How an air source heat pump actually heats your home

An air source heat pump works like a fridge in reverse. It absorbs low-grade heat from the outside air into a refrigerant, compresses it to raise the temperature, and releases that heat into your radiators or underfloor heating via water. Because it moves heat rather than burning fuel, it can deliver three or four units of heat for every unit of electricity it consumes — but only when it is set up to run gently and steadily.

That last point is the key difference from a boiler. A gas boiler is sized to blast water out at 70–80°C, heat the house quickly and switch off. A heat pump is happiest running long, low and slow, typically at 35–55°C flow temperature. The lower the flow temperature it can use, the less electricity it needs. Everything that follows flows from that single fact.

Insulation and draught-proofing come first

Because your radiators will be cooler than you are used to, the fabric of the house has to hold heat better. If your home leaks heat quickly, the heat pump has to run harder and hotter, and your running costs suffer. Before you choose a heat pump, work through the basics:

• Loft insulation — aim for around 270mm of mineral wool. If you can see the joists, you need more.
• Cavity wall insulation — if your home has unfilled cavities and is suitable, this is one of the highest-return improvements available.
• Draughts — chimney balloons, letterbox flaps, and sealing around loft hatches and skirting boards make a surprising difference.
• Glazing — double glazing throughout, or secondary glazing on period windows, reduces the heat you lose and the cold you feel.
• Floors — suspended timber floors over a cold void benefit from insulation between the joists.

None of this is wasted money. It lowers your heat demand, which means a smaller and cheaper heat pump, and it makes any heating system feel more comfortable.

Radiators and emitters: sizing matters more than you think

A radiator’s quoted output is usually given at a 50°C difference between the water and the room. Drop that to 30°C, which is where heat pumps often sit, and a radiator gives out roughly half as much heat. That is why installers so often recommend larger radiators, or more of them.

In practice you have options:

• Upsize existing radiators — swapping a single-panel for a double or triple-panel convector of the same width is often enough.
• Add radiators in cold rooms, especially north-facing bedrooms and rooms over garages or open passages.
• Underfloor heating — where you are renovating, it is ideal, because it runs at very low flow temperatures and feels lovely underfoot.
• Pipework — narrow microbore pipework can struggle to carry enough flow. Some homes need a partial upgrade to 15mm runs.

A proper room-by-room heat loss calculation should decide all of this. If you are quoted a heat pump without one, ask why.

Controls, hot water and the way you live

Heat pumps reward good controls. Weather compensation adjusts the flow temperature according to the outside temperature, so the system runs cooler on mild days and warmer on frosty ones. Combined with a sensible heating curve and thermostatic radiator valves, this keeps the house stable without the boiler-style peaks and troughs.

Setback periods are worth rethinking. Rather than letting the house cool right down overnight and reheating it at 6am, a gentle two-degree reduction usually works better and costs less. Hot water is stored in a cylinder, typically heated to around 48–50°C, which means you need a cylinder big enough for your household — usually 180 to 250 litres for a family. Reheat times are longer than with a boiler, so talk through your morning routine with your installer.

Practical checks before you commit

• Space outside — the unit needs a spot with good airflow, ideally not boxed in or directly under a bedroom window.
• Noise — modern units are quiet, but check the sound power rating and think about where you and your neighbours sit in the garden.
• Planning — in England, a single domestic unit is often permitted development if it sits at least a metre from your boundary and meets other conditions. Check your local rules if you are in a flat, a listed building or a conservation area.
• Electrical supply — some installations need a dedicated circuit, and older fuse boards may need attention.
• Grants — government grant schemes have supported heat pump installations in recent years, so it is worth checking what is currently available before you budget.

Making the switch work in an older home

Heat pumps are not just for new builds. Solid-walled cottages, Victorian terraces and 1970s semis can all work well, provided the heat loss is understood and the emitters and controls are matched to the house. The trick is to treat the whole system as one package: insulation, radiators, cylinder, pipework and controls, all chosen together.

Do that, and you get a warm home that runs quietly, evenly and predictably, with running costs that compete with gas — particularly if you shift some usage to cheaper overnight electricity. Rush it, skip the survey, or keep undersized radiators, and you will end up with a system that works hard for mediocre results. Take your time, ask for the calculations, and it will serve you well for twenty years.