Fan coils and heat pumps: efficient heating and, at last, cooling in summer

Anyone weighing up an air-to-water heat pump eventually hits the same worry: radiators don't cool. It's a physical limit —circulating cold water through a radiator would make it condense and drip— and for many people it's the reservation that stalls the switch: "and in summer, then what?". The answer has a name: the fan coil (or fan coil unit). It's the piece that turns a "heating-only" heat pump into a complete year-round climate system, with a single outdoor machine. Today we explain it in full: how it works, what it costs and when it pays off.

What a fan coil is, and why it gets on so well with a heat pump

A fan coil is, in essence, a water heat exchanger with a fan: the heat pump sends it water —hot in winter, cold in summer— and it turns that into conditioned air for the room. The key is that it works exactly in the regimes where a heat pump performs best.

For heating, a fan coil warms with water at 35-45 °C, the same range as underfloor heating. That means the heat pump works with small temperature lifts and the SCOP stays high —very close to underfloor heating, and clearly above what you get by forcing conventional radiators—, with one added advantage underfloor heating doesn't have: a response measured in minutes, not hours. You get home, switch on, and within a quarter of an hour the room is comfortable. For homes with irregular occupancy, that agility is worth its weight in gold.

For cooling, the fan coil receives water at 7-12 °C and does what no radiator can: it cools. It comes with a condensate tray and drain built in, so the condensation that would ruin a radiator is here part of the design. The efficiency of the heat pump–fan coil pairing in cooling (EER around 2.5-3.5) is somewhat lower than that of a good direct-expansion split, and we're the first to say so. But that figure, on its own, is misleading, and we'll soon see why.

The cooling that's paid for with sunshine

Household cooling has a quirk that makes it unique among all the loads in a home: demand coincides almost exactly with solar production. It's hotter when there's more sun. A house with solar PV and a battery can cool itself at midday and early afternoon with surplus that would otherwise be exported at a low price, and can even pre-cool by a couple of degrees before the late-afternoon peak, using the building's own thermal inertia as a cold store. With that strategy, the system's theoretically lower EER is offset by an energy cost close to zero: the cheapest cooling isn't from the most efficient machine, but the cooling produced when energy costs nothing —one of the three ways a solar battery saves you money.

The condition, of course, is having enough summer solar production, and that can be estimated before deciding anything: at the Fans del Sol portal you'll find a free roof analysis with PVGIS satellite data, month by month, telling you exactly how much energy to expect in July and August — the figure you need to know whether your summer cooling can come from the sun.

Types of fan coil and ballpark prices

The most common format in homes is the floor console or low-profile unit: it looks like a modern radiator, installs where the radiator used to be and reuses the existing pipe runs. It's the natural route for a retrofit. Ballpark equipment price: between €700 and €1,400 per unit depending on capacity and range. The alternatives: the wall-mounted unit (similar to a split, but water-based; €400-900), the ducted unit (hidden in a false ceiling with grilles, €900-1,800 plus distribution; a single unit can condition a whole floor and is the most invisible option where there's a false ceiling in the corridors) and the ceiling cassette, more common in commercial premises than in homes.

On the spec sheet, three things separate a good fan coil from a cheap one: the modulating EC fan (consumption from 5 to 30 W and far quieter than classic motors), the sound level (good ones run at 20-25 dB(A) on minimum speed: practically inaudible) and control (a valve and thermostat per unit, letting you condition only the occupied rooms). The standard domestic configuration is two-pipe: the whole system does heat in winter and cold in summer, with a seasonal changeover; four-pipe systems, which do both at once, are rarely justified in a home.

Installed and all-in, replacing a radiator with a console while reusing the existing two-pipe circuit usually comes to between €1,000 and €1,600 per unit. For a reference 200 m² house, conditioning it entirely with 8-10 fan coils is between €9,000 and €14,000: more than low-temperature radiators, yes, but with cooling included and without having to maintain (or power) a second split installation. The move that often works out best is the mixed one: fan coils in the living room and main bedrooms, low-temperature radiators in bathrooms and circulation areas.

The three details that decide a good retrofit

Experience has taught us that fan coil quotes don't differ by the brand of the unit, but by three details that have to be checked on site.

The first is the condensate drain: each fan coil needs one, and where there's no downpipe nearby you need a condensate pump or exposed pipework. It's the first question we ask on any visit. The second is pipe insulation: if you reuse the radiator pipes to circulate water at 7 °C, the old thermal insulation is useless —you need closed-cell anti-condensation insulation along the whole accessible run, or condensation will appear inside the walls, which is the last place you want it. And the third is a buffer tank on the circuit (40-100 litres), which stops the heat pump from short-cycling when only one or two units are open and extends its life.

None of these three points adds much to the project if it's planned from the design stage; all three get expensive if they're discovered once the work is done.

The numbers versus gas: 200 m² in inland Catalonia

So far we've talked about the coast, which is an easy climate. Now let's put the same 200 m² house in a mid-range inland area —the plains of Vic, Bages, Ponent—, with frosty winters and summers that genuinely call for cooling too. Here gas heating comfortably reaches 18,000-20,000 kWh a year, which with the fixed charge included is around €1,700-2,000 a year. And here's the first counterintuitive point: the harsher the climate, the more attractive the switch, because every kWh of gas displaced is a kWh saved, and there are many more to displace.

With a heat pump working at low temperature against fan coils, the SCOP in this climate lands around 4.0-4.2 (a little below the coast: winter shows up in the machine too). The same heat comes from about 4,300-4,800 electrical kWh which, with solar PV and the battery covering a majority share —in winter solar coverage drops, let's be honest: we're talking about 50-60% well managed—, leave a residual heating electricity bill of around €350-500 a year. Summer cooling, fed by surplus, adds an almost negligible marginal cost. The annual running saving versus gas thus lands between €1,300 and €1,600 —numbers that line up with what we cover on the real cost of self-consumed energy.

On the investment side, the full job —heat pump, DHW tank, replacing the emitters with fan coils, hydraulics and paperwork— comes to €28,000-31,000 gross for a house this size. The subsidy package available in 2026 (a 40% income-tax deduction, energy-savings certificates for boiler replacement —generous in this case—, and local building-tax rebates) can return between €4,500 and €5,500, leaving a net cost of around €23,000-26,000.

The "raw" payback against gas thus comes to about 14-16 years: longer than the coastal case, and we say so plainly. But that calculation compares apples with oranges, because it includes a complete cooling system that the gas house doesn't have. If the fair comparison is against "stay on gas and add a multi-split for cooling" —which in a house this size is €6,000-8,000 plus its running cost— the investment attributable to heating drops to €17,000-19,000 net and the real payback lands at 10-13 years, for equipment with a 15-20 year service life. And with two variables working in your favour over time: the price of gas, which has never been a friend to those who stay on it, and the fixed costs of the gas grid, which will be shared among ever fewer users as homes disconnect.

In short

If the heat pump is the engine of the electrified home, fan coils are the drivetrain that gets everything out of it: low-temperature heating with the SCOP at its peak and an immediate response, and summer cooling from the same machine, fed by midday solar surplus. If you have a heat pump in mind —or one already installed that just watches the summer go by— and you want to know what it would cost to complete it, you know where we are: the analysis is free and the numbers come without any make-up.

Frequently asked questions

Do fan coils make noise?

Models with a modulating EC fan run at 20-25 dB(A) on minimum speed: practically inaudible. Annoying noise usually comes from old motors or oversized units, not from the technology itself.

Can I reuse my existing radiator pipes?

Often yes for fitting a console, but if cold water will circulate in summer you need to renew the insulation with closed-cell anti-condensation material. Reusing the old thermal insulation causes condensation inside the walls.

Do I need a battery to cool with fan coils?

It isn't mandatory, but the battery and time-of-use control are what make the afternoon cooling come from solar surplus rather than the grid. Without a battery, you cool at near-zero cost only during sunlight hours.

How much does a fan coil consume?

The EC fan draws between 5 and 30 W. The bulk of the consumption is the heat pump, which with water at 35-45 °C runs at a high SCOP and turns each electrical kilowatt into 4 or more of heat.

SolarBox Team