is good practice

is good practice to provide supplementary heat for all heat
pump installations in areas where the temperature drops
below 45°F. It is also good practice to provide sufficient
supplementary heat to handle the entire heating requirements
if there should be a failure of heat pump, such as a
compressor failure, or refrigerant leak, etc.
Since the temperature of the liquid refrigerant in the outdoor
coil on the heating cycle is generally below the freezing point,
frost forms on the surfaces of the outdoor coil under certain
weather conditions of temperature and relative humidity,
Therefore, it is necessary to reverse the flow of the refrigerant
to provide hot gas in the outdoor coil to melt the frost
accumulation. This is accomplished by reversing the heat
pump to the cooling cycle. At the same time, the outdoor fan
stops to hasten the temperature rise of the outdoor coil and
lessen the time required for defrosting. The indoor blower
continues to run and the supplementary heaters are
energized