A heat pump works by extracting heat from outside air, ground or water and transferring it indoors using a refrigeration cycle, achieving efficiencies of 300-400% compared to a gas boiler’s 90%. The Energy Saving Trust confirms that for every 1kWh of electricity used, a heat pump can deliver 3-4kWh of heat (Energy Saving Trust, 2026).
- Heat pumps achieve 300-400% efficiency vs a gas boiler's 90%.
- Four main components: evaporator, compressor, condenser and expansion valve.
- Even at -15°C, air source heat pumps can extract useful heat.
The key variable is the system’s coefficient of performance (COP), which determines running costs. A diagram typically shows four main components: an evaporator, a compressor, a condenser and an expansion valve. This applies to air source and ground source heat pumps, but not to direct electric heating or gas boilers, which lack the refrigerant cycle that makes heat pumps so efficient.
Evaporator absorbs heat from the outside air or ground
In an air source heat pump, the evaporator contains a liquid refrigerant that boils at very low temperatures, often below -20°C. As outside air passes over the evaporator coils, the refrigerant absorbs heat and turns into a gas. The Energy Saving Trust notes that even at -15°C outside, the refrigerant can still absorb useful heat (Energy Saving Trust, 2026). Ground source systems use buried pipes filled with a water-antifreeze mix to capture stable ground temperatures of around 10-12°C year-round.
Compressor raises the temperature of the refrigerant gas
The gaseous refrigerant enters an electrically powered compressor, which pressurises it. This rapid compression dramatically raises the temperature of the gas, often to 50-80°C. The UK government’s Heat Pump Ready programme states that modern compressors can achieve COPs of 3.5 to 4.5 under standard test conditions (GOV.UK, 2026). The compressor is the main consumer of electricity in the system, but the heat output far exceeds the electrical input.
Condenser releases heat into your home heating system
Hot refrigerant gas passes through the condenser, where it transfers heat to your home’s water or central heating circuit. As the gas cools, it condenses back into a liquid, releasing latent heat. The liquid then passes through an expansion valve, which drops its pressure and temperature, ready to start the cycle again. Ofgem confirms that heat pumps can provide space heating and hot water, with flow temperatures typically 35-55°C for radiators or underfloor heating (Ofgem, 2026).
A worked example
For a typical 1930s semi-detached house in Manchester, installing an air source heat pump under the BUS grant costs around £3,500 after the £7,500 government contribution. This homeowner previously spent £1,450 a year on gas heating, but the heat pump now costs just £580 annually to run, saving £870 per year. The payback period on the upfront cost is roughly four years, and over a 25-year lifespan the total savings reach £21,750, assuming current energy prices. The Energy Saving Trust confirms that a well-installed heat pump can achieve a coefficient of performance of 3.5, meaning for every £1 of electricity used, you get £3.50 worth of heat. With 0% VAT on heat pumps until March 2027, this homeowner also saved an extra £175 at installation.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,500 |
| Yearly savings | £870 |
| Payback period | 4 years |
| 25-year lifetime savings | £21,750 |
What homeowners often get wrong
Many people mistakenly believe a heat pump cannot keep a home warm in a British winter. Here are three common misunderstandings that can lead to wasted money or poor performance.
- Thinking a heat pump works like a boiler Heat pumps run at lower temperatures for longer periods, so turning them on and off like a gas boiler actually wastes energy and reduces comfort. The right approach is to set a steady temperature and leave the system running, which can cut running costs by up to 20%.
- Assuming all radiators must be replaced Many older homes can keep existing radiators if the heat pump is correctly sized and the property has basic insulation. Unnecessarily replacing all radiators can add £2,000 to £4,000 to the installation cost, and most homes need only a few upgraded units.
- Ignoring the need for proper insulation first A heat pump performs best in a well-insulated home, so fitting one in a draughty property with no loft or cavity wall insulation can double running costs. The ECO4 scheme can fully fund insulation for eligible households. That makes this a free first step before even considering a heat pump.
Quick reference
- A heat pump diagram shows four key parts: evaporator, compressor, condenser and expansion valve.
- For every £1 spent on electricity, a heat pump can deliver £3 to £4 worth of heat energy.
- You qualify for the £7,500 BUS grant if you own your home and have a valid Energy Performance Certificate with no outstanding recommendations for loft or cavity wall insulation.
- Ground source heat pumps achieve a coefficient of performance of 4.0 or higher, but cost £15,000 to £25,000 to install before grants.
- Installing a heat pump without first sealing draughts can increase annual running costs by up to £300.
Frequently Asked Questions
A diagram typically shows four main components: an evaporator, a compressor, a condenser and an expansion valve. The Energy Saving Trust confirms that for every 1kWh of electricity used, a heat pump can deliver 3-4kWh of heat (Energy Saving Trust, 2026).
The coefficient of performance (COP) for a heat pump typically ranges from 3 to 4. This means for every 1kWh of electricity input, the system outputs 3-4kWh of heat, as confirmed by the Energy Saving Trust.
Yes, air source heat pumps can still extract useful heat even at -15°C outside. The Energy Saving Trust notes that the refrigerant in the evaporator can absorb heat at very low temperatures (Energy Saving Trust, 2026).
