A heat pump is an electric appliance that heats your home (and often your hot water) by moving heat that already exists outside — in the air, the ground, or a body of water — rather than burning fuel to create heat. It's the same technology as a fridge, just running in reverse.
A fridge pulls heat out of the cold inside of the box and dumps it into your warm kitchen — that's why the back of a fridge feels warm. A heat pump does exactly the same thing, but on a much bigger scale: it pulls heat out of the cold outdoors, however chilly, and pumps it into your warm home. It can do this even when it's close to freezing outside, because there is still usable heat energy in the air down to well below 0 °C.
How it works
A small amount of electricity runs a compressor, which is used to concentrate and move heat rather than generate it directly. That's why a heat pump can deliver several times more heat energy than the electrical energy it consumes — typically 3–4 units of heat for every 1 unit of electricity.
Behind that simple picture is a repeating loop: a cold refrigerant gas absorbs heat from outside (the evaporator), a compressor squeezes it to raise its temperature further, the hot gas gives up its heat to your home's water or air (the condenser), and an expansion valve drops the pressure so the cycle can start again. You don't need to understand this to use the database — it just explains why figures like COP, SCOP and flow temperature (covered in our other guides) matter so much.
The three main types
Air source (ASHP)
Draws heat from the outside air via an outdoor unit. Cheapest to install and by far the most common choice in UK homes.
Ground source (GSHP)
Draws heat from buried pipes (a "ground loop"). Higher install cost and needs garden space, but very steady, efficient performance year-round.
Water source (WSHP)
Draws heat from a lake, river, borehole or similar. Excellent efficiency where a suitable water source is available, though this is rare in practice.
A gas boiler burns fuel on-site to generate heat directly; a heat pump uses electricity to move heat that's already there. That fundamental difference shows up across almost every point of comparison:
Heat pump
Gas boiler
How it makes heat
Moves existing heat from outside air/ground/water
Burns natural gas on-site
Typical efficiency
300–400% (COP/SCOP of 3–4)
~90% (some heat lost up the flue)
Running cost driver
Electricity price ÷ efficiency
Gas price ÷ efficiency
Typical running cost
Often on par with or cheaper than gas, but it depends on your electricity-to-gas price ratio and the system's real-world COP — see What is COP and why does it matter?
Tracks the gas price directly; electricity has historically cost several times more than gas per kWh, which is why a heat pump's efficiency matters so much
Emissions (running)
None on-site; depends on the electricity grid mix, which keeps getting cleaner
Direct CO₂ emissions every time it fires, for the boiler's whole life
Output temperature
Most efficient at 35–55 °C — suits underfloor heating and larger radiators
Can run hot, 60–80 °C — copes with small/older radiators
Upfront cost
Higher, though UK grants can offset a significant chunk
Lower
Space needed
Outdoor unit (or ground loop/borehole), often a larger hot water cylinder
Compact; usually wall-mounted
Noise
Outdoor unit makes a low hum, similar to an air conditioner
Near-silent
Typical lifespan
15–20 years
10–15 years
Figures are typical UK ranges for guidance only — actual performance depends on the specific product, installation and property. See our COP & SCOP guide for how efficiency is measured.
Why "300–400% efficient" sounds strange
A gas boiler can never be more than 100% efficient — it can only release the energy locked in the gas it burns, and some is always lost. A heat pump isn't creating extra energy; it's moving free heat from outside, so the "output" can be several times the electricity "input". The diagram below shows roughly what happens to one unit of input energy in each case.
Key considerations — is a heat pump right for your home?
Heat pumps can heat pretty much any UK home, but a good result depends on getting a few things right beforehand. These are the main factors worth thinking through before you commit:
Insulation & heat loss
The better insulated your home, the smaller and cheaper the heat pump needed, and the easier it is to run efficiently. Loft, wall and floor insulation are worth tackling first.
Radiators & emitters
Heat pumps work best at lower flow temperatures than a boiler. Many homes need larger radiators, underfloor heating, or more radiators, to heat rooms comfortably at that lower temperature.
Siting & placement
Outdoor units typically need more space than a boiler cupboard — a firm, level base, clearance from walls, boundaries and windows, airflow on at least one side, and a route for condensate drainage. Ground source needs room for a buried loop or a borehole, plus rig access during installation.
Noise
Outdoor units make a low hum, similar to an air conditioner. Modern models run quietly, but tight gardens or close neighbours are worth discussing with your installer.
Hot water cylinder
Most heat pump systems need a hot water cylinder — if you currently have a combi boiler with no cylinder, you'll need to find space for one.
Upfront cost & grants
Installation typically costs more than a gas boiler, though UK grant schemes can reduce this significantly. Running costs depend heavily on your electricity tariff.
Property type & permissions
Most houses are straightforward. Flats and listed or conservation-area properties may need extra care siting the outdoor unit, and occasionally planning permission.
Getting started
Start with a room-by-room heat loss survey from an MCS-certified installer — it tells you the size of heat pump you need and whether your radiators or pipework need upgrading.
Get quotes from more than one installer, and ask what flow temperature the system is designed around.
Check current UK grant schemes (such as the Boiler Upgrade Scheme) — support and eligibility rules change, so confirm the latest position before budgeting.
Reviewed July 2026. This guide is general information, not professional or installation advice — always get a proper heat loss survey and installer quote for your specific property.