Abstract: An improved heat pump having supplementary electric heat and a bypass feature protecting the outside coil from icing. The heat pump operates conventionally during space cooling. When employed for heating purposes, operation is conventional above the critical ambient temperature below which the heat pump is ineffective. Below this critical ambient temperature, an electric heater heats refrigerant which then proceeds through a conventional vapor compression and heat exchange cycle. However, frigid gaseous refrigerant coming from the indoor coil then bypasses the outdoor coil, and is conducted directly to the heater. Valves isolate the bypass when it is not in use. The bypass prevents the outdoor coil from icing, so that no energy need be expended for de-icing. It also enables rapid switching from supplementary to air based heat source operation, when weather conditions fluctuate. The heater preferably comprises a heating element which threads into a liquid chamber formed in the heater body.