How Does a Heat Pump Work? Plain-English Guide for UK Homeowners

A heat pump does not burn fuel to create warmth — it moves heat from outside air or the ground into your home using a refrigerant cycle. That distinction matters enormously: moving heat is far more efficient than generating it from scratch, which is why a modern air source heat pump can deliver three units of warmth for every one unit of electricity it consumes. This plain-English guide explains how a heat pump works, what the efficiency numbers mean, and what you need to know before choosing one for a UK home.

The refrigerant cycle: how a heat pump actually moves heat

The heart of every heat pump is a closed loop of refrigerant — a fluid engineered to boil and condense at very low temperatures. The cycle has four stages:

1. Evaporator. Outdoor air (or ground-loop fluid) passes over the evaporator coil. Even on a cold UK day — at 0 °C or below — the refrigerant inside the coil is colder still, so it absorbs heat from the air and evaporates into a low-pressure gas.
2. Compressor. The electric compressor raises the pressure of the refrigerant gas. Compressing a gas raises its temperature: the refrigerant can now reach 60–80 °C, far above what the evaporator captured.
3. Condenser. Hot, high-pressure refrigerant flows into the condenser (a heat exchanger), where it transfers its heat to the water circulating in your central heating and hot water cylinder. As it gives up heat, it condenses back into a liquid.
4. Expansion valve. The liquid refrigerant passes through an expansion valve, dropping in pressure and temperature, ready to absorb heat from outside again. The cycle repeats continuously.

The same cycle, run in reverse, is what makes a refrigerator or air conditioner work — a heat pump is simply a refrigerator pointed the other way.

COP and SCOP: what the efficiency numbers mean

Efficiency in a heat pump is expressed as the Coefficient of Performance (COP): the ratio of heat output to electricity input at a given moment. A COP of 3.0 means 1 kWh of electricity produces 3 kWh of heat. For comparison, a gas boiler at 90% efficiency produces 0.9 kWh of heat per kWh of gas burned — a ratio of 0.9, not 3.

COP varies with outdoor temperature and flow temperature, so manufacturers also publish SCOP (Seasonal COP), which averages performance across a full heating season. SCOP is the figure that matters for calculating your annual running costs.

Real-world SCOP in UK retrofits typically falls between 2.5 and 3.5. The Energy Saving Trust’s heat pump field trial, published on GOV.UK, recorded an average SPFH4 (a whole-system efficiency metric) of 2.45 for air source heat pumps in its Phase I dataset. A later BEIS Electrification of Heat demonstration project — monitoring 742 homes — recorded an average SPF of 2.81. Well-optimised systems with good insulation and low flow temperatures regularly achieve SCOP 3.5 or above.

For a full breakdown of how SCOP drives your energy bills, see our guide to air source heat pump running costs.

Why heat pumps are more efficient than any fossil fuel

A gas boiler converts chemical energy in gas into heat. Because of thermodynamic limits and flue losses, even a premium A-rated condensing boiler tops out at around 92% efficiency. No amount of engineering can push a combustion system above 100% — it cannot generate more energy than it burns.

A heat pump is not subject to the same limit because it moves heat rather than creating it. The electricity drives the compressor; the heat delivered to your home comes mostly from outdoor air or the ground, which are effectively free energy sources. As long as there is a temperature difference to exploit, the system delivers more energy as heat than it consumes as electricity. On a mild UK autumn day, ASHP COP can exceed 5.0.

This principle is also why heat pumps lose efficiency on very cold days: a smaller temperature difference between outside air and the evaporator means the refrigerant picks up less heat per cycle. At −10 °C, most air source heat pumps still operate, but COP may drop to 1.5–2.0. In the UK, temperatures rarely stay that low for extended periods, so seasonal averages remain well above 2.

Air source vs ground source heat pumps

The refrigerant cycle is identical in both types — the difference is where the heat comes from.

Air source heat pumps (ASHP)

An ASHP extracts heat from outdoor air using an outdoor unit with a large fan and evaporator coil. Installation is relatively straightforward: the outdoor unit sits on a concrete plinth or wall brackets, connected to the indoor hydraulic unit and hot water cylinder by refrigerant pipes. Most UK houses can accommodate an ASHP without major groundworks. The system runs on R32 or R290 refrigerant: R32 has a Global Warming Potential (GWP) of 675 and is classified A2L (mildly flammable); R290 (propane) has a GWP of just 3 and is A3 (highly flammable but used in sealed, factory-charged units). Manufacturers including Vaillant, Mitsubishi, and Daikin have shifted heavily toward R290 as the UK and EU tighten F-gas regulations.

Typical installed cost: £9,000–£14,000, reduced to £2,000–£7,000 after the Ofgem-administered Boiler Upgrade Scheme (BUS) grant of £7,500.

The outdoor unit produces a low hum during operation — typically 40–50 dB at one metre, similar to a domestic fridge. UK permitted development rules require the noise level not to exceed 42 dB(A) at the nearest neighbour’s window (MCS 020 standard). For a detailed look at how heat pump noise is assessed and which models are quietest, see our heat pump noise guide.

Ground source heat pumps (GSHP)

A GSHP circulates a water-glycol mixture through pipes buried in the ground (horizontal trenches or vertical boreholes), where the ground stays at a stable 10–12 °C year-round in the UK. Because the heat source temperature is higher and more constant than outdoor air, GSHPs achieve consistently higher SCOP — typically 3.5–4.5. The tradeoff is installation cost and land requirement: horizontal collectors need roughly 2–3 times the heated floor area; boreholes go 80–150 m deep and cost £1,500–£2,500 each. Total installed costs typically run £20,000–£35,000 before the £7,500 BUS grant.

For most UK homeowners, an ASHP is the practical starting point. GSHPs make most sense where land is available and the higher efficiency is needed to offset a high heat demand.

How heat is distributed around the home

Heat pumps work best when delivering heat at low flow temperatures — typically 35–55 °C — rather than the 65–80 °C a gas boiler might use. This has two implications for your home:

• Underfloor heating is the ideal partner: it operates at 30–40 °C flow temperature and has a large surface area, so a lower-temperature fluid heats the room just as effectively. SCOP is maximised.
• Radiators need to be larger than standard. If your existing radiators were sized for a 70 °C boiler flow, they will feel barely warm at 45 °C. An MCS-certified installer should carry out room-by-room heat loss calculations and specify radiator upgrades where needed. Oversized radiators are the single most common route to improving a retrofit’s real-world SCOP.

A heat pump also heats hot water, stored in an unvented cylinder at around 55–60 °C. Most heat pumps run a weekly legionella cycle to briefly heat the cylinder to 60 °C, ensuring safe water temperatures.

Is a heat pump right for your home?

A heat pump works in almost any UK property — the BEIS Electrification of Heat project confirmed this across a wide range of house types and ages. Performance depends on three things: insulation quality (lower heat demand = lower flow temperature = higher SCOP), emitter sizing (larger radiators or underfloor heating), and correct commissioning (weather compensation, correct refrigerant charge).

If your home is EPC C or above, or you are planning insulation improvements alongside the installation, an ASHP is likely to deliver running costs competitive with a gas boiler at current energy prices. If your home is EPC D or below with no insulation planned, address the fabric first or carefully model the running costs at realistic SCOP figures before committing. For a detailed comparison of the two options, read our heat pump vs gas boiler guide, which covers upfront costs, running costs, and the cases where each system wins.