How a heat pump works

A refrigerator pulls heat out of its interior and dumps it into the room behind it — that's why the back of a fridge feels warm. A heat pump does exactly the same thing, just in reverse and at a larger scale: it pulls heat out of the outdoor air (or the ground) and delivers it into your heating water. Even cold air still holds usable heat energy — well below freezing.

The cycle, in four steps

A refrigerant runs through the same loop, over and over:

1. Evaporation (outside): the refrigerant has a very low boiling point. Outdoors, it evaporates even in cold air, absorbing heat as it does.
2. Compression: a compressor squeezes the now-gaseous refrigerant. This sharply raises its pressure and temperature — and this is the step that costs the electricity a heat pump consumes.
3. Condensation (inside): the hot, compressed refrigerant gives up its heat to your heating water and condenses back into a liquid.
4. Expansion: an expansion valve drops the pressure and temperature back down, cooling the refrigerant sharply — and the cycle starts again.

The result: heat travels from outside (cold) to inside (warm) — exactly the direction heat doesn't flow on its own. That takes energy, and the compressor supplies it.

Why this doesn't break physics

A heat pump doesn't create energy — it moves existing heat energy from outside to inside, using electricity only to drive that movement (the compressor). That's why more heat can come out than electricity goes in: part of the heat comes from the environment, not from the socket.

In practice, good heat pumps often deliver three to five times as much heat as the electrical energy put in — exactly how that ratio gets measured and compared (COP, SCOP, JAZ) is the subject of the next chapter.

Remember: the colder it is outside, the harder the compressor has to work to "pump" the same amount of heat — which is why a heat pump's efficiency drops at very cold outdoor temperatures. That's normal and accounted for in sizing, not a fault.

Refrigerant: why R290 matters

Modern heat pumps increasingly use R290 (propane) as the refrigerant — very low global-warming potential (GWP ≈ 3), and efficient even at the higher flow temperatures older, radiator-heated houses need. Older refrigerants like R410A (GWP ≈ 2088) are being phased out.

Monobloc vs split

• Monobloc: the entire refrigerant loop sits in the outdoor unit; only water enters the house. No on-site refrigerant work — usually the simpler install.
• Split: indoor and outdoor units are connected by refrigerant lines — more effort to install and service, though sometimes more efficient in specific setups.

What this means for you

This principle is also why flow temperature (how hot the water needs to be that the heat pump delivers to your radiators) is the single biggest lever on efficiency — more on that in the flow-temperature chapter. And it's why a heat pump behaves fundamentally differently from the oil boiler it replaces (see "Why a heat pump doesn't behave like your oil boiler").