A heat pump moves existing heat from outdoor air into your home. It works in cold weather, but correct sizing and a well-designed heating system determine how comfortably and efficiently it runs.
Think of a fridge working backwards
A fridge takes heat from inside its cabinet and releases it into your kitchen. An air source heat pump takes heat from outside air and delivers it indoors. Its fan draws air across a heat exchanger. A refrigerant absorbs that heat, a compressor raises its temperature, and a second heat exchanger passes the heat into your home's heating system.
Most UK installations are air-to-water systems: they warm water for radiators or underfloor heating and usually a separate hot-water cylinder. Air-to-air models blow warmed air into rooms and generally do not provide domestic hot water. This distinction matters when comparing quotes or checking grant eligibility.
What happens on a cold day?
There is still usable heat in outdoor air below 0°C. A correctly sized unit can extract it, though efficiency falls as the outdoor temperature drops and the system has to work harder. Frost on the outdoor coil is normal in some conditions; the unit briefly reverses operation to defrost.
The design should be based on your property's room-by-room heat loss, not a guess from floor area. The installer then checks the heat emitters and chooses water temperatures that meet the heat demand. Larger radiators, suitable underfloor heating and lower flow temperatures can help the system work efficiently.
Why can one unit of electricity deliver three or four units of heat?
The electricity mainly powers the compressor and fan. It does not create all the delivered heat. Energy Saving Trust explains that a heat pump can deliver around three to four units of heat for each unit of electricity under suitable conditions. This ratio is called the coefficient of performance, or COP.
A showroom COP is measured at particular test conditions. The seasonal coefficient of performance, or SCOP, is a more useful annual comparison. Your actual performance also depends on weather, insulation, system design, controls and how much hot water you use. Ask an installer for the expected seasonal performance at your property and the assumptions behind it.
What to ask before you buy
Get a survey-led design showing heat loss, proposed radiator changes, outdoor unit position, hot-water plan and estimated annual electricity use. A good explanation should show how the system will keep each room warm on a cold day. If the quote gives only a unit size and a price, ask for the design detail before committing.
- Is this an air-to-water or air-to-air system?
- What seasonal efficiency have you assumed?
- Which radiators, pipework and hot-water components are included?
- Who will teach us to use the controls after installation?
A note about the numbers
Prices, grants and tariffs can change. The dated figures above come from the linked sources and are examples unless explicitly described as a published average. Your installer should assess your property before giving a final design or quotation.