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

1. 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.
2. 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.
3. 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.
4. 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.

The refrigerants that make it possible

Modern UK heat pumps mostly use R32 or the more recent R290 (propane) refrigerant, both chosen for a low global warming potential compared with older refrigerants. R32 boils at around −52°C and R290 at around −42°C, far below any UK outdoor temperature, which is precisely why the evaporator coil can still pull heat out of air that feels cold to a person. The refrigerant doesn't need the outside air to be warm, it just needs to be warmer than the refrigerant itself, and at −42°C to −52°C that condition holds even in a hard UK frost.

Why this matters for your bills

Because the compressor is only "lifting" heat rather than creating it from scratch, a heat pump can deliver more energy as heat than it consumes as electricity, a gas boiler by contrast converts fuel to heat at roughly 88% efficiency at best, always less energy out than in. This is the entire basis of a heat pump's running cost advantage over gas, provided the electricity price per kWh isn't too many multiples of the gas price per kWh. See our running costs guide for the exact maths on when that holds.

Why the same principle powers your fridge and air conditioner

A fridge is a heat pump that moves heat out of its insulated box and dumps it into your kitchen, that's why the coils on the back feel warm. An air conditioner is the same cycle again, moving heat from inside a room to outside. A heat pump for home heating is the same machine running in the useful direction: moving heat from outside (even cold outside) into your home. Once you've understood one of these three appliances, you've understood all of them, only the direction of heat flow and the temperatures involved change.

What size the compressor needs to be

The compressor is sized to match your property's heat loss on the coldest day it's realistically likely to see (the "design day"), not the average UK winter day. A compressor that's too small struggles to keep up during a cold snap; one that's too large cycles on and off more than it should, which wastes energy and wears components faster. This is the core reason a proper heat-loss survey matters more than picking a heat pump by brand or headline kW rating, the number on the box only means something once it's matched against your specific property's heat loss calculation.

Why heat pumps sound different from a gas boiler

A gas boiler is largely silent because combustion happens inside a sealed unit with a small fan. A heat pump's outdoor unit has a genuinely larger fan moving a much greater volume of air across the evaporator coil, plus the compressor itself, so it produces continuous background sound rather than the boiler's occasional hum. Manufacturers have reduced this significantly over the past decade, modern units run at 40 to 45 dB at 1 metre, but it's a genuinely different noise profile from a gas boiler and worth being aware of, particularly for units sited near bedroom windows or close to a neighbour's boundary.

The MCS Permitted Development Rights standard caps outdoor unit noise at 42 dB measured at the nearest neighbour's window, which is why installers run a noise assessment as part of the design, not just the heat-loss survey. A well-sited unit on a reasonably sized plot rarely comes close to that limit in practice, it mostly becomes a real design constraint on very tight terraced sites where the nearest window sits close to the only viable outdoor wall.

Understanding the physics also explains why heat pumps sometimes get an unfair reputation from early, poorly designed installs. A heat pump sized and commissioned correctly for its property performs close to its rated SCOP; one bolted onto an undersized radiator system with a flow temperature set too low performs poorly not because the underlying physics failed, but because the design didn't account for it. This is the single most common cause of disappointing heat pump experiences reported in the UK, and it's almost always fixable with a proper redesign rather than a reason to distrust the technology itself.