Why Won’t My Furnace Turn Off?

Does turning off your furnace not work? Or have you noticed your furnace turning on and off in a strange pattern? If these issues sound familiar, your furnace has an airflow issue or it’s short cycling.

Determining the pattern at which your furnace is shutting on and off is important to diagnosing which cause is likely the culprit.

• If your furnace is turning on and off a few times before finally shutting off
• If your furnace is turning on and off on its own without ever shutting off at all

We’ll talk about both of these patterns and break them down for you below. We’ll also link you to our professional technician services so you can get these problems fixed as soon as possible.

If your furnace eventually shuts off for good after turning back on and off a few times, this is likely an airflow issue. Furnaces need a steady supply of cold air coming in to heat and pump back out into your home.

Without proper airflow, your furnace will heat only itself and overheat your system. If the internal temperature of your furnace gets too high, the protective measure known as the limit switch is triggered, shutting it down entirely.

When the limit switch is triggered, furnaces will attempt to turn on again a few times before finally shutting off for good. This problem should not normally persist, but if it does, here are a few reasons that could be causing airflow issues with your furnace:

• Dirty air filter
• Closed supply air vents
• Clogged or blocked intake/exhaust vent or pipe

What can you do on your own to fix these issues?

Checking and replacing a dirty air filter is relatively easy on your own and should be done every 1 to 3 months—even a fine layer of dust and debris on a filter can cause issues for the overall airflow in your home. Below is a comparison of what a dirty and clean air filter looks like

Healthy airflow is dependent on open supply air vents. These vents are the ones you have all over your home that distribute heated air into each room from your furnace. Make sure they are open to prevent overheating and blockage from your furnace.

Your furnace pulls in cool outside air from something called an intake/exhaust vent or intake/exhaust pipe. The entrances of these vents face outside, and if they get clogged, this can inhibit your furnace from having proper airflow, and therefore it cannot warm your home or keep itself from overheating.

You can identify your intake pipes via the white PVC pipes that connect and lead away from your furnace system. If it leads to the side of your house, you can check the entrance for any debris or blockage that could keep it from efficiently pulling in air. If the pipe leads to your roof, call a professional to take a look. In any case, if you’re concerned that the blockage may be deeper, always contact a professional who has the proper equipment to view and clean these pipes carefully.

If your furnace is continuously turning on and off without turning off for good, this is what’s known as short cycling. The term means exactly what it implies–your furnace is running shorter, more strenuous cycles that can be expensive and damaging to your system overall.

These two issues usually cause short cycling:

• A thermostat problem
• An oversized furnace

Your thermostat controls your overall HVAC system and the temperature of your home. If it cannot correctly detect the indoor temperature—even as it rises—because of poor placement in your home or a malfunction, it will tell the furnace to keep running.

Solution:

Examine the location of your thermostat to make sure there are no drafty windows or corners near it that could affect its ability to measure your home's temperature properly. If it is ideally placed in your home and is otherwise still malfunctioning, you should have a professional come and run a diagnostic test on it.

An oversized furnace means that your furnace is too large for the size of your home.

Since it’s oversized, it will be able to heat your home quickly before shutting off. While quickly warming your home may seem like the ideal solution for cold weather, it can actually be quite damaging to your furnace to run shorter and more strenuous cycles over and over.

Essentially the energy it takes to turn on and off and then heat up quickly in shorter intervals is significantly less efficient for your furnace and wallet. More gradual but thorough heating of your home keeps you comfortable and doesn’t overwhelm your overall HVAC system.

Solution:

A professional can run a load calculation on your home to know just what size furnace you need for the amount of space you have and replace your current system.