The technology explained — without the jargon. Why heat pumps are efficient, what the components do, and what a good installation looks like.
The key insight: A heat pump doesn't generate heat — it moves it. That's why it can deliver 3–4 units of heat for every 1 unit of electricity it uses. You can't do that with combustion.
Your kitchen fridge works by pumping heat out of the food compartment and releasing it into your kitchen. Touch the back of a running fridge — it's warm. That warmth came from inside the fridge.
An air source heat pump does the same thing in reverse: it pumps heat in from the outdoor air and releases it into your home via your radiators or underfloor heating. The outdoor unit contains a refrigerant that evaporates at low temperature, absorbs heat from the air, is compressed (which raises its temperature), and then releases that heat inside your home.
Even when outdoor temperatures drop to −10°C, there is still significant heat energy in the air. Modern heat pumps are designed to extract it efficiently, which is why they work well in the UK climate.
Contains the evaporator (which absorbs heat from the air), a fan to draw air across it, and the compressor. This is the box you see outside the house — typically about the size of a large suitcase.
The condenser transfers heat from the refrigerant to your home's water circuit. A hot water cylinder stores domestic hot water. Some systems combine these into a compact indoor unit.
Unlike gas boilers, heat pumps should run continuously at low flow temperatures rather than blasting heat in short bursts. Weather compensation adjusts the flow temperature automatically based on how cold it is outside.
Radiators or underfloor heating that distribute heat around your home. Heat pumps typically run at 35–50°C flow temperature (versus 70–80°C for a gas boiler), so radiators may need to be larger.
COP stands for Coefficient of Performance. It's the ratio of heat delivered to electricity consumed. A COP of 3.5 means the pump delivers 3.5 kWh of heat for every 1 kWh of electricity used.
The SCOP (Seasonal COP) is the average efficiency across an entire heating season — a more realistic figure. Modern UK-installed heat pumps typically achieve an SCOP of 2.8–4.0 depending on the home's insulation and the design of the heating system.
Flow temperature matters enormously. A heat pump running at 35°C flow temperature will have a SCOP around 3.5–4.0. The same pump at 55°C might achieve only 2.5. Proper system design — matching the flow temperature to properly sized radiators — is what separates a great installation from a disappointing one.
Every good heat pump installation starts with a heat loss calculation — a calculation of exactly how much heat your home loses on the coldest day of the year in your region. This determines the correct size of pump to install.
Oversizing is the biggest cause of bad installations. An oversized heat pump short-cycles — it reaches target temperature quickly, turns off, then cycles back on repeatedly. This wastes energy, causes wear, and produces the "my heat pump doesn't work" stories that damage the technology's reputation. The problem is poor design, not poor technology.
Before agreeing to any installation, ask your installer: "Can you show me the heat loss calculation for my home?" If they can't or won't, find someone else.
Our free calculator gives you a simplified heat loss estimate based on your home's type, age, size, and location. It uses CIBSE regional design temperatures — the same data professional engineers use. It's not a substitute for a full survey, but it gives you a credible ballpark to go into installer conversations with.
| Factor | Gas boiler | Air source heat pump |
|---|---|---|
| Fuel | Natural gas | Electricity |
| Efficiency | 85–95% | 300–400% (SCOP 3–4) |
| Running cost* | ~£900–1,400/yr | ~£700–1,100/yr |
| Carbon emissions | High (gas) | Low (grid electricity) |
| Installation cost | £2,000–4,000 | £8,000–15,000 (before grant) |
| Grant available | No | £7,500 BUS grant |
| Typical lifespan | 12–15 years | 20–25 years |
| Maintenance | Annual service | Annual service |
| Planning permission | No | Usually permitted development |
*Based on typical 3-bed semi-detached. Ofgem Q1 2025 rates. Actual savings vary.
Run our free calculator to get a heat loss estimate, recommended pump size, and running cost comparison for your specific home.
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