HVAC Troubleshooting

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Furnace Limit Switch: A Small Part with a Big Job

What is a limit switch on a furnace?

A furnace limit switch is a built-in safety feature in your furnace. It’s responsible for keeping your furnace from overheating, which could result in damage to your furnace and your home.

What does a furnace limit switch do?

When your thermostat signals the gas furnace to start heating, it’s not an instant process. Generally, the furnace burners need a little bit of time to warm up the heat exchanger before sending heated air through your ductwork.

Once the furnace reaches the right temperature, the limit switch signals that it is safe to distribute the air. The limit switch works to monitor the temperature inside the supply plenum, and if it gets too high, the limit switch shuts off the gas supply to the burners.

This is why it’s also known as the furnace high limit switch. It also activates the furnace blower fan to cool the furnace down. The gas valve will only reopen when the temperature has fallen to a safe level.

What are the symptoms of a bad limit switch on a furnace?

A failing or failed furnace limit switch can cause big problems, including:

The furnace stops abruptly in the middle of operation.
The furnace won’t shut off.
The furnace is not blowing hot air.
The furnace is short-cycling – switching on and off in short cycles.

If your furnace is exhibiting any of the symptoms above, a faulty limit switch could be to blame. Don’t attempt to bypass or reset the limit switch. Call an HVAC professional.

Schedule furnace repair

What does furnace limit switch replacement cost?

If the limit switch has failed, you can expect the replacement to cost from $100-$375. Your actual cost may vary based on the furnace model, your location, the HVAC service call fee, and whether you need emergency furnace repair.

Schedule furnace repair now

What causes a furnace limit switch to fail?

Several things can cause a furnace limit switch to fail, including:

Years of wear and tear
Restricted airflow
Overheating

You can’t prevent years of wear and tear, but you can address restricted airflow and overheating with regular maintenance. More on that below.

Prevent problems with furnace maintenance

Don’t get left in the cold. Replace your furnace filter every 1-3 months, depending on the quality of the filter. A clogged air filter can adversely impact airflow and cause your furnace to overheat. Make sure that furniture and curtains are not blocking registers or return air vents.

In addition, schedule a professional furnace tune-up each fall before heating season begins. At this visit, the HVAC technician will inspect, clean, and adjust internal components to promote safe and optimal performance. By ensuring proper furnace maintenance, you can help your heating system run at peak performance and even extend the life of your system.

Take advantage of Trane Diagnostics

For peace of mind and to catch problems while they’re still small, opt into Trane Diagnostics via the Trane Home App. When you opt in, you give your dealer remote monitoring capabilities, much like home security monitoring, only for your HVAC system.

Your Trane smart thermostat sends real-time data to your dealer, alerting them to problems with your system or when it’s time for HVAC maintenance. If you give them one-time dealer remote access (DRA), they can run troubleshooting tests to pinpoint a problem. Sometimes, they can solve the problem remotely.

If the issue requires an on-site service call, the HVAC technician will arrive with the knowledge collected during remote troubleshooting. This also means they will arrive with the right replacement parts, which can save both you and the HVAC company time and money.

By Anne Fonda

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer