Heat Pump

A heat pump is a device that works like a refrigerator, but 'operating inside out' — instead of being used to cool a small chamber (like a refrigerator) or a room (like an air conditioner), it is used to heat a building or a similarly large structure, by extracting heat from the cold air outside it.

In a heat pump, the evaporator coils are placed outside the building, where they absorb heat from the outside air (even cold outside air still contains some extractable heat, at a temperature above absolute zero); the condenser coils are placed inside the building, where they release the heat absorbed outside into the room air, warming the building's interior. This is, structurally, the exact mirror image of an air conditioner's coil placement (Section 4.9.4) — the same underlying refrigeration cycle, just run to move heat in the opposite direction relative to the building. …