Here is a heat pump from todays inspection. Temps. around mid. 20’s.
The thermostat was operating in aux. mode. I would think that the defrost mode would cycle for the unit outside. Any thoughts.


Most heat pumps will have a timed defrost mode when temperatures are really cold outside (below 40F). What happened in “heat” mode? Sometimes you have to wait 20 minutes for a defrost cycle to kick in, maybe longer. You’ll know when the defrost mode turns on, it sounds different, and the frost melts almost instantly. If it never goes into defrost mode, there are numerous things that could be wrong. Bad sensors, something wrong on the board, valve bad, etc. You can force a defrost by turning on the air conditioning mode for a little while, that’s all a defrost cycle is actually, a short spurt of reverse flow. I’ve never had the opportunity to force a defrost, and I’ve only seen frost on an outside coil a handful of times. We go over defrost cycles in the HVAC Training Course.

It will not go in the defrost cycle when in Aux Mode. Nothing on the heat pump side works in that mode.

Defrost timers do not cycle in 20 min. intervals.
To force a defrost accelerate the timer at the circuit board. You may have to jump the ODT so it doesn’t bypass.