A heat pump is a HVAC compressor that cools outside and heats inside. The unit has a reversing valve that changes the direction of the Freon flow. It is cheaper to operate than a resistive heating circuit alone. It will work efficiently until the temperature drops below 38 degrees. When the temperature drops below the magic number, the restive heat is more effective. In theory the heat pump will work at 32 degrees, but the heat produced is very little.