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Why Won’t My Furnace Turn Off? A Phoenix Tech Explains.

“Ugh! Why won’t my heat turn off?”

We know it’s frustrating to have your furnace run non-stop. So we’ll help you troubleshoot this problem quickly.

If your furnace won’t shut off, you likely have one of the following problems:

Unfortunately, you can only fix the first 2 problems by yourself. You’ll need to have a furnace repair tech fix the rest.

Let’s start with the problems you can fix...

The easy problems you can fix

You may either have a wrong thermostat setting or a dirty air filter. We’ll show you how to troubleshoot each problem:

Step #1: Check the thermostat

Sometimes your thermostat accidentally gets switched to the wrong setting.

A thermostat set to AUTO in a home

Make sure your thermostat is set to AUTO, not ON

Check to make sure the thermostat is set to AUTO, not ON. ON makes your furnace’s blower run non-stop but AUTO will tell your furnace’s blower to run only when heat is being produced, so it should shut off once your home reaches the desired temperature.

Note: If your thermostat fan is set to ON, your furnace isn’t actually running non-stop—just the blower fan is. But you’ll still need to change to AUTO to save money and prevent repairs.

If that doesn’t fix the issue, then you’ll want to try Step #2.

Step #2: Check the air filter

A clean air filter next to a dirty air filter

Does your filter look like the one on the right? If so, it’s time to change it.

Believe it or not, a dirty air filter can actually make your furnace run non-stop because it restricts air coming into your furnace.

Let’s take a closer look at why that’s the case: When your furnace gets the call for heat, air from your home is pulled into your heating/cooling system and through an air filter. The filter’s job is to trap and stop any large dust and debris from entering the system.

BUT, if your filter gets too dirty, little or no air will pass into the system (think of this as breathing with your mouth covered). And the less air your furnace pulls in, the less hot air it can breathe out, meaning it has to run longer to heat your home.

Solution: Check your air filter. If it’s dirty, replace it with a new one then wait an hour or so. If you still hear the furnace run after an hour, then you’ll want to contact a furnace contractor for help...

Problems you need a professional to fix

The following furnace problems are a little trickier, so you should contact a professional to fix them for you.

Problem #1: Broken blower fan motor

Your furnace has a fan (or blower) that’s powered by a motor. The purpose of this fan is to pull in air from your home and push heated air back into your home.

Motor and wheel parts of a furnace

Blower wheel assembly

Sometimes these motors glitch or have mechanical failure which causes them to run non-stop. The most likely culprit here is a faulty fan limit switch, which is the part that basically tells your furnace fan when to turn on and off.

A professional can repair or troubleshoot any blower motor issues. Usually, it’s a matter of installing a new switch or repairing wiring to the switch.

Problem #2: Malfunctioning thermostat

Over time, thermostats can malfunction and the wiring that transmits electrical signals to the furnace can short or wear out.

Since this type of problem requires electrical expertise, you should have an expert handle this problem.

Problem #3: Leaky ductwork

Has your furnace run constantly for a long time? If so, the problem may be leaky ductwork.

You see, your home has 2 types of ductwork: supply and return.

  • Supply ducts blow heated air to your home
  • Return ducts suck air into the furnace to be heated

Now, if you have leaks in the supply ducts, heated air is leaking into your attic instead of into your home. That means your furnace runs constantly trying to heat your home.

A diagram of how a furnace can have supply duct leakage

Supply duct leakage.

If you have supply duct leakage, you’ll need a professional to fix them for you. Repairing duct leaks requires a process called duct sealing, where professionals use mastic sealant or adhesive or metal tape to cover any holes.

Need a Phoenix tech to repair your furnace?

Give us a call or schedule online. We’ll send one of our trusted heating technicians to your home and troubleshoot the problem quickly so you can have a normal, working furnace again.

Visit our furnace repair service page for more information on scheduling and what is expected from your team at George Brazil.