What Is a Fan Limit Switch?

What is a fan limit switch?

What Is a Fan Limit Switch?

(Last Updated On: May 30, 2017)

Definition of a Fan Limit Switch

A fan limit switch is a component of a furnace system that determines when the furnace blower assembly for hot air turns on and off. Essentially, this means that it determines when the hot air should be blown through your ventilation system. The fan limit switch also has two other functions. Just as it determines when the blower assembly should turn on, it will also determine when it should turn off. It also determines if the furnace should be shut down in the case of overheating.

How a Fan Limit Switch Works

A fan limit switch consists of the following components:

  • An on-off switch for the blower fan.
  • A temperature sensor within the furnace.
  • A control which turns the blower fan on or off depending on the air temperature within the furnace.
  • A high limit setting, which turns off the heat source or burner if the furnace overheats to dangerous temperatures.
  • A blower fan Manual ON switch is also included on many fan limit controls. This allows the blower fan to be set to run continuously.

When the furnace calls for heat, the burners turn on and heat up the air in the air plenum, before it is blown across the heat exchanger and out to various rooms through the ductwork of the home. The fan limit switch tells the blower assembly when it is OK to blow the heated air. Once the fan limit switch has sensed that the air in the plenum is a suitable temperature (usually around 20 degrees above the room temperature) it engages the blower fan to distribute air. When the room temperature reaches the thermostat setting, the furnace will turn off. The blower will continue blowing until the fan limit switch has determined that the temperature within the furnace has fallen low enough to disengage.

In the event that the furnace fan breaks down or malfunctions, but the burner continues burning and increasing the temperature within the furnace, the fan limit switch will cut the power to the gas valve. This is a safety feature that helps to prevent possible fires or explosions.

Benefits of Fan Limit Switches

A fan limit switch is beneficial for a few reasons, both safety- and comfort-related. From a safety standpoint, it helps to prevent the furnace from overheating. This helps to prevent potential home fires and explosions, as well as damage to the furnace. Because it controls the blower assembly of the furnace, it also helps with temperature regulation in the home. This means there is a reduction in cold air drafts as the system runs, which helps to make the home more comfortable for its occupants.

Problems With Fan Limit Switches

Problems that commonly occur with fan limit switches include:

  • The switch’s sensor becomes dirty and stops working correctly.
  • The furnace filter is clogged and raises the temperature within the plenum prematurely, tripping the sensor.
  • The blower motor stops working for some reason, again overheating the furnace prematurely and tripping the switch to shut off the furnace.
  • There could also be problems within the contactors, circuit board, gas valve, or even the thermostat, which may cause the fan limit switch to stop working properly.

You can tell there might be a problem with your furnace’s fan limit switch if the furnace continually runs, yet no heat comes through the system. You may also notice a problem if the blower does not run, or does not stop running when it normally would. If your furnace continually locks out or does not function as expected, it’s best to contact a qualified HVAC professional to help troubleshoot and repair the issue.

Ways to Prevent Fan Limit Switch Problems

The best way to maintain your fan limit switch to make regular HVAC maintenance an aspect of your home maintenance. Regular cleanings and inspections can help to ensure that your HVAC system is running in tip top shape for years to come, and to catch and fix any problems before they become major issues.

You can replace a faulty fan limit switch yourself, but the  job is likely beyond the skills of most homeowners and it’s best to contact a professional for assistance.

While most moderately competent homeowners should be able to troubleshoot the fan limit switch on their furnace, it’s best to call in an HVAC professional if you have any doubts or are unsure of the source of the problem. They are experts who can get your HVAC system back up and running quickly and safely.

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