How heat pumps actually work

"Heat pump" is a slightly odd name for something that also cools a room, so it's worth clearing up the confusion first: a heat pump doesn't create heat or cold. It moves heat from one place to another. In summer, it moves heat from inside your home to outside. In winter, it reverses and moves heat from outside to inside — yes, even when it's cold out there. That single trick, run in two directions, is the whole system.

The four-step loop that makes it work

Every heat pump runs the same continuous cycle, using a refrigerant — in our installations, R32 — that changes between liquid and gas at a very low temperature. Here's the loop, in heating mode:

Run that same loop backwards — using a component called a reversing valve to flip the flow direction — and you get cooling instead: the indoor coil absorbs heat from the room, and the outdoor coil dumps it outside. Same refrigerant, same compressor, same physics. It's genuinely just running in reverse.

Why R32 is well suited to this job

Refrigerants are chosen partly for their boiling point at atmospheric pressure — R32's is around −52°C, much colder than a UK winter's day. In the sealed system, the compressor and expansion device control the actual operating pressure at each stage, which is what allows the evaporator to keep absorbing heat from outside air even when it's close to freezing outdoors. That's the step most people assume can't work in winter, and it's exactly why it does.

Isn't this just... a fridge?

Yes, essentially. A refrigerator uses precisely this cycle to keep its interior cold by pumping heat out into your kitchen — which is why the back or underside of a fridge always feels warm. A heat pump air conditioning system is the same mechanism, just built at a larger scale, with the ability to reverse direction, and with an outdoor unit instead of a kitchen to dump the heat into (or draw it from).

How is this different from a gas boiler?

A boiler burns fuel to generate heat directly — you get out roughly the same amount of energy you put in, at best. A heat pump doesn't generate heat at all; it relocates heat that already exists in the outside air. That's why heat pumps can deliver 4 to 5 times more heating energy than the electrical energy they consume — there's no equivalent multiplier for burning fuel directly.