Heat pumps explained: heating that runs on electricity
A heat pump is the most consequential and most misunderstood low-carbon upgrade. It does not burn fuel to make heat; it moves existing heat from outside into your home, which is why it can deliver more heat energy than the electricity it consumes. Get the design right and it is cheap and comfortable; get it wrong and it is neither, which is the whole story of heat pumps in Britain.
The one-sentence version
A heat pump uses electricity to move heat from the outside air (or ground) into your home, typically delivering around three units of heat per unit of electricity, so it heats for less energy than a direct-electric heater and, on the right tariff, less cost than a gas boiler, with far lower carbon.
How it moves heat uphill
It runs the same refrigeration cycle as a fridge, in reverse. A refrigerant fluid with a very low boiling point is pumped around a loop. Outside, it passes through an evaporator and absorbs heat from the air, even cold air contains plenty of heat energy, and boils into a gas. A compressor then squeezes that gas, which raises its temperature sharply. The hot gas passes through a condenser indoors, gives up its heat to your heating water, and condenses back to a liquid. An expansion valve drops its pressure and temperature, and the cycle repeats. The clever part is that you are paying only to move heat (running the compressor and fans), not to create it, which is why the output beats the input.
COP and SCOP: the efficiency numbers
The coefficient of performance (COP) is heat out divided by electricity in at a given moment: a COP of 3 means three units of heat per unit of power. Because COP changes with the weather and the temperature you ask for, the number that matters is the seasonal version, SCOP, an average across a heating season. A well-designed UK air-source system reaches a SCOP of about 3.2 to 3.6 on radiators and higher on underfloor heating. The single biggest lever on SCOP is flow temperature: the cooler the water the system circulates, the more efficient the pump. A heat pump asked to run at 55C works far harder than one running at 40C.
Why sizing and emitters matter so much
A gas boiler is cheap to oversize, so installers often do. A heat pump is the opposite: it works best sized accurately to the home's heat loss and run steadily at a low flow temperature for long periods, rather than blasting on and off. To deliver enough heat at a low flow temperature, the radiators (emitters) must be large enough; underfloor heating, with its huge surface area, is ideal. This is why a proper room-by-room heat-loss survey, not a like-for-like boiler swap, is the difference between a cheap-to-run system and an expensive one. Insulating first lets the whole system, and the bill, be smaller.
Air source vs ground source
An air-source heat pump (ASHP) takes heat from outside air, is cheaper to install and is what most homes fit. A ground-source heat pump (GSHP) draws heat from buried pipes, where the ground stays at a stable temperature year-round, so it achieves a higher, steadier SCOP, but the groundworks make it much dearer and suited to larger or rural properties.
Running costs and grants
Because electricity costs more per unit than gas, the running-cost comparison hinges on the SCOP and the tariff. On a standard tariff a heat pump lands close to level with a gas boiler; on a dedicated heat-pump tariff with cheaper electricity it tips clearly cheaper, and it cuts carbon sharply either way. The Boiler Upgrade Scheme grant covers a large slice of the upfront cost: £7,500 in England and Wales, rising to £9,000 for off-gas oil and LPG homes from 21 July 2026. Scotland has its own grant and loan.
The details an expert checks
A heat-loss calculation done properly, not a rule of thumb; a design flow temperature of 45C or lower; emitters upsized where needed; weather compensation enabled so the system varies flow temperature with the outdoor conditions; a sensible hot-water cylinder; and an installer who commissions and tunes it rather than leaving factory defaults. A heat pump left running like a boiler, hot bursts then off, will disappoint; run low and slow, it shines.
Where to go next
Compare your boiler against a heat pump with the heat pump calculator, read the running-cost case in are heat pumps worth it, and check the grant detail in the Boiler Upgrade Scheme explained.
Hot water, defrost and the cold-weather question
Two practical points decide everyday satisfaction. First, hot water: most heat-pump homes use a hot-water cylinder, because a heat pump heats water steadily rather than on demand like a combi boiler. The pump raises the cylinder to a usable temperature efficiently, and periodically runs a brief hotter cycle to keep it safe. Sizing the cylinder to the household avoids running short. Second, defrost: in cold, damp weather frost can form on the outdoor unit, and the pump briefly reverses to melt it, which is normal and built into the seasonal efficiency figure. Heat pumps work well below freezing (they are standard across Scandinavia); output falls as it gets colder, which is exactly why an accurate heat-loss design, sized for a cold day, matters so much.
Noise, space and planning
An outdoor unit needs a position with airflow and a sensible distance from boundaries and windows. Modern units are quiet, around the level of a fridge at a few metres, and most installs proceed under permitted development provided siting and noise conditions are met, though it is worth checking for your property. Indoors you need room for the cylinder where an old tank or airing cupboard may already sit. None of this is onerous, but it is the kind of detail a good installer surveys up front rather than discovering on the day, and it is part of why a proper survey beats a phone quote.