heat pump

A heat pump is the same device as a refrigerator, but you care about the warm side instead of the cold side. Rather than cooling an interior, it heats a building by pumping heat from the chilly outdoors into the cozy indoors. It sounds backward — extracting warmth from cold winter air — but even cold air holds plenty of thermal energy, and a heat pump uses a modest amount of electrical work to shovel that energy indoors where you want it.

In one cycle a heat pump absorbs heat Q_c from the cold outside, adds work W from electricity, and delivers Q_h = Q_c + W into the warm house. Its rating is the coefficient of performance for heating, COP = Q_h / W: how much heat you deliver indoors per joule of work paid. Because Q_h = Q_c + W is always larger than W alone, this COP is greater than 1 — a heat pump delivers more heat energy to your home than the electrical energy it consumes, which no ordinary electric heater can do.

This is the heat pump's superpower and why it is central to efficient heating: an electric resistance heater turns 100 joules of electricity into 100 joules of heat, but a heat pump can deliver 300 or more, because most of that heat is scooped free from the outside air rather than manufactured. The honest caveat: the COP falls as the outdoor temperature drops and the gap to the indoor target widens, so heat pumps work hardest, and least efficiently, on the coldest days.