Underfloor heating for swimming pool rooms and spas

A swimming pool room is the most demanding space in a house for a heating designer. The air is kept warmer than anywhere else in the building, the humidity is high all year, everyone walks on the floor barefoot, and the floor is wet for hours at a time. Get the heating wrong and you get cold tiles, puddles that never dry and condensation running down every cool surface.

This guide covers indoor domestic pools, spa rooms and hot tub rooms: what temperature to hold, why the floor behaves differently in a 30°C room, which kit survives the atmosphere, and how to design the circuits. Start with a proper room-by-room calculation, as our guide to heat loss calculations explains, because a glazed pool hall rarely behaves like the rooms around it.

What temperature should a pool room be?

Keep the air about 1°C above the pool water temperature, which for most private pools means 29 to 31°C. CIBSE's guidance on servicing pool buildings explains that holding the air just above the water temperature minimises evaporation from the pool, which is the biggest energy loss and the main source of humidity.

These water temperatures are the PWTAG maximums quoted in CIBSE's pool guidance, which also recommends relative humidity of 60% plus or minus 10% and around 4 to 10 air changes per hour of ventilation. In practice a domestic pool room needs a dehumidification and air handling unit as well as heating. The UFH is one part of a system, not the whole answer.

Does underfloor heating actually heat a 30°C room?

Not much, and that is by design. BS EN 1264 limits the floor surface to 29°C in occupied areas, and a floor at 29°C in a 30°C room gives out almost no net heat. Floor output depends on the gap between floor surface and room air, so the warmer the room, the less a compliant floor can deliver.

So in a pool hall the UFH has three jobs that matter more than raw output:

• Barefoot comfort. A tiled surround at room temperature feels acceptable; an unheated slab bridged to cold ground feels cold even in warm air. The heated floor keeps every tile consistently warm underfoot.
• Drying the surround. A gently warm floor evaporates splashes and wet footprints quickly, which matters for slip risk and for the fabric of the building.
• Condensation control. Warm, humid air condenses on any surface cooler than its dew point. A heated floor stays above it; a cold slab edge or threshold does not.

The bulk of the room's heat load, especially the ventilation load, is normally carried by the air handling plant. Treat the UFH as the comfort layer and size the air side with the pool contractor.

Should a pool room floor use water or electric UFH?

Water UFH is the usual choice for a pool surround. The room is heated for long hours all year, which suits the low running cost of a wet system, and it avoids putting electric heating elements into a wet, barefoot area. BS 7671 places extra restrictions on electrical equipment in and around swimming pools, so if you are considering electric UFH anywhere near a pool, involve a qualified electrician at design stage, not after the floor is down. Any electric UFH connection must be made by a qualified electrician in any case (Part P, BS 7671).

For the wet side, a standard screeded system works: PERT-EVOH oxygen barrier pipe clipped to insulation, a full screed and tiles. The pipe is plastic and unaffected by the humid atmosphere. Browse the water underfloor heating range for the parts.

Which components survive a pool atmosphere?

Choose stainless steel and plastics, and keep anything you can out of the humid room. A pool hall's warm, damp, chemically treated air is hard on bare mild steel and on electronics.

• Manifold: use a stainless steel manifold such as the Dualmix stainless steel manifold, and site it in a dry adjoining cupboard or plant room rather than in the pool hall itself. Our guide to where to put the manifold covers cabinet options.
• Mixing and pump: a blending valve pump pack, such as the 20 to 55°C blending valve pack with Grundfos UPM3 Flex, lets you hold a steady low flow temperature for the surround. Site it with the manifold in the dry space.
• Thermostat and sensors: put the room thermostat body outside the splash zone and use a floor probe in the screed so the floor temperature is controlled directly. Floor-sensing control suits pool surrounds because the air setpoint is handled by the air plant.
• Pipe: PERT-EVOH or multilayer UFH pipe is unaffected; there is nothing to corrode within the floor.

How do you stop condensation in a pool room?

Hold the humidity at about 60%, ventilate, and keep surfaces warm. CIBSE's pool guidance recommends effective vapour barriers, low-level air supply under glazing to wash the glass with warm air, and keeping the pool hall at a slightly negative pressure so moist air does not migrate into the rest of the house.

The floor helps because it is the one surface you can guarantee stays warm. Pay attention to the edges: an uninsulated slab edge, a door threshold to outside or a strip of floor beyond the heated area will sit cooler than the rest and is where condensation and cold-feet complaints start. Run the heated area to the walls, fit edge insulation, and insulate the slab to current standards before the pipe goes down.

How should the circuits be designed?

Give the pool hall its own zone, run it continuously and keep it off the household schedule. A pool room holds its temperature all year, so it should never share a thermostat or a schedule with living rooms that set back overnight.

1. Separate manifold or dedicated ports. Keep pool hall loops on their own ports, with their own actuators, so the zone can run 24/7.
2. Floor-sensing control. Set the floor to a steady comfortable surface temperature and let the air plant manage the air.
3. Mind the surface limit. BS EN 1264 allows up to 33°C surface in bathrooms and wet barefoot areas, against 29°C in normal occupied rooms. Even so, most pool surrounds feel right well below the limit.
4. Tile properly. Full-bed adhesive, movement joints and a suitable primer matter more here than anywhere; see tiling over underfloor heating.
5. Think about the wet rooms around it. Changing areas and showers next to the pool are standard wet room designs; our wet room guide covers them.

Frequently asked questions

What temperature should an indoor pool room be kept at?

About 1°C above the water temperature. For a typical private pool at 29°C water, that means around 30°C air, with relative humidity held at about 60%. These figures come from CIBSE guidance on pool buildings.

Is underfloor heating worth it in a pool room if it adds little heat?

Yes, for comfort and moisture control rather than output. It keeps tiles warm under bare feet, dries the wet surround quickly and keeps the floor above the dew point, which an unheated slab in a humid room will not manage.

Can I use electric underfloor heating around a swimming pool?

Only with specialist electrical design. BS 7671 restricts electrical equipment in and around pools, so speak to a qualified electrician first. Most pool surrounds use water UFH instead, which has no electrical parts in the floor.

How hot can a pool surround floor run?

BS EN 1264 allows up to 33°C surface temperature in wet barefoot areas such as bathrooms, against 29°C in normal occupied rooms. Most pool surrounds are comfortable well below 33°C.

Where should the manifold go for a pool room?

In a dry adjoining plant room or cupboard, not in the pool hall. Use a stainless steel manifold, and keep the pump, blending valve and wiring centre in the dry space with it.

Does a pool room still need a dehumidifier if the floor is heated?

Yes. The floor cannot remove moisture from the air. CIBSE guidance is built around ventilation and humidity control at about 60% RH; the heated floor works alongside that plant, not instead of it.

Sources: CIBSE: Swimming pools, servicing the building (air and water temperatures, humidity, ventilation)