Overheating Or Limit Switch Failure — How Can You Tell The Difference?

If you own a combustion furnace that uses natural gas, oil, or propane for heating, then your furnace will contain multiple critical safety switches to detect and prevent unsafe operation. The limit switch is an essential part of this safety system. Limit switches detect out-of-range temperature conditions at both ends of the scale. In other words, your limit switch determines if your furnace is too hot or cold.

The lower end of the range prevents the furnace blower from running when the heat exchanger isn't yet up to temperature. On the other hand, the high-end range is crucial to prevent the heat exchanger from overheating and damaging itself. Since a damaged heat exchanger can emit toxic carbon monoxide gas, this role is vital to safely operating your home's furnace.

What Happens When Your Limit Switch Trips?

When operating normally, your furnace will cycle to meet your thermostat's setpoint. If you set your thermostat to 70 degrees Fahrenheit, the furnace will turn on when the temperature falls below 70 degrees and turn off when it reaches 70 degrees. A typical furnace may cycle multiple times per hour as your home loses heat and cools down.

However, the limit switch will trip if the internal temperature of your furnace gets too warm. A tripped limit switch will cause your furnace to shut down, resulting in a situation known as short cycling. In the above example, you'd notice your furnace shutting off before the temperature reaches 70 degrees. The furnace may attempt to run several more times, shutting off prematurely in some or all cycles.

Why Do Limit Switch Trips Occur?

There are only two reasons why your furnace's limit switch would trip:

  • Your furnace is overheating
  • Your limit switch is faulty

The former case is normal operation for a combustion furnace. Your furnace needs to shut down when overheating because the consequences of running too hot are too severe. Fortunately, overheating is often due to simple airflow issues. Replacing your furnace filter or clearing obstructions near vents will sometimes solve the problem.

If correcting basic airflow issues doesn't resolve the problem, the next step is to diagnose the limit switch. This step is necessary to rule out a sensor fault and determine if another issue is causing an overheating condition. Unfortunately, making this determination can be tricky since testing the limit switch requires electrical skills and equipment.

What Should You Do About a Tripping Limit Switch?

If your limit switch trips once or twice, don't panic. In many cases, checking and replacing your air filter is the only step necessary to resolve the issue. However, don't ignore a persistent problem that causes your furnace to short cycle even after resolving basic airflow issues. In these cases, stop using your furnace and contact a professional to investigate the problem.

Contact a company that offers furnace services to learn more.