How to wire electric underfloor heating (UK)

Helpful video reference. We use Warmup's video "How To Install Warmup Electric Underfloor Heating Mats" as the video reference here. Warmup is a UK underfloor heating manufacturer with a long track record in the British market. This guide covers the mat installation side of the job; the electrical connection steps below bring in the BS 7671 requirements that any UK installation must meet.

1. Calculate the load and plan the supply

Most electric underfloor heating mats are rated at 150 W/m² or 200 W/m². Multiply the mat area in square metres by the wattage per square metre to get the total load in watts. Divide by 230 to get the current in amps.

Examples:

• Bathroom, 3 m² at 150 W/m² = 450 W, roughly 2 A. Fine on a 13A spur from the ring.
• Kitchen, 8 m² at 150 W/m² = 1200 W, roughly 5.2 A. Possible on a spur, but check the ring is not already heavily loaded.
• Large open-plan floor, 20 m² at 150 W/m² = 3000 W, 13 A. Approaching the ring spur limit — a dedicated circuit is the right answer.

For anything above 2 kW, or where the calculation is borderline, run a dedicated circuit from the consumer unit rather than adding load to an existing ring.

2. RCD protection: this is not optional

BS 7671 Section 753 (Electric heating units for surface heating) is clear: every electric floor heating system must have 30 mA RCD protection. This is not a matter of opinion or installer preference — it is a requirement in the wiring regulations.

In most modern homes the circuit will already be protected by an RCBO at the board or by an RCD covering the relevant zone. Before you connect the mat, confirm that 30 mA RCD protection is in place. Test the RCD using the test button on the device — the circuit should trip within 300 ms. If the circuit has only an MCB and no RCD, stop and sort the protection first.

In a bathroom, the spur that feeds the UFH will typically share the bathroom circuit, which in a modern installation is RCBO-protected. That meets the requirement. In a kitchen or living room, check carefully.

3. Fit the fused connection unit or prepare the dedicated circuit

For a bathroom mat fed from the ring, the cleanest approach is a switched fused connection unit (FCU) with a 13A fuse, positioned outside the bathroom zone or in Zone 2 at minimum if IP-rated. The FCU gives a local isolation point without touching the consumer unit.

Wire the FCU from the ring using 2.5 mm² twin and earth. The outgoing side of the FCU connects to the thermostat supply terminals. Keep the FCU and thermostat in the same wall chase run so the cable route is tidy and traceable.

For a dedicated circuit, an electrician will run 2.5 mm² twin and earth from a spare way in the consumer unit (or an added RCBO) to the thermostat position. This is notifiable work under Part P.

4. Chase out and lay the mat

Chase out a channel in the wall from the thermostat position down to floor level for the cold tail and sensor cables. The channel should be deep enough to accommodate the cables without proud edges — the tiles will come up to this wall.

Unroll the mat and position it over the area to be heated, keeping elements at least 50 mm away from fixed furniture, toilets, basins and shower trays. Do not cut the heating element. You can cut the mesh carrier to change direction, but never the wire itself.

The cold tail — the non-heating lead at the mat's end — should route up the wall channel to the thermostat position. Route it through the channel and into the back box before the mat is embedded.

5. Install the floor sensor

The floor sensor probe monitors the temperature of the floor surface and signals the thermostat to cycle on or off. It must be positioned between the heating elements — not touching them, and not closer than 150 mm to the wall.

Feed the sensor down its own conduit tube (usually supplied with the mat kit), which sits between the heating elements and can be withdrawn and replaced if the sensor fails. Route the sensor cable up the same wall channel as the cold tail.

Note the position of the sensor on a plan or photograph before tiling. If it fails in ten years, whoever replaces it needs to know where to pull.

6. Wire the thermostat

The thermostat sits in a standard pattress box in the wall, typically at switch height (1.2 m) or at a convenient position near the door. Most thermostats have clearly labelled terminal blocks:

• Mains input: Live, Neutral, Earth from the FCU or dedicated circuit.
• Load output: Live and Neutral to the cold tail of the heating mat. (Earth from the mat goes to the thermostat earth terminal.)
• Sensor: The two sensor wires (polarity usually not critical — check the manufacturer's data sheet).

With the circuit isolated, connect each terminal in order. Take a photograph before fitting the thermostat into the box — if you need to revisit this in future, the photo saves significant time.

Set the thermostat to the maximum floor temperature recommended by the floor covering manufacturer. For ceramic tile this is typically 27°C; for engineered wood or LVT, the limit may be lower (often 27°C maximum as well, but check).

7. Cold resistance test before tiling

This is the most important step in the whole process. Before the mat is covered by tile adhesive, measure its resistance with a multimeter. Set the meter to the ohms range and measure between the two cold tail conductors (live and neutral of the mat — not earth).

Compare your reading to the value on the mat datasheet. It should be within 5% either side. Most 150 W/m² mats have a resistance of around 50 to 150 ohms depending on area — the exact value will be on the label or in the instructions.

A reading significantly lower than expected: possible short between conductors, perhaps from a nail through the mat. A reading of infinite resistance (open circuit): the heating element is broken. Either way, do not tile. Find and fix the fault first.

Some electricians and heating engineers also do an insulation resistance test using a 500V Megger at this stage, measuring between the conductors and the earth braid of the mat. A healthy mat will show many megohms. This test is best practice, though less often done on domestic installations.

Repeat the resistance check after tiling is complete and after the adhesive has cured. If the reading has changed significantly, something went wrong during tiling.

8. Commission and test

Wait for the tile adhesive or screed to cure fully before switching on the heating. Tile adhesive typically requires 24 to 48 hours; standard floor screed requires up to 28 days. Running the heating before the adhesive has cured can cause cracking or delamination.

Once cured, power up the system and confirm the thermostat responds — the display should show a sensible floor temperature, and the heating should cycle on and off as the floor warms. Walk across the floor area after an hour to confirm even warmth across the heated zone.

When to call us

The mat laying and sensor installation are reasonable DIY tasks. The supply circuit — especially anything involving the consumer unit, a new dedicated circuit, or RCD verification — is best handled by a qualified electrician. Richard covers this kind of work across Sandwich and east Kent, and can confirm the protection is in place before you lay anything.