Abstract: A system and method for defrosting an air source heat pump are presented. A defrost conduit extends from the discharge from a system compressor to the outdoor coil. The defrost conduit includes a normally closed defrost valve. When the need for defrost exists, the defrost valve is opened, and warm refrigerant is delivered from the compressor discharge to the outdoor coil to effect defrost of the outdoor coil.

Abstract: A heat pump system is presented having a primary compressor, a booster compressor, a multi-step thermostat, an indoor coil, an outdoor coil, a refrigerant flow conduit connecting these elements is a closed loop system, and at least one pressure sensor positioned to always sense the refrigerant system low side pressure, which is commensurate with outdoor ambient air temperature in all heating modes of operation. The pressure sensor functions as an override and safety control to prevent or permit operation of the compressors, and also other system components, depending on outside air temperature. The primary compressor is preferably a multi-capacity compressor. A defrost energy transfer fluid is used in a defrost cycle.