What is a Furnace Limit Switch? How it Works and How to Troubleshoot

What is a Furnace Limit Switch

🔥 What Does a Furnace Limit Switch Do?

The furnace limit switch either shuts the gas furnace down if the heat exchanger is getting too hot or it cycles the blower fan on and off based on the heat exchanger temperature.

Most fan-limit switches do both.

The two types of furnace limit switches that we’ll discuss in this article are the furnace high-limit switch and the furnace fan-limit switch.

🔥 What Does a Furnace Fan Limit Switch Looks Like?

The furnace high-limit switch looks like a flat, silver rectangle and has two wires going to it.

The fan-limit switch looks like a black rectangular box with a clock-looking dial on the top portion and wires going into the bottom portion.

Furnace High Limit Switch

🔥 Where is the Limit Switch on a Furnace?

Both limit switches would be installed in the burner section on the front wall, and they would be above the burners in most cases.

Both limit switches have an arm that extends into the heat exchanger section with a temperature sensor on the end of it.

🔥 Why Do You Need a Furnace High-Limit Switch?

You need a furnace limit switch to protect your home from the heat exchanger overheating and causing a fire.

The furnace blower fan is meant to remove the heat from that section, but the furnace will overheat if there is an issue with the airflow.

🔥 Why Do You Need a Furnace Fan-Limit Switch?

A furnace fan-limit switch is needed to do two things; turn the blower fan on once the heat exchanger gets warm enough and protect your home from the furnace overheating and catching fire.

🔥 Adjust the Furnace Fan Limit Switch to Control the Blower

The furnace fan limit switch can be adjusted to turn the blower on at different temperatures and trip the furnace off at different temperatures.

Please remember that the manufacturer sets the temperature when the furnace limit switch should trip the furnace off and that temperature should not be adjusted.

🔥 Common Problems With a Furnace Limit Switch

furnace high limit switch behind gas valve
furnace high limit switch behind the gas valve

As the furnace gets older, it is typical for the furnace limit switch to get dirt, grime, and other materials built up on it.

If your gas furnace is cycling on and off, it could be due to various issues, ranging from dirt and grime build-up to blower motor issues or a dirty furnace filter.

Any of these issues would cause the furnace limit switch to cycle the furnace on and off before the end of the heating cycle.

If this is the case, you’ll need to troubleshoot each problem I mentioned.

Other issues that could cause the furnace limit switch to trip would be a faulty gas valve, a hole in the heat exchanger, or a blocked supply or cold air return registers.

The first place to check will always be your furnace filter, as people usually forget to change them regularly.

Furnace Limit Switch vs. Furnace Pressure Switch

They have very similar names with very different use cases.

The furnace limit switch protects the furnace from overheating, while the furnace pressure switch protects the home from leaking products of combustion into the home.

🔥 Furnace Limit Switch Repair and Replacement Cost

phyxter furnace repair

You cannot repair a furnace limit switch unless it’s simply a broken wire leading to it.

To replace a defective furnace limit switch, you’ll need the model and the serial number of your furnace.

Then call a local HVAC supplier to get a price on it.

The cost is usually around $35-$135 dollars depending on the furnace brand and whether it’s a high-limit switch or a fan switch.

The above price does not include labor.

The labor cost to hire a professional HVAC contractor to install your new furnace limit switch will be based on your geographic area.

I suspect the hourly rate could range from $75.00 to $300 per hour.

🔥 How Do I Know If My Limit Switch is Bad?

To determine if your furnace limit switch works or if your furnace has other issues causing it to trip, you’ll need a temperature probe.

First, remove the defective limit switch from the furnace housing, leave the wires connected, and put the temperature probe into the furnace.

You will not want to leave the furnace this way as it creates a very dangerous operating condition.

Now, turn the heating system up on the thermostat and wait for it to begin heating.

With the furnace running, you’ll be monitoring the temperature probe to see how hot the air temperature gets.

If the furnace temperature gets hotter than the furnace limit switch setting, you could have an issue with the airflow, the air filter, or other internal components.

🔥 Can I Replace a Furnace Limit Switch Myself?

Yes, you can replace the furnace limit switch yourself.

However, performing furnace repair on your own can be dangerous as you’re working with combustible gas and electricity, so be careful as the entire furnace has gas and electricity throughout.

🔥 When to Hire a Furnace Repair Specialist

Phyxter Approved Logo on Service Truck

If your heating system isn’t supplying enough warm air to your home, and you’re not confident that you can work safely with gas and electricity, then you should definitely hire a professional furnace repair company.

If you want to learn more about DIY furnace repair, you should check out our article on exactly that. Click here to read more about DIY furnace repair.

Jake Gibson

CEO @ Phyxter and HVAC Guru

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Phyxter (pronounced Fix-ter) is a national home services brand specializing in Residential HVAC, Electrical and Plumbing solutions.

Phyxter is laser-focused on writing the best articles and guides to empower homeowners to get the best out of their homes.

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