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Heating / January 5, 2021

An Overview of a Fan Limit Switch for a Furnace

A furnace is a useful appliance, but only if it’s running as it should. Otherwise, the furnace becomes dangerous. Furnaces come with checks and balances for safety and efficiency. The fan limit switch, located near the furnace’s heat exchanger, provides some of these checks and balances. Below is an overview of the fan limit switch and the importance of it.

Functions

The fan limit switch has two major functions.

  • Turning the furnace fan on and off
  • Shutting down the furnace in case of overheating

Both functions of the limit switch are critical to the furnace’s safe and efficient operation.

First, the limit switch keeps the air handler’s fan off when the furnace’s heating cycle commences. Otherwise, the fan would blow cold air throughout the house. The limit switch turns on the fan once the air becomes appropriately warm.

The furnace’s burners switch off once the room temperature reaches the required level. However, the fan limit switch keeps the fan running to continue to blow residual hot air. The limit switch only turns off the fan once the temperature falls to a preset level. That way, you continue to enjoy comfortable temperatures even with the burners off.

Secondly, the limit switch keeps sensing the furnace temperature and turns off the gas supply if the temperature rises above a dangerous level. Otherwise, the furnace might overheat and suffer catastrophic damage. For example, overheating can crack the heat exchanger.

Temperature Settings

The limit switch has different settings for different functions. Below are some of these settings.

Fan Off

The setting determines the temperature at which the blower fan turns off. For example, if the Fan Off setting is set to 90 degrees Fahrenheit, the blower fan will run until the temperature falls to 90 degrees Fahrenheit.

Fan On

The setting determines when the blower fan turns on. For example, if the setting reads 130 degrees Fahrenheit, the blower fan will stay off until the temperature reaches 130 degrees Fahrenheit.

Limit Off

The Limit Off setting determines the temperature at which the furnace switches off. For example, if the setting is 200 degrees Fahrenheit, the furnace will turn off if it overheats to that temperature.

Limit Stop

The Limit Stop setting is critical since it prevents the furnace from overheating and its dangerous consequences. Thus, it would be catastrophic to set the Limit Stop setting to a dangerously high temperature. The fan limit switch comes with a setting, the Limit Off setting, which prevents raising the Limit Stop setting to a dangerously high temperature.

Malfunctions

The fan limit switch can malfunction just like other parts of the furnace. Below are some potential malfunctions.

Sensor Clogging

The limit switch has a temperature sensor that detects the plenum temperature. Dust or debris can clog the sensor and insulate it. Such a dirty sensor might not detect the correct plenum temperature. If that happens, the furnace might continue to run even after the temperature reaches the preset levels.

Incorrect Installation

The limit switch is an electrical device that requires proper installation. Incorrect installation will interfere with the limit switch’s operations.

Incorrect Settings

As mentioned above, the limit switch has multiple temperature settings. The settings determine what happens in the furnace at each temperature. Incorrect settings will interfere with the limit switch’s functions.

Physical Damage

The temperature sensor is a delicate bimetallic spring. The sensor can bend or suffer damage during installation, causing the switch’s malfunction.

Hopefully, you won’t experience any issues with your fan limit switch. Ensure that is the case by using a professional technician to install, set, and maintain the fan limit switch. Contact Mauzy Heating, Air & Solar if you suspect that your furnace’s fan limit switch has malfunctioned. We will diagnose and fix the issue, whether it is a limit switch malfunction or any other issue.

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