Wet vs Electric Underfloor Heating: Costs, Floor Builds and Which to Choose

Underfloor heating (UFH) spreads gentle warmth across the whole floor instead of concentrating it in radiators on the wall. It frees up wall space, feels pleasant under bare feet and works especially well with heat pumps. But there are two quite different technologies behind the same label, and picking the wrong one for a room is an expensive error to unpick once the screed or tiles are down. This guide explains how each type works, what each needs from the floor construction, how they compare on running cost, and which suits a bathroom, an extension or a whole-house retrofit.

The two types in brief

Wet (hydronic) UFH circulates warm water through plastic pipes (usually PEX or multilayer composite) laid in or under the floor. The water comes from your boiler, heat pump or other heat source through a manifold that serves each loop. It is designed as part of the heating system.

Electric UFH uses resistance heating cables or thin heating mats connected to the mains through a thermostat. Each circuit simply heats up when switched on. There is no boiler, manifold or pump.

How they compare

The figures for floor build-ups are indicative and vary by manufacturer. Always check the data sheet for the exact system you buy.

Running costs: the maths

An electric system turns each kWh of electricity into one kWh of heat, so the cost is the unit rate multiplied by hours of use. As an illustration with assumed numbers, take a bathroom of 5 m² with a mat rated at 150 W per m². Total output is 5 × 150 = 750 W, or 0.75 kW. If it runs for 4 hours a day at an assumed electricity price of 24p per kWh, daily cost is 0.75 × 4 × 0.24 = £0.72, or about £260 a year if left on every day all year. In practice thermostats cycle the mat on and off, so the actual figure is usually lower, and using it only in cold months drops it further. A cheap overnight tariff reduces the cost again if you have enough thermal mass in the floor to hold heat.

Wet UFH has no unique running cost. It uses whatever heat source feeds it. With a gas boiler, the cost per kWh of heat follows gas price divided by efficiency; with a heat pump, it follows electricity price divided by SCOP. Because UFH works with water at 30–45°C, it lets a condensing boiler condense more of the time and a heat pump reach a higher SCOP. If you plan a heat pump, wet UFH is often the most efficient emitter, a point developed in our heat pump and gas boiler comparison. Our running cost calculator can put figures on your own tariff.

Heat output and floor surface temperature

A floor can only give out a limited amount of heat before it becomes uncomfortably warm. For occupied areas, BS EN 1264 and common practice limit the floor surface temperature to around 29°C (with higher limits in bathrooms and perimeter zones). A typical UFH floor delivers somewhere on the order of 50 to 100 W per square metre. That sounds low, but a well-insulated modern room has a heat loss of perhaps 30–50 W per square metre, so the floor can often cover it on its own. In an old, draughty house with a high heat loss, the floor may not be able to cover the full load, and you may still need a radiator or other top-up. Our pipe spacing and flow calculator helps estimate the pipe centres and flow rates for a given load.

Example, labelled illustrative: a living room of 20 m² with a design heat loss of 900 W needs 900 ÷ 20 = 45 W/m² from the floor. That is within normal range for UFH with moderate flow temperatures. If the same room lost 2,400 W, it would need 120 W/m², beyond what most floors can safely deliver at normal surface temperatures, and extra heat or better insulation would be needed.

Pipe spacing and flow temperature (wet systems)

Pipes are laid in loops at centres typically between 100 mm and 300 mm. Closer spacing gives more output and more even surface temperature but uses more pipe. Most manufacturers recommend loops no longer than about 100 m for 16 mm pipe, because longer loops need higher pump pressure and flow becomes unbalanced; the exact limit depends on pipe diameter and the manufacturer. Flow temperatures of around 35–50°C are normal. The floor covering matters a great deal: floor tiles and stone conduct heat well, while carpet and underlay add thermal resistance and reduce output. Manufacturers state a maximum combined resistance for coverings, often expressed as a tog or R value.

A good installer will provide a layout drawing showing each loop's length, pipe spacing and flow rate. Insist on this, and on a pressure test of the pipework before the screed is laid or the floor is covered. The test is typically at a higher pressure than working pressure and held for a period specified by the manufacturer.

Insulation is not optional

Without insulation under the heating layer, much of the heat will go down into the ground or the void below. New ground floors in England must meet the U-value requirements of Approved Document L, and the insulation beneath a UFH system forms part of that build-up. When retrofitting over an existing concrete floor, rigid insulation boards with routed channels or overlay panels are laid first. When retrofitting over a timber floor, you can fit pipes between the joists with spreader plates or use low-profile overlay panels, but the floor construction, loading and moisture must be checked. A solid floor with a damp problem needs a damp-proof membrane before the build-up. For floor structures, see our guide to timber joist notching before cutting into joists for any reason.

Screed, curing and commissioning

Where pipes are embedded in sand and cement or liquid (anhydrite or cementitious) screed, the screed must be allowed to cure, then brought up to temperature gradually before floor coverings are laid. The usual practice, set out in screed and UFH manufacturers' guidance, is to start the heating at a low flow temperature (commonly around 20–25°C) for a few days, raise it in steps to the maximum design flow temperature, hold it, and then step it back down. Skipping this stage can cause cracks, curling of screed or failures in tile adhesive. Wooden flooring needs a moisture content check on the screed before it is laid, since the heat drives moisture out of the build.

Electric systems: when they make sense

• Small rooms such as bathrooms or kitchens where installing a wet loop would not be worthwhile.
• Retrofit under tile where only a few millimetres of floor height can be spared.
• Comfort heating alongside radiators, taking the chill off a tiled floor rather than heating the room.

Electric systems must be installed to BS 7671, with circuit protection by a 30 mA RCD, and the installation must comply with Part P of the Building Regulations in England (see our Part P guide). Heating mats installed in a bathroom fall within special-location rules, so the final connection is typically by a qualified electrician. A floor sensor and a thermostat with a floor limit are standard. Never cut a heating mat's cable: you may cut the mesh to turn corners, but the heating element must remain continuous. Test the resistance with a multimeter before, during and after laying, and write the readings down on the warranty card.

Mats should not be installed under fixed furniture or built-in units with no airflow, and cables should not cross each other or touch. Check the manufacturer's instructions for the thermal insulation requirements; with no insulation underneath, the system will be slow and costly to run.

Which should you choose?

1. Whole ground floor in a new extension, or a heat pump house: wet UFH, designed as part of the heating system.
2. A single cold bathroom or kitchen floor: electric mat under tile, with insulation board beneath.
3. A whole-house retrofit in an existing home with a gas boiler: wet UFH is possible with low-profile overlay panels but may be disruptive; radiators on a lower flow temperature may be more practical.
4. Rooms over timber floors: check the structure and the weight of any screed with a structural engineer or building control before choosing.

Wet UFH laid as part of a new extension or other significant works will fall under the Building Regulations, including Approved Document L and, where relevant, Part G for hot water systems. Gas boiler work must be done by a Gas Safe registered engineer. This guide offers general information, not professional design advice; ask a qualified heating designer to produce a layout and heat loss calculation for your home.

Sources

• Approved Document L: Conservation of fuel and power (GOV.UK)
• Planning Portal: Approved Document P Electrical safety
• Approved Document F: Ventilation (GOV.UK)
• Ofgem: Boiler Upgrade Scheme