How to Install a Defrost Board

The defrost board in a heat pump is the electronic circuitry that determines when the pump goes into the defrost mode.
Heat pumps are essential for minimizing electrical bills.Heat pumps are essential for minimizing electrical bills.
When the coils freeze up and will not defrost, it is likely that the heat pump defrost board is defective. Other possibilities include the two temperature sensors that attach to the board. The defrost board costs less than $100 and it can be replaced in less than half an hour. You can easily save at least $200 in repair labor costs if you do it yourself. No special tools are required. The defrost circuit board is the same part across a number of heat pump brands, including Lennox, Kenmore, Tempstar, Whirlpool and at least seven other manufacturers.

Step 1

Shut off the power to the heat pump. The 220 VAC breaker should be located on the building wall next to the heat pump. Remove the screws that secure the access panel to the electronic controls, then remove the access panel.

Step 2

Cut the two sensor wires that attach to the defrost circuit board, if they are soldered onto the board, with the dykes. Strip the ends of the wires and install connectors with the crimpers on the wire ends as needed to connect to the board and transfer the wires to the new part. Remove the other wires one by one from the old board with the needle-nose pliers and, if needed, a small screwdriver. Reinstall them onto the new board one by one.

Step 3

Unscrew the screws that mount the old board in place and remove it. Place the new board over the mount holes and reinstall the screws. Replace the cover of the electrical controls on the heat pump and then turn on the power at the circuit breaker.

Things You Will Need

  • Screwdriver
  • Dykes
  • Wire stripper or razor knife
  • Wire plug connectors (optional)
  • Crimper tool
  • Needle-nose pliers
  • Small screwdriver (optional)


  • Read the heat pump technical manual before you start the job to determine where the defrost board is located.
  • Sometimes, more than two wires will need to be cut to change the terminal ends.
  • Board mounts may also need to be replaced in some cases. A drill and bit may be required to make the mounting holes larger.
  • Replace the coil and air temperature sensors if you are unsure they are working properly.
  • When you replace the connectors on the board, make sure that they are not loose. If they are, then crimp them to make them attach securely.


  • If you are not comfortable doing electrical work, hire a licensed electrician.
  • Make sure that the new replacement board is the right part before starting the job.

About the Author

As a scientist, Randy McLaughlin has been a professional technical writer since 1980. He has a Master of Science from Texas A&M University and a Ph.D. in plant pathology from the University of Wisconsin. McLaughlin covers diverse topics, including Costa Rica, technical guides, alternative healing and spiritual development.