, first listen for frequent starts and stops. Most motors have thermal overloads that will shut off power when it overheats. You’ll also know it’s too hot if you place your hand near (but DON’T touch!) the motor and feel heat. If that’s the case, or you see it looks dirty, contact an HVAC specialist to clean and repair it.

#4: Clogged evaporator

A dirty evaporator coil (the A-shape coil) can block airflow over the heat exchanger, causing it to overheat.

However, this problem only happens if the evaporator was installed in the wrong spot.

To explain what we mean, you first should know where the evaporator should be installed relative to your furnace.

Correct order of heating/cooling parts:

  1. Return duct
  2. Blower motor
  3. Furnace (heat exchanger)
  4. Evaporator coil
  5. Supply duct

As you can see, the evaporator coil should be installed AFTER the blower motor and heat exchanger. Unfortunately, some contractors install the evaporator coil like this:

Incorrect order of heating/cooling parts:

  1. Return duct
  2. Evaporator coil
  3. Blower motor
  4. Furnace (heat exchanger)
  5. Supply duct

Installing the evaporator boil BEFORE the blower and heat exchanger is usually a bad idea for  2 reasons:

  1. Blocked airflow— If the evaporator coil gets dirty, it will blocked airflow over the heat exchanger, causing it to overheat.
  2. Can cause rust the heat exchanger—When your evaporator cools air in the summer, that humid air is pushed from the blower to the heat exchanger. The cool, moist air can rust your heat exchanger and corrode other parts of your furnace.

A heating & cooling expert can help you know if your system was installed correctly, and if your evaporator is causing your furnace to overheat.

#5: A broken gas valve


If your furnace gas valve is supplying too much gas pressure to your burners, it could cause your system to overheat.

When too much gas flows into your furnace’s burner, the pilot flame enlarges, creating more heat. This extra heat then conducts through a thin layer of sheet metal to the heat exchanger. If there’s too much heat (caused by too much gas) reaching the heat exchanger, then your furnace will overheat.

The part of your gas valve most likely responsible for an overheating problem is the regulator. The regulator controls the gas pressure that reaches the burners in your furnace.

If you suspect a problem, you should contact a furnace contractor to inspect and repair your gas valve if you suspect this is the problem.

Need a professional to look at your furnace?

Contact On Time Service Pros to schedule a furnace repair today. We’ll make sure your furnace doesn’t overheat so you and your family can safely stay warm.

Related articles

Share |
View by Month

View by Category