Defrosting Thermostats: Learning & Understanding Heat Pumps

Now that we understand what the defrost thermostat does, let’s follow the service manual on what ways we can troubleshoot the board and defrost thermostat.

First, we know that on residential and small package products the defrost thermostat closes at 32° ± 3°F and opens at 65° ± 5°F so if you find one of our heat pumps running on heat mode but has a mass of ice or frost around the coil as well as at the defrost thermostat but it is still not going on defrost mode, then you need to determine what is causing this issue using your multimeter. Start by removing the small Molex connector with the two pink wires attached to the DFT from the defrost board and check what resistance or ohms you have between those two wires. OL or infinite means the thermostat still open. If that’s the case, replace the defrost thermostat which we know, at the temperature that surrounds the defrost thermostat, should be closed.

Second, we need to know if the defrost board works properly. With the same scenario, heat pump still running on heat mode, we need to test the board. With the Molex or pink wires removed from the DFT, you have two pins exposed. Attach an alligator jumper wire between the pins. Next, utilize a metal tool, like a flat screw driver, to short or connect the two speed up pins on the board. Make sure you count 5 to 7 seconds. It depends on the type of defrost board, but if you have the low voltage on the red wire from inside the unit, the logic will switch to defrost mode. When you hear the “swoosh” from the reversing valve, remove the screw driver from the speed up pins, otherwise will have a very short defrost. Once on defrost mode, you can confirm that the board is reacting to the defrost thermostat by removing the alligator jumper wire and the heat pump should switch to heat mode again.

An example below of one defrost board with the jumper on DFT terminals and with the screwdriver shorting the speed up pins. Wires were removed for clarity.