← All articles

Why Won’t My Furnace Turn Off? A Texas HVAC Technician Explains

September 30, 2019

cost to replace furnace in san marcos

Frustrated by a furnace that won’t turn off?

We understand your frustration—and we’ll do our best to explain what causes this annoying problem and how you can fix it.

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

  • Thermostat issue
  • Dirty air filter
  • Faulty blower
  • Leaky air ducts

We’ll go into more detail about each of these issues below...

Want a professional to repair your furnace ASAP? Let us help!

Cause #1: Thermostat issue

Two thermostat problems could be causing your furnace to run continuously:

  1. Your thermostat is set incorrectly
  2. Your thermostat has a wiring issue

Incorrect thermostat setting

Your thermostat controls your furnace fan. Sometimes the fan setting accidentally gets changed to the wrong position.

The correct setting is AUTO. Make sure your thermostat is set to AUTO instead of ON.

Thermostat to AUTO not ON

Set your thermostat to AUTO instead of ON

Here’s the difference between the two fan settings:

  • When your thermostat is set to ON, your furnace fan will run non-stop, which could be what is causing your furnace to sound like it’s not turning off
  • When your thermostat is set to AUTO, your furnace fan will run only while your furnace is heating your home’s air

Wiring issue

If your thermostat is set to AUTO and everything else seems to be working, the issue could be with the thermostat itself.

Over time, the wiring that connects the thermostat to your furnace can start to wear out. This could prevent your furnace from turning off.

Instead of attempting to fix the wiring issue yourself, we recommend contacting a professional to inspect your thermostat. Electrical issues are often complex and potentially dangerous to fix, which is why it’s best to let a pro handle this job.

Cause #2: Dirty air filter

A dirty air filter blocks air from entering your furnace, which could be causing your furnace to run non-stop.

Here’s why: When you set your thermostat to HEAT, your furnace starts pulling in cold air from inside your home to be heated. This air passes through the air filter where any large particles are removed from the air before it reaches your furnace.

However, if your filter accumulates too much dirt and debris, air will have a difficult time passing through the filter, which means little or no air will reach your furnace. The less air your furnace pulls in, the less hot air it can breathe out. This means your furnace will have to run longer to get your home to your desired temperature, which may be the reason your furnace is running continuously.

Do this: Check your air filter. If it is dirty, replace it with a new one. Then, run your furnace as normal and wait a couple of hours. If you still hear your furnace running non-stop during that time, contact a professional for help.

If your filter looks like the one on the right, it’s dirty and needs to be replaced

Cause #3: Faulty blower

Like we mentioned above, your furnace has a blower that pulls in cold air to be heated and pushes out warm air into your home.

This blower is powered by a motor, which can start to malfunction over time. The most common blower problem that makes your furnace run non-stop is a faulty limit switch. A limit switch is the part of your furnace that tells your system when to turn on and off. If it’s broken, your furnace won’t know when to shut off and will continue to run.

If this is your issue, you’ll need to hire an HVAC professional to fix it. Fixing a broken fan limit switch requires electrical skills and know-how, which is why we recommend letting a professional handle this job.

Cause #4: Leaky air ducts

If you have leaky air ducts (specifically on the supply side, seen in the image below), then heated air might be leaking into the attic instead of going into your home.

Leaky ducts

A leak on the supply side of your ductwork means warm air is escaping into your attic

If you’re losing heated air, your furnace will have to produce more heated air to warm your home, which means longer run times.

You might have leaky ducts if you notice the following signs:

  • Uneven heating throughout the house
  • Lots of dust in your home
  • Higher-than-normal utility bills

If you notice these signs, have a duct specialist inspect your ductwork for leaks. If they find that your ducts have leaks, you’ll need them to be repaired (sealed).

Need a furnace repair from a Texas pro?

We offer same-day service! Our technicians are standing by to help.

For more information about what to expect when you hire us, visit our furnace repair page.

Related reading: