How heat pumps work

Heat pumps don't make heat — they move it. The same thermodynamic cycle that makes your fridge cold makes a heat pump warm. Once you see the cycle, the "but how does it work below zero?" question answers itself.

The four-step cycle

Evaporation. A refrigerant liquid passes through a coil that's exposed to the outside (air, ground, or water). The refrigerant is much colder than the outside source, so heat flows into it and turns the liquid into a gas. Refrigerants used in modern UK heat pumps (R32, R290 propane) boil at around −30 to −40 °C, which is why a heat pump can extract heat from sub-zero air.
Compression. An electric compressor squeezes the gas, which raises its temperature dramatically. The compressor is where the heat pump uses electricity — typically one unit of electricity in, three to four units of heat out.
Condensation. The hot gas passes through a heat exchanger inside the house. Your central heating water flows on the other side of the exchanger and absorbs the heat. The refrigerant gives up its heat and condenses back to a liquid.
Expansion. The high-pressure liquid passes through an expansion valve, which drops its pressure and temperature back down. The cycle repeats.

Why one unit of electricity moves three to four units of heat

This ratio is the Coefficient of Performance (COP). A COP of 3.5 means 1 kWh of electricity at the wall produces 3.5 kWh of heat in your radiators. The "extra" energy isn't created; it's pulled from the outdoor air, ground or water. The compressor provides the "lift" that moves heat from cold outside to warm inside.

The further you ask the heat pump to lift — i.e. the colder the outside and the hotter your radiator water — the harder the compressor works and the lower the COP. This is why low flow temperatures matter and why a heat pump designed for 35 °C flow runs at roughly twice the efficiency of one designed for 60 °C flow.

SCOP: efficiency over a year

COP is a moment-in-time measurement. SCOP — Seasonal COP — averages the heat pump's efficiency across a typical UK heating season, weighted for the temperatures it actually meets. SCOP is what affects your bills.

Well-designed ASHP install with modern radiators: SCOP 3.5 – 4.5
Retrofit ASHP with mixed radiator sizes: SCOP 2.8 – 3.5
GSHP install with underfloor heating: SCOP 4.0 – 5.0

"What about when it's below freezing?"

Modern heat pumps work down to −15 °C or lower. Output drops with temperature, so the system has to be sized for the coldest day you'll realistically see (the "design day"). For most of the UK that's around −2 to −4 °C, falling to −10 °C in highland Scotland.

At the coldest hours of the coldest days, a heat pump may need to switch on its defrost cycle — briefly running in reverse to clear ice off the outdoor coil. Manufacturers manage this automatically. You'll see a few minutes of mist near the unit; nothing more.

What heat pumps do not do well

Sudden heat-up. A heat pump heats a property slowly and steadily. It's not designed to recover from a cold house in two hours — that's a gas-boiler workflow. Modern thermostats with weather compensation manage this transparently, but you'll want to learn the system's rhythm rather than fight it.
Heating poorly-insulated properties cost-effectively. The colder the radiator water you need, the higher the SCOP, but that demands larger radiators and decent insulation. Bring those up first if needed.
Producing hot water at boiler-flash speeds. A heat pump heats a cylinder over an hour or two. If you want a 90 °C bath instantly, a heat pump isn't your tool; if you're happy with a stored cylinder at 50 °C, it's perfect.