What Is A Furnace Limit Switch? How It Operates and Troubleshooting

A Furnace Limit Switch

The furnace is essential to a home's heating and cooling system. There are different furnace systems, and all of them are equipped with a limit switch. For example, natural gas furnaces have a limit switch that regulates the temperature to prevent the unit from overheating.

Read more: What Are The Different Types Of Furnaces?

The furnace limit switch is a safety device that monitors the temperature inside the furnace. It shuts off power to the burner when it reaches a certain level, preventing the system from overheating or overworking itself. We will explain how the limit switch operates and how it can be tested and troubleshot.

How Does the Furnace Switch Work?

The furnace limit switch is a safety device that prevents the furnace from overheating. In short, this switch works by monitoring the temperature of your home's air near the furnace. The switch will shut off the burner if the temperature exceeds what has been predetermined as safe levels. This helps to prevent dangerous temperatures from occurring and possible fire hazards.

The switch is typically located near the furnace unit and resembles a long metal tube connected to the unit's side. Inside the switch, there is a bimetallic strip that consists of two different metals that are joined together. As air temperature increases, this strip will bend, allowing for a connection between two contacts and triggering the limit switch to shut off power to the furnace.

Common Issues With a Furnace Switch

This switch may become faulty over time or be damaged by debris or extreme temperatures, resulting in malfunction. Common issues with a furnace switch include:

a) Incorrectly wired:

Furnace switches must be carefully wired to function correctly, and incorrect wiring can lead to operational issues.

b) Dirty switch contacts:

Over time, the switch contacts may become dirty and corroded, reducing the functionality of the switch. Cleaning them with a soft cloth or brush will help restore their contact.

c) Broken switch:

If the switch is broken or damaged, it will need to be replaced.

d) Blown fuse:

The furnace limit switch may have a blown fuse and require replacement.

e) Improperly calibrated:

The furnace limit switch may require recalibration if it needs to be fixed.

Troubleshooting a Furnace Limit Switch

Whether you have a gas or electric furnace, troubleshooting your limit switch is not difficult. Here are some tips on how to test and diagnose your furnace limit switch:

1. Check the air filter:

Make sure that the air filter is clean to allow for proper airflow. A dirty or clogged-up filter can lead to problems with the limit switch.

2. Check the wiring:

Ensure all wires connected to the limit switch are correctly connected and in good condition.

3. Check the settings:

Verify that the switch is set at the correct temperature and on the correct fan speed settings.

4. Try resetting it:

If you’ve checked all of these items, try to reset the switch by turning it off for a few minutes and then turning it back on again.

5. Check the fan:

The furnace limit switch may only work if the fan is spinning correctly, so check to ensure it's in good condition.

Moncrief is a leading hvac company in Atlanta. We provide furnace installation, maintenance, and repair services.