Furnace Fan Limit Switch Wiring Diagram

By | March 5, 2023

A furnace fan limit switch is an important part of the heating system in your home. It helps to regulate the amount of heat that the furnace produces by preventing it from getting too hot and overworking. The fan limit switch plays an important role in keeping your home comfortable and safe.

The fan limit switch is a thermostatically-controlled device that is connected to the blower motor of the furnace. It is usually located right above the blower access door. It works by controlling the blower fan speed, allowing the furnace to cycle between on and off. When the temperature limit is reached, the fan limit switch closes, shutting off the blower fan. This helps prevent the furnace from overheating and damaging other components.

There are several common problems with a furnace fan limit switch. These include a broken limit switch, a tripped limit switch, or a limit switch that is not calibrated correctly. If the switch is broken, it may either be stuck in the closed position, or it may be stuck in the open position. If it is tripped, it will shut off the blower fan even though the temperature of the furnace is still within the acceptable range. If the limit switch is not calibrated correctly, it may either be set too high or too low, resulting in the blower fan cycling frequently.

How to Troubleshoot a Faulty Furnace Fan Limit Switch

Troubleshooting a faulty furnace fan limit switch can be a complicated process. The first step is to determine whether or not the switch is functioning properly. If it is not, you will need to replace it. The following steps can help you troubleshoot a faulty furnace fan limit switch:

  • Check for power. Check to make sure that the power to the furnace is turned on and that the circuit breaker is in the "on" position.
  • Test the limit switch. Set your multimeter to read ohms. Remove the cover of the fan limit switch and connect one end of the meter to the two terminals. If the switch is working correctly, the meter should read about 20 ohms. If not, the switch needs to be replaced.
  • Reset the limit switch. If the switch is tripped, reset it by turning the knob counterclockwise until it clicks.
  • Check the wiring. Make sure that all of the wiring is connected correctly and that there are no loose connections.
  • Calibrate the limit switch. Adjust the temperature setting on the switch to match the temperature of the room.

If none of these steps resolve the issue, it is best to call a professional HVAC technician to inspect the furnace and replace the fan limit switch if necessary.

How to Replace a Furnace Fan Limit Switch

If troubleshooting doesn't resolve the problem, you will need to replace the fan limit switch. Replacing the fan limit switch is relatively simple and can be done with a few tools and some basic knowledge. The process involves disconnecting the old switch and connecting the new one. Here are the steps for replacing a furnace fan limit switch:

  1. Turn off the power to the furnace. Turn off the power at the main breaker box.
  2. Remove the old switch. Remove the screws that hold the switch in place and carefully pull it out of the furnace.
  3. Connect the new switch. Connect the wires to the new switch and tighten the screws. Make sure the connections are secure.
  4. Replace the cover. Place the cover back on the switch and replace the screws to hold it in place.
  5. Turn on the power. Turn on the power at the main breaker box and test the furnace to make sure the fan limit switch is working properly.

Safety Precautions when Working with a Furnace Fan Limit Switch

It is important to take safety precautions when handling a furnace fan limit switch. Always wear protective gloves, goggles, and a dust mask when working on the furnace. It is also important to turn off the power to the furnace at the main breaker box before beginning any work. Additionally, make sure that all of the wiring is connected correctly and securely and that no wires are frayed or broken.

Can I Bypass a Faulty Fan Limit Switch in a Furnace?

Although it is possible to bypass a faulty fan limit switch in a furnace, it is not recommended. Bypassing the limit switch can cause damage to the furnace and lead to serious safety issues. Additionally, bypassing the limit switch may void your warranty. It is best to replace the limit switch with a new one to ensure that the furnace is functioning properly and safely.

Is It Possible to Reset a Tripped Fan Limit Switch in a Furnace?

Yes, it is possible to reset a tripped fan limit switch in a furnace. To reset the switch, turn the knob counterclockwise until it clicks. After resetting the switch, wait a few minutes and then turn on the power to the furnace to see if it is working properly. If it is still tripped, you may need to replace the switch.

A furnace fan limit switch is an important part of the heating system in your home. It helps to regulate the amount of heat that the furnace produces by preventing it from getting too hot and overworking. If the switch is malfunctioning, it is important to troubleshoot and replace it as soon as possible. It is also important to take safety precautions when handling the furnace fan limit switch to avoid injury or damage to the furnace.


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