Defrost Mode

When a homeowner sees a puff of steam coming out of their heat pump condensing unit, and it is a cold day, the heat pump is probably in the "defrost mode". The defrost mode is a normal function of a heat pump system in colder weather. Icing on a heat pump condenser is normal in cooler weather. The outdoor unit, which is normally an air conditioner, reverses the process and becomes a heater. In the heating mode, the outdoor coil becomes cooler than the outdoor air in order to absorb heat from the outdoor air and pump it indoors. The heat pump has a defrost cycle that will reverse the heat pump back to the air conditioning mode when it detects ice on the outdoor coil. In the cooling mode, the outdoor coil is hot from the heat absorbed from the indoor air. The hot outdoor coil melts the ice and then the defrost control returns the heat pump to the heating mode. During the defrost cycle, the indoor coil is cold, so the auxiliary heat is used to temper the cooler air.