If you already have, or are planning, an air source heat pump (ASHP) for heating, a fan coil system lets the same equipment cool your home in summer — without installing a separate air conditioner.
How it actually works
Your ASHP normally moves heat from outside air into water that runs through radiators or underfloor heating. Run in reverse, it can chill that water instead, which is then circulated to fan coil units. The fan coils themselves are small units, similar in size to a slim radiator or ceiling cassette, that blow air across a cold coil into the room. They can be mounted on walls, ceilings, or even in the floor, and are usually installed in the rooms that need cooling most (typically bedrooms and the main living space).
Why bother, if you have radiators already
Radiators are built to emit heat, not absorb it efficiently — they’re a poor match for cooling. Fan coils are designed to do both, which is why most ASHP-cooling installations pair the heat pump with a small number of fan coils in the rooms that need it most (typically bedrooms and the main living space), rather than replacing whole-house heating.
The condensation problem
This is also the reason you can’t just run cold water through your existing radiators or underfloor pipework instead of installing fan coils. Any surface colder than the surrounding air’s dew point will pull moisture straight out of the air and turn it into liquid water on that surface, the same way a cold drink glass beads up on a warm day.
Radiators and underfloor loops are built into the fabric of the house: bare pipes, screed, plaster, skirting boards. Chill them below the dew point and you get condensation forming inside walls, under floors and behind fittings where it can’t evaporate away, leading to damp, mould and slow damage to the building itself. Underfloor systems are especially risky, since any moisture is trapped under flooring with no airflow to clear it.
Fan coil units solve this by design. The cold coil sits inside a casing with a condensate tray and drain underneath, and the fan blowing air across it keeps moisture moving rather than sitting still, so any condensation is caught and drained away safely rather than seeping into the building. It’s a small, contained, and deliberately designed process, not an accident waiting to happen in your walls.
What it costs to run
Because the heat pump is already sized for winter heating, running it for summer cooling adds relatively little to the electricity bill — a modern ASHP-fan coil pairing typically achieves a seasonal coefficient of performance (SCOP) above 4, meaning over 4 units of cooling for every unit of electricity used.
Where it fits in the layers of cooling
We only recommend an ASHP fan coil system after establishing that shading, reflectivity and ventilation are already doing what they can. Sized correctly against a smaller remaining heat load, it runs quieter, cheaper, and less often than a system installed as a first resort.