Cooling with an air-to-water heat pump: underfloor loops, fan coils and the dew point

Updated September 10, 2026 · Claude AC · 2 sources cited

Contents (7)

Three different things get called heat pump cooling
It has to be specified before the order, not after
The emitter decides what you get
The dew point is the hard ceiling
What it actually feels like
Running cost, and the paperwork nobody mentions
Frequently asked questions

Europe has spent the past few years replacing gas boilers with air-to-water heat pumps, and a predictable question follows every installation into its first summer: the machine moves heat, so can it move it the other way? The salesman’s answer is often a confident yes. The honest answer is that it depends on three things — whether the unit was built to reverse, what the water flows through once it does, and how humid the air in the room is — and that the third one sets a ceiling nothing else can raise.

This is a different question from the one most European households face. A reversible air-to-air split is straightforward air conditioning, rated with a SEER, blowing cold dehumidified air into a room; that case is covered in our guide to cooling with heat pumps. A hydronic system — one that heats water and sends it to radiators, floor loops or fan coils — is a machine with genuinely different physics on the cooling side.

Three different things get called heat pump cooling

Active cooling. The compressor runs, a four-way valve reverses the cycle and the unit chills water instead of heating it — a small domestic chiller. Flow temperatures range from about 18 °C for surfaces down to 7–12 °C for fan coils, and the electricity consumption is real, though modest by air conditioning standards.
Passive or natural cooling. Available only on ground-source and water-source systems: the compressor stays off and a heat exchanger simply transfers house heat into the borehole or ground loop, which sits at roughly 10–14 °C all summer. Only the circulation pumps consume electricity — often under 100 W — which makes it the cheapest cooling in Europe by a wide margin, and also the weakest, since the ground warms as the season progresses.
Nothing at all. A large share of the air-to-water units sold across Europe are heating-only, with no reversing valve and no cooling logic in the controller. There is no software unlock and no economical retrofit; the correct answer for those households is a separate machine.

It has to be specified before the order, not after

Cooling on a hydronic system is a set of decisions taken at design stage, and adding them later usually means opening a screed or replacing a controller.

A reversible unit with a declared cooling capacity and a cooling efficiency figure on its datasheet.
A controller that supports a cooling mode, with its own setpoint, and with the zone actuators configured to open on a cooling demand — many underfloor manifolds are wired to close when there is no heat demand, which silently disables the whole thing.
A room humidity sensor and dew-point logic — not an accessory but the safety device that stops the system putting condensation into your floor.
Insulated pipework, manifolds and valves throughout, because chilled pipes sweat: uninsulated runs through a screed, a riser or a cupboard produce damp where nobody looks until a stain appears. And a decision about which rooms are in the cooling zone at all.

Specified at the outset, the cooling side often adds a modest amount to a system being installed anyway — a valve kit, a sensor, controller configuration. Adding fan coils to rooms that have none is a separate per-room cost, closer to fitting emitters than to configuring software. Treat any figure quoted as indicative, and get it itemised.

The emitter decides what you get

Emitter	Typical flow temperature	Indicative cooling output	Removes humidity?	Verdict
Radiators	—	Negligible	No	Do not try. The surface area is designed for a large temperature difference in the other direction, and a cold radiator simply drips
Underfloor loops	18–20 °C	Roughly 15–25 W/m² in practice	No	Gentle, silent, invisible. Capacity is limited by the dew point and by floor coverings — tile and stone perform, thick carpet and some wood floors halve the output
Chilled ceilings or wall panels	16–18 °C	Two to three times the floor figure per m²	No	Better physics — cool air falls, so a ceiling cools a room more effectively than a floor. Rare in retrofits, excellent in new build
Fan coils (hydronic units, wall, floor or ducted)	7–12 °C	1–5 kW per unit	Yes, with a condensate drain	The only hydronic emitter that handles latent load and responds in minutes. Needs a drain, and it makes fan noise like any air conditioner

The pattern in that table is the whole subject. Surfaces are comfortable and quiet but can only ever shave the peak; fan coils behave like air conditioning because they do what air conditioning does — they get cold enough to condense water out of the air, and they have somewhere for it to go.

The dew point is the hard ceiling

Cool a surface below the dew point of the air touching it and water forms on it. Air at 26 °C and 60% relative humidity has a dew point close to 17.5 °C, which is why floor cooling systems limit flow to about 18 °C and modulate it against a measured room humidity: a screed circulating 15 °C water in an August kitchen is a condensation problem written into the building.

Three consequences follow, and they explain why the same system delights an owner in Munich and disappoints one in Rimini.

Humid climates get less cooling. The wetter the indoor air, the higher the dew point, the warmer the surface has to stay, the smaller the temperature difference and the lower the output. Coastal and Mediterranean summers are exactly where surface cooling has least to give — and where the demand is greatest.
Wet rooms are excluded. Bathrooms, shower rooms and utility spaces are normally taken out of the cooling zone entirely, and drying laundry indoors can push a whole floor’s humidity high enough for the controls to back off.
No dehumidification means no relief from a clammy night. Surface cooling lowers air temperature without removing moisture, so relative humidity actually rises as the room cools. If your discomfort is stickiness rather than heat, the machine you need is a fan coil, a split or a dehumidifier — the comparison is in our guide to dehumidifier or air conditioner.

What it actually feels like

Managed properly, floor cooling is the most comfortable cooling in the house: no draught, no noise, nothing blowing on anyone, an even temperature from ankle to head. Its realistic effect is to hold indoor peaks 2–4 °C below where they would otherwise land — in a continental summer, the difference between a usable house and an unusable one; in a Mediterranean heatwave, a useful base layer rather than a solution.

It must also be operated as a slow system. Surfaces have hours of inertia, so the strategy is a steady setpoint held through the day rather than a burst of cooling once the room already feels hot — the logic behind pre-cooling in our guide to when to run your air conditioning. The hybrid is often the right answer: surface cooling as the background across the living space, plus one fan coil or air-to-air split in the bedroom, where humidity and response time genuinely matter.

Running cost, and the paperwork nobody mentions

Cooling water to 18 °C is easy work for a heat pump, so efficiency on the cooling side is high — indicatively several units of cooling per unit of electricity. A house taking 1.5–2.5 kW of floor cooling might draw 250–500 W including pumps, so 2.5–5 kWh over ten hours: well under two euros a day at most European tariffs, and a fraction of that on a passive ground-source system, as our guide to running costs in Europe puts in context. The caveat is hours rather than watts, since surface systems run all day through a hot spell.

Two regulatory points. A monobloc heat pump is factory-sealed, so no refrigerant is handled on site; a split installation opens a circuit and must be done by personnel certified under the F-gas Regulation. And active cooling changes how the dwelling is assessed — it appears in the home’s energy assessment, covered in our guide to air conditioning and the energy performance certificate, and in the Netherlands it removes the dwelling from the overheating test described in our guide to why new European homes overheat.

Frequently asked questions

Can my existing heating-only heat pump be made to cool?

Almost never economically. Cooling requires a reversing valve in the refrigerant circuit and cooling logic in the controller, and a heating-only unit has neither. Some manufacturers offer a cooling-capable variant of the same model, which is a different appliance rather than an upgrade. If cooling matters and the unit is already installed, price a separate air-to-air split against replacing a working heat pump — the split will usually win.

Will underfloor cooling make my floor wet?

Not if it is controlled. The system measures room humidity, calculates the dew point and keeps the flow temperature above it, typically around 18 °C. Condensation appears when that control is missing, when a humidity sensor was never fitted, or when pipework and manifolds outside the room were left uninsulated. Ask specifically what dew-point protection is fitted before commissioning.

Is underfloor cooling enough in a heatwave?

In central and northern Europe, often yes — holding the peak 2–4 °C lower keeps a well-shaded house tolerable. In southern Europe, or in a top-floor flat, or on a run of tropical nights, no: the output is capped by the dew point and there is no dehumidification. Shading and night ventilation raise its effectiveness substantially, which is why the passive measures in our guide to cooling without air conditioning come first here too.

Can I put fan coils in and keep the radiators?

Yes, and it is a common retrofit shape: radiators heat in winter, fan coils in the rooms that matter heat and cool, and the cooling circuit is isolated from the radiator circuit so no chilled water reaches them. It requires pipework and a condensate drain per unit, so it is a plumbing project rather than an accessory — and worth comparing on cost against a multi-split, which is set out in our guide to multi-split systems.