How an air source heat pump actually works

The short answer

An air source heat pump takes low grade heat out of the outside air, concentrates it by compressing a refrigerant, and hands it over to the water in your heating system. It uses electricity to run the compressor and the fans, not to create the heat itself. That is why the output can be several times larger than the electricity input, and why comparing a heat pump to an electric immersion heater is the wrong mental model.

In the government funded Electrification of Heat demonstration project, which monitored 742 heat pumps in real British homes, the median in situ seasonal performance factor for air source systems was 2.80. In plain terms, the median home in that trial got 2.8 units of heat out for every unit of electricity in. Well designed installations do better than that, and poor ones do worse, which is the real story of this technology.

The cycle, in four moves

Every air source heat pump does the same four things in a loop. The refrigerant inside is chosen because it boils at a very low temperature, so even cold winter air is warm enough to make it evaporate.

Where the heat ends up

The water leaving the heat pump goes to whatever emitters your house has. Radiators, underfloor heating, or a mix of the two. It also heats a hot water cylinder, because a heat pump does not work like a combi boiler that heats water on demand at the tap.

• Radiators work, but they were almost certainly sized for boiler temperatures and some rooms may need larger ones. Our guide to whether you need new radiators covers the room by room arithmetic.
• Underfloor heating is the natural partner, because it already runs at the temperatures a heat pump likes. See underfloor heating and heat pumps.
• Hot water is stored in a cylinder rather than made on demand. Vaillant states that its aroTHERM plus reaches a 75C flow temperature, which the manufacturer says allows cylinder hot water without an immersion heater top up for legionella protection.

Why temperature is the whole game

A heat pump gets less efficient the harder you ask it to work, and how hard it works is set by the gap between the outside air and the water temperature you demand. Heat pumps run most efficiently at flow temperatures of roughly 35 to 45C, against 70 to 80C for a conventional boiler system.

Samsung's own published figures make the cost of pushing that temperature up very visible. Its EHS Mono R290 5 kW model is quoted at SCOP up to 4.84 at a 35C flow temperature. Its EHS Mono HT Quiet range at a 65C flow temperature is quoted at SCOP 2.82 for the 8 kW, 3.01 for the 12 kW and 3.03 for the 14 kW. Same manufacturer, same technology, roughly 40 percent less efficient at the higher temperature.

This is why flow temperature is the number to argue about in a quote, and why the emitters in your rooms matter more than the badge on the box outside.

Does it work in a British winter

Yes, and the manufacturer operating ranges are far below anything a UK winter produces. Vaillant quotes the aroTHERM plus as operating down to minus 25C outdoor temperature. The Mitsubishi Electric Ecodan R290 datasheet quotes the same minus 25C limit, and Daikin quotes operation down to minus 25C on the Altherma 3 R. Efficiency drops as it gets colder, because the temperature gap widens, but the unit keeps running.

The more useful winter question is whether the system was sized for your actual heat loss on the coldest design day, which is what a proper survey establishes. That is covered in heat pump sizing and what a proper survey covers.

R290 and why 2026 units run hotter

The industry has shifted its flagship models to R290, which is propane, with a global warming potential of 3 as quoted by Vaillant. Vaillant, Mitsubishi Electric, Samsung, Grant and Octopus all now lead with R290 lines. The practical consequence for a homeowner is temperature: R290 units reach 70 to 75C flow, which is what makes keeping most of your existing radiators viable and what lets the heat pump handle hot water without help.

What decides whether yours is any good

The Electrification of Heat data showed high variation in performance between installations, which is why design quality and the heat loss survey matter more than the age or style of the property. The same unit, fitted two different ways, is effectively two different products.

1. A room by room heat loss calculation, not a rule of thumb from the size of your old boiler.
2. Emitters checked room by room against the design flow temperature, with only the rooms that fail getting new radiators.
3. The lowest design flow temperature the house allows, because every degree costs efficiency.
4. A hot water strategy that does not lean on the immersion heater every day.
5. Controls set up so the system runs steadily rather than blasting on and off.

Every county page on this site carries a local calculator and the current grant position for that area, so start at our coverage map if you want the picture for where you actually live. If you are also weighing up solar panels or battery storage, the interaction with a heat pump is worth planning in one go rather than in three separate visits.

Clearline gives independent guidance. Installations are carried out by an MCS-certified installation team for your area, and we do not sell or supply any of the products named on this page.