Learn & Understand

Beating 100%: Why a Heat Pump Isn't Magic

A heat pump can deliver several times more heat energy than the electrical energy it consumes — a claim that sounds like a violation of physics until you understand what it is really doing. It does not create energy; it moves it. The thermodynamics behind this, tracing back to a young French engineer, reveals why moving heat beats making it.

Moving Heat, Not Making It

An electric resistance heater turns electricity into heat at best one-for-one — its coefficient of performance is fixed at one. A heat pump does something cleverer: it uses electricity to pump heat that already exists in the outside air or ground into the house. Because it relocates far more heat than the electricity alone could produce, its output-to-input ratio can be three, four, or more — no energy created, only transported.

Carnot's Insight

In the nineteenth century Sadi Carnot analyzed the ultimate limits of heat engines and, by extension, heat pumps. He showed that the best possible performance depends on the temperature difference across which heat is moved: the smaller the gap between the source and the destination, the more efficiently heat can be pumped. This Carnot limit is the theoretical ceiling no real machine can exceed.

Why Cold Weather Hurts

Carnot's temperature-difference rule has a direct consequence: as the outdoor air grows colder, a heat pump must move heat across a larger gap to warm the house, and its efficiency falls. There is also simply less ambient heat to extract from frigid air. This is why air-source heat pumps lose performance in deep cold, and why manufacturers publish efficiency at specific outdoor temperatures.

The Second Law's Blessing and Limit

The second law of thermodynamics says heat naturally flows from hot to cold; a heat pump pays energy to push it the other way, from cold outdoors to warm indoors. The law permits this — at a cost set by the temperature difference — and forbids doing it for free. A heat pump's remarkable efficiency and its cold-weather decline are two faces of the same fundamental physics.

Calculating Performance

To compute a heat pump's coefficient of performance, use the COP Calculator. Compare cooling efficiency with the EER Calculator, and size the equipment with the Heat Pump Sizing Calculator.