L4064B Fan & Limit Switch Replacement - DANGER

Article ID: 103
Last updated: 18 Oct, 2017

Gas Furnaces

Re: L4064B Fan & Limit Switch

The L4064B controls the on and off operation of the heating unit’s fan motor and provides high limit control of the main burner. It is suitable for all types of forced air heating systems.

The L4064B’s bimetal sensing element turns fan on and off according to plenum temperature.

The L4064B has a manual switch (white push button) to provide continuous fan operation, mounting adapters for replacing competitive devices, adapters for wiring convenience, and a strain relief bushing for protecting the wiring from damage due to field abuse.

Limit contacts are suitable for line voltage, low voltage or millivoltage circuits. The fan-on timing can vary depending on applied voltage and switch ambient.

  • The FAN contacts are "Normally-Open" and close on temperature rise.
  • The LIMIT contacts are "Normally-Closed" and open on temperature rise.


Failure to remove brass jumper, if limit switch is in low voltage circuit (4-wire configuration), WILL cause electrical shock hazard and damage low voltage controls.

To remove jumper, break with needlenose pliers and remove completely. Once removed, it is not replaceable.


When adjusting the fan and limit set point levers, hold the scalepale dial to keep it from turning and straining the sensing element. Move each indicator lever to the control point recommended by the burner or furnace manufacturer. Use gentle finger pressure.


  • Move the FAN OFF lever to the temperature at which the fan is to stop to prevent circulation of cool air.
    • Setting the FAN OFF temperature too high can cause the blower to cycle off and back on after a heating cycle.
    • Setting the FAN OFF temperature too low can cause the blower to stick on after a heating cycle.
  • L4064B - Move the FAN ON lever to the temperature at which the fan is to come on.

MANUAL FAN SWITCH: For constant fan operation, push the FAN switch button in. For fan to cycle automatically, pull button out.

  • It is very easy to accidentally push the Manual Fan Switch in when installing the metal cover.
  • Be sure to pull the switch out and test on/off operation after installing cover.

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Article ID: 103
Last updated: 18 Oct, 2017
Revision: 17
Access: Public
Views: 533
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