Explainer · Updated monthly

How Does a Heat Pump Work? A Plain-English UK Explainer

Last reviewed · 9 minute read

Heat pumps look like air conditioners but feel like magic. For every kWh of electricity they use, they deliver 3-4 kWh of heat. That sounds like it breaks physics, and it kind of does — until you realise it's not generating heat at all. It's moving heat that was already there.

We're independent — we don't sell heat pumps. What follows is the actual physics, the practical implications for your home, and the things installers often skip explaining.

The refrigerant cycle in four steps

Every heat pump uses a sealed loop of refrigerant — a fluid with a low boiling point. The loop has four stages that repeat continuously while the heat pump runs:

Step 1: Evaporation (outdoor unit)

The refrigerant is in liquid form, very cold — about −20°C. Outdoor air at, say, +5°C feels positively warm by comparison. As the air passes over the outdoor coil, it warms the refrigerant. The refrigerant absorbs this heat and starts to boil, becoming a low-pressure gas.

This is the surprising bit: even cold outdoor air contains usable heat. The refrigerant just needs to be colder than the air for heat to flow into it. This is why heat pumps work in winter — physics doesn't care that the air feels cold to you, only that the refrigerant is colder.

Step 2: Compression

The low-pressure gas is sucked into a compressor — the noisy bit of a heat pump and where almost all the electricity gets used. Compressing the gas raises both its pressure and its temperature, dramatically. A refrigerant that boiled at −20°C ends up at +70°C to +90°C after compression.

This is the key trick: you've spent electricity to compress the gas, but the gas was already carrying heat from the outside air. The compression concentrates that heat to a useful temperature for your radiators.

Step 3: Condensation (indoor unit / heat exchanger)

The hot, high-pressure gas flows into an indoor heat exchanger — typically a coil next to your water cylinder, or against the indoor side of the system. Your radiator water (or in air-to-air heat pumps, your room air) passes over this coil and absorbs the heat. As the refrigerant gives up its heat, it condenses back into a liquid.

This is where you get warm radiators and hot water. The refrigerant is still under high pressure but now at moderate temperature.

Step 4: Expansion (returning to the start)

The liquid refrigerant passes through an expansion valve which suddenly drops its pressure. As the pressure drops, so does the temperature — back to that −20°C starting point. The refrigerant returns to the outdoor unit ready to absorb heat from the outside air again, and the cycle repeats.

Why heat pumps can be "300% efficient"

A gas boiler is, by definition, less than 100% efficient. You burn 1 kWh of gas, you get something like 0.85 kWh of useful heat (the rest goes up the flue or is lost as latent heat).

A heat pump's "efficiency" is measured differently — as a Coefficient of Performance (COP) or, more usefully, a Seasonal Coefficient of Performance (SCOP). It's the ratio of heat delivered to electricity consumed.

If a heat pump delivers 3 kWh of heat for 1 kWh of electricity, its COP is 3.0. Or, if you prefer, it's "300% efficient." This isn't violating any law of physics — the heat pump doesn't generate 3 kWh of heat from 1 kWh of electricity. It uses 1 kWh of electricity to move 2 kWh of heat that was already outside, then delivers 3 kWh total to the inside.

The energy balance works out: 1 kWh electricity (in) + 2 kWh outdoor heat (in) = 3 kWh delivered (out). Conservation of energy holds.

COP vs SCOP — the important distinction

When an installer or brand quotes "COP", they're usually quoting it at one fixed operating condition — typically 7°C outdoor and 35°C flow temperature. That's a flattering condition for the heat pump and gives high numbers (often 4.0–5.0).

SCOP is the annual real-world efficiency, weighted across UK climate variation (warm days when efficiency is high, cold days when it drops). SCOP is what determines your bill. Energy Saving Trust's 2024 field trial of 742 UK heat pump installations found:

  • UK average SCOP: 2.9
  • Well-insulated homes: 3.4
  • Bottom quartile (poorly insulated, oversized, bad install): 2.3

When you're comparing quotes or brands, insist on SCOP figures, not COP. If an installer can't give you SCOP for your specific property design, they don't fully understand their own product.

"But what about cold weather?"

The most-asked question about heat pumps in the UK. Three things to understand:

1. Modern heat pumps work at low temperatures

Most UK-sold air-source heat pumps maintain full heating output down to:

  • Mitsubishi Ecodan: −15°C
  • Vaillant aroTHERM Plus: −15°C
  • Daikin Altherma 3: −28°C
  • Samsung EHS Mono Gen7: −20°C

UK average winter daytime minimum is around +2°C; only the coldest snaps approach −10°C in southern England (slightly colder in northern Scotland). Heat pumps designed for −15°C operation have substantial headroom.

2. Efficiency does drop as it gets colder

A heat pump's COP falls as the gap between outdoor and indoor temperatures widens. At +7°C outdoor, COP might be 4.0; at 0°C, 3.0; at −10°C, 2.2. This is what the SCOP averages across — winter performance is built into the annual figure.

Crucially, this efficiency drop is gradual, not a cliff. A heat pump that runs at COP 3.5 at +7°C will run at COP 2.5 at −5°C — still cheaper per kWh of heat than a gas boiler.

3. The other UK heating story is the demand spike

On the coldest UK day of the year, your home needs more heat — and the grid as a whole sees a national demand spike. Heat pump tariffs like Cosy Octopus and EDF Heat Pump Tracker are designed to incentivise heat pump owners to shift consumption away from those peak demand windows where possible (by pre-heating the home in cheaper windows). This is grid-friendly behaviour and is rewarded with cheaper electricity.

Air source vs ground source vs air-to-air

Air source (the standard)

The outdoor unit pulls heat from outdoor air. Cheapest to install, suitable for ~95% of UK homes. Output drops slightly in cold weather. £10,000–£15,000 typical install before BUS grant.

Ground source (the premium)

The "outdoor" unit pulls heat from buried pipes (either trenches or boreholes). Ground temperatures are far more stable than air (~10°C year-round), giving consistently higher SCOPs (4.0–4.5). Much more expensive to install (£20,000–£35,000) because of groundworks. Payback only stacks if you'll stay 15+ years.

Air-to-air (the new BUS option)

The same air-source principle, but instead of heating water in radiators, it blows warm air directly into rooms via wall-mounted indoor units. Same as a split-system air conditioner running in reverse. No hot water — you'd need a separate immersion or hot-water heat pump. Live under BUS at £2,500 since 28 April 2026; cheaper to install (£4,000–£8,000) and faster to retrofit.

What this means for your home

Lower flow temperatures

Heat pumps work best when the water leaving the unit ("flow temperature") is between 35°C and 55°C. Gas boilers typically ran at 65–75°C. Cooler radiator water means:

  • Radiators feel warm rather than hot. They still heat the room, they just don't need to be scalding.
  • Some radiators may need to be larger to deliver the same heat output at the lower temperature. Typically 2–4 radiator changes per house — budget £200–£400 each.
  • You'll need a hot water cylinder (typically 250–300 litres). If you have a combi boiler, this means adding the cylinder somewhere.

Continuous operation, not on/off

Gas boilers are designed to blast heat fast and switch off. Heat pumps prefer to run continuously at lower output, holding the house at temperature rather than letting it cool and then reheating. This often surprises people switching from gas — leaving the heat pump on all day is usually cheaper than turning it off when you go to work.

Weather compensation

Modern heat pumps adjust their flow temperature based on outdoor temperature — colder outside, slightly warmer flow. This is called "weather compensation" and it's key to getting good SCOPs. A badly-tuned weather compensation curve is one of the most common causes of disappointing real-world performance.

What the installation actually involves

A typical UK air source heat pump installation takes 3–5 days and includes:

  1. Removing the old gas boiler (or oil/LPG boiler, or storage heaters).
  2. Installing the outdoor unit on an external wall or ground stand, with ~1m clearance.
  3. Installing the indoor unit, hot water cylinder, and any buffer tank.
  4. Replacing 2–4 radiators (identified during the MCS HP-001 heat-loss survey).
  5. Updating controls — replacing your boiler thermostat with a heat-pump-friendly one.
  6. Commissioning — setting weather compensation curves, tuning flow temperatures, training you on the controls.

The Boiler Upgrade Scheme grant goes to the installer; you pay only the difference. See our BUS guide for the full process and the upcoming £9,000 oil/LPG uplift.

Next steps

Frequently asked questions

Is "300% efficient" really true?

Yes, but it's not generating energy from nothing. The 300% number is the ratio of heat delivered (in kWh) to electricity used (in kWh). The extra heat comes from outdoor air. Total energy balance: 1 kWh electricity in + 2 kWh outdoor heat in = 3 kWh heat out. Conservation of energy is preserved.

Do heat pumps work in really cold weather?

Yes. UK-sold heat pumps work down to −15°C (Mitsubishi, Vaillant) or −28°C (Daikin). Their efficiency drops as it gets colder, but they keep producing heat. The "heat pumps don't work in winter" story originates from earlier US/Canadian installations using technology from the 1990s.

Are heat pumps noisy?

Modern units run at 40–50 dB(A) at 1 metre. Mitsubishi Ecodan is the quietest at 45 dB(A). That's quieter than a fridge. You'll hear them outside if you stand next to them; you rarely hear them inside the house.

How long does a heat pump last?

15–20 years for the main unit; longer for the indoor components. Gas boilers typically last 10–15 years. Heat pump warranties are usually 5–7 years on the manufacturer side, with insurance-backed workmanship warranties of 6–10 years from the installer.

Will my electricity bill go up?

Yes — electricity consumption rises by 7,000–9,000 kWh/year compared to a non-heat-pump household. But your gas bill drops to zero (or oil/LPG bill drops to zero) and the net effect on your annual heating cost is a £200–£400 reduction on the right tariff. Use our running cost calculator for your specific numbers.

Can a heat pump replace my combi boiler?

Yes, but you'll need to add a hot water cylinder. Combi boilers heat water on demand; heat pumps need a storage tank for hot water. This is the main reason flats often need air-to-air rather than full air-to-water heat pumps — combi boiler installs without cylinder space.

How big is the outdoor unit?

About the size of a large suitcase, typically 1m wide × 0.7m tall × 0.4m deep. It's mounted at ground level next to an external wall or on a small frame against the house.

Sources

Page changelog

  • 19 May 2026 — Initial publication.

See what a heat pump would actually cost you to run

Our calculator uses your current fuel use and home insulation to model annual cost on the standard cap, Cosy Octopus, EDF Heat Pump Tracker and the standard gas comparison.

Try the running cost calculator →