A New Defrost Methodology... 1

[KFLACK]

Is there an easy way to initiate a defrost cycle with a heat pump from an external source ?

I.E. Can a defrost cycle be initiated by simply jumpering 24 V AC to the DFT terminal on a Defrost Card ?

What signally does it need and what signalling is needed to terminate the defrost request...

 

[imok2]

Time-temperature defrost controls typically are equipped with a field check capability that includes speeding up the defrost cycle. Most are similar in operation. The typical steps to a “forced” defrost may include:
•Turn off the power to the outdoor unit
•Disconnect the outdoor fan motor lead from the fan board or defrost control board and prevent it from grounding
•Restart the unit in the heating mode and allow frost to accumulate on the outdoor coil
•After a few minutes in the heating mode, the liquid line temperature should drop to the closing temperature setting of the defrost thermostat (approximately 32 degrees)
•Short between any “speed-up” terminals on the defrost board or make any appropriate switch setting if so equipped
•The shorting or switching usually accelerates the timing sequence around 256 times (so 30 minutes would become 7 or 8 seconds, etc.)
•You will hear the reversing valve change position as you will be beside it during this test
•When it reverses, you should remove the shorting device from the probes or move the switch back to normal setting immediately (otherwise the control board will terminate the normal defrost cycle in approximately 2 to 3 seconds)
•Note: the length of the defrost cycle is dependent on the length of time it takes to remove the shorting device from the test probes or moving the test switch after the reversing valve has shifted position
•The unit should remain in the defrost mode for the remainder of the defrost cycle or until the defrost thermostat opens at approximately 65 degrees outdoor coil temperature
•You will then de-energize the power to the outdoor unit and reconnect the outdoor fan motor lead back on the fan board or defrost board/

Some trade schools in the past have taught that to close the defrost thermostat, you can simply disconnect the wires to the defrost thermostat from the control board and short them together. This method is not recommended by manufacturers.