A Heat Pump operates like a reversible Air conditioner. In summer, air flowing across the colder, heat-absorbing side of a Refrigeration apparatus is chilled and ducted through the building, and the heat is vented through an apparatus outdoors. In winter, the flow of the refrigerant is reversed. It absorbs heat through the outdoor apparatus, and the heat is released using the indoor apparatus and ducted through the building.
Heat pumps are inefficient when the outside temperature is below freezing.