heat pumps

not wanting to test the AC function when the outside temp is below 40 degrees, is it safe to assume the compressor is working correctly if the sytem is providing proper heat without using the emergency or back up furnace ?

To start with, in my opinion it’s never safe to assume anything.
However, I’m going to have to assume that this is a heat pump! :slight_smile:

There is no reason you cannot operate a heat pump below 40° (or any other low ambient temperature). During normal operation the heat pump goes into the air-conditioning mode every 45 minutes to perform a defrost operation on the outdoor coil. The heat pump is designed to operate in these adverse conditions and have special components to protect the compressor and allow efficient operation of the air-conditioner in low ambient conditions.

If you can isolate the use of the emergency/auxiliary/backup heaters, then it is safe to say that the unit is operating sufficiently in the wintertime mode. It should be noted that during the winter, the air-conditioning system requires a small percentage of the refrigerant and capacity to perform its operation.
It is not safe to say that the unit will operate correctly in the summer when tested in the winter, under any circumstances. There are different load requirements. You can only say that the unit is operating in both modes and that there is no apparent deficiency when tested in the winter months.

Even when I modify an air-conditioning system (not a heat pump) to operate during low ambient conditions, I can only verify that the compressor and other components are “functioning”. No assessment as to capacity/efficiency can be made about the cooling capacity of the equipment.