Heat Pumps use refrigerant to both heat and cool. The refrigeration cycle is changed by a valve in the outdoor section called the reversing valve. I service customers in the Wilmington, NC area where it is just beginning to get cold in November. As the air gets colder you may start hearing heat pumps in your area defrosting. When a heat pump defrosts it switches into cooling mode. The reason this happens is because the hot refrigerant coming out of the compressor in cooling mode travels to the outdoor coil. This is the area of the system that forms ice during the winter. The outdoor fan motor will also shut off during defrost to increase the amount of heat the refrigerant retains. The process will send steam into the air, which on many occasions has prompted homeowners to believe their unit is “blowing up”. It does not help that after the defrost cycle is over there is also a large whooshing sound caused by the reversing valve switching the system back into heating mode. Even though this process may look and sound like a system failure, it is a planned and necessary part of the heat pump heating process.