How to Adjust a Honeywell Fan Limit Switch

The Honeywell L4064 is a combination fan limit switch installed in a wide variety of forced air furnace applications. The switch senses temperature in the furnace plenum and controls on/off actuation of the blower fan. When the thermostat ignites the gas burners, the fan limit switch delays blower start-up until a preset temperature is reached in the furnace plenum. After proper room temperature is achieved and the thermostat shuts down the gas burners, the fan limit switch continues to run the blower until the heat exchanger has reached a preset minimum to prevent damage from residual heat. The Honeywell L4064 also incorporates a Limit Off setting to shut off the gas burners if plenum temperature exceeds safe maximums at any point in the furnace cycle. Symptoms of switch misadjustment include a blower that runs continuously, cycles off and on at rapid intervals or shuts off too soon. Honeywell provides limited adjustment of Fan Off and Fan On parameters with two adjustment tabs on the dial face of the switch. The Limit Off setting is a safety function and is not user adjustable.

Screwdriver

Instructions

- 1
  Turn off electrical power to the furnace.
- 2
  Remove the two screws securing the cover to the fan limit switch and remove the cover.
- 3
  Locate the Fan Off adjustment tab. It's the furthest tab to the left on the round silver adjustment dial of the fan limit switch.
- 4
  Hold the silver adjustment dial in place while sliding the Fan Off adjustment tab to the proper temperature setting inscribed on the dial. The Honeywell factory temperature setting for the Fan Off limit is approximately 95 degrees Fahrenheit.
- 5
  Locate the Fan On adjustment tab. It's the second tab from the left on the round silver adjustment dial of the fan limit switch.
- 6
  Hold the silver adjustment dial in place while sliding the Fan On adjustment tab to the proper temperature setting inscribed on the dial. Depending upon furnace manufacturer's specifications, Fan On settings cover a range from 20 degrees Fahrenheit above the Fan Off setting to a maximum of 30 degrees Fahrenheit below the Limit Off setting. The Honeywell factory temperature setting for the Fan On tab is approximately 130 degrees Fahrenheit.
- 7
  Replace the fan limit switch cover and secure with two screws.
- 8
  Restore electrical power to the furnace.

How to Calibrate a Honeywell Anticipator

How to Change the Ignitor on a Trane Heater

Heating & Cooling Repair