Domestic heat pump and defrost cycle below 42 degrees

[howardwheeler]

I had a Duo Therm heat pump that wasn’t going into defrost in temps below 42 and above 30. I repositioned the ambient sensor to one of the vent holes to the outside air and it started working. The factory position apparently lets enough heat come up through the box that it affects the sensor, which the factory screwed right to the base near the evaporator which acts like the condenser when in heat mode. Everything seemed great but later on when the temperature reached very close to 42, the defrost cycle went through two strange cycles. The first only lasted 2 minutes instead of the normal five, and the second lasted only a few seconds then the fan came back on. My question is what controls the length of time the defrost cycle lasts and if the sensor reached 42 while running in heat would it immediately shut of the defrost cycle? If the sensor has no bearing on the length of the defrost cycle then I also have a bad control box. Hopefully the ambient sensor has something to do with it, which would simply mean that the ambient temperature had gone above the set point and properly shut down the heat pump. Anyone know what controls the duration of the defrost cycle?

[howardwheeler]

That is what I feared. So I also have an erratic control board. Moving the ambient sensor definitely made a big difference. Before I couldn’t get it to go into defrost until it was in the low 30s. Now it starts in the low forties but I have definitely had two short cycles of the defrost segment. So I need another control box. Thanks Doug

[sayoung]

dougrainer wrote:
The Main Control board is what determines the length of Defrost. Once the Sensor indicates it needs the defrost cycle, the control board then starts the sequence and stops it. Doug

The control board should also have a max defrost time limit . I have never serviced a rv heat pump but on residential/commercial units the defrost sensor should be mounted on the outside coil ( condenser ) as it gets cold in heating mode, 15 to 20 degrees below ambient temperature.

[dougrainer]

The Main Control board is what determines the length of Defrost. Once the Sensor indicates it needs the defrost cycle, the control board then starts the sequence and stops it. Doug

[PaulJ2]

howardwheeler wrote:
I had a Duo Therm heat pump that wasn’t going into defrost in temps below 42 and above 30. I repositioned the ambient sensor to one of the vent holes to the outside air and it started working. The factory position apparently lets enough heat come up through the box that it affects the sensor, which the factory screwed right to the base near the evaporator which acts like the condenser when in heat mode. Everything seemed great but later on when the temperature reached very close to 42, the defrost cycle went through two strange cycles. The first only lasted 2 minutes instead of the normal five, and the second lasted only a few seconds then the fan came back on. My question is what controls the length of time the defrost cycle lasts and if the sensor reached 42 while running in heat would it immediately shut of the defrost cycle? If the sensor has no bearing on the length of the defrost cycle then I also have a bad control box. Hopefully the ambient sensor has something to do with it, which would simply mean that the ambient temperature had gone above the set point and properly shut down the heat pump. Anyone know what controls the duration of the defrost cycle?

I only know that on my stick and brick home unit the defrost cycle is activated when the air flow through the condenser is restricted enough by ice build up on the coils. Don't know how it knows to stop the defrost cycle, but i suspect it may be a timed thing.