Do I need a dedicated circuit for electric underfloor heating?

For small bathroom mats (up to about 3 m2 at 150 W/m2, drawing under 450 W), you can spur from the bathroom lighting circuit or a nearby socket circuit. For larger areas or kitchen/hallway installs drawing over 3 kW, a dedicated 16 A or 20 A radial from the consumer unit is required. Always calculate the total wattage before choosing the circuit.

Does wiring underfloor heating need to be notified under Part P?

A new radial circuit from the consumer unit is notifiable under Part P in England. Spurring from an existing circuit is not notifiable. Work in a bathroom (any new or replacement circuit) is also notifiable. In all cases the work must comply with BS 7671 and be certified by a competent person.

What is a floor sensor probe and why does it matter?

The floor sensor is a thermistor probe that sits in conduit under the heated area and feeds the floor temperature reading to the thermostat. It allows the thermostat to prevent the floor temperature exceeding a set limit (usually 27 C for flexible tile adhesives). Without it, the thermostat can only measure air temperature and cannot protect the adhesive or tiles from overheating.

When should I test the mat resistance?

Test before laying adhesive (baseline reading), after laying adhesive but before tiling, and after tiling. The mat resistance should remain within the manufacturer's specified tolerance (typically plus or minus 10%) at each stage. Any drop in resistance or increase in leakage current indicates damaged cable; replacing a cable under tiles is a far bigger job than replacing one before they go down.

Can electric underfloor heating be fitted in a bathroom?

Yes. A cable-mat electric underfloor heating system under bathroom tiles is a common and well-established installation. The thermostat must be outside bathroom zones 0, 1, and 2, or rated to the appropriate IPX classification for its zone. The supply circuit must be RCD-protected at 30 mA. For bathrooms with a very small footprint, the thermostat can go on the landing wall outside the bathroom.

How long does electric underfloor heating take to heat up?

Under tiles in adhesive, a cable-mat system typically brings the floor to temperature in 20 to 40 minutes from cold. A programmable thermostat with a boost function is worth fitting so the floor is warm before the household gets up. The heating mat itself produces heat; the tiles act as thermal mass and can hold warmth for some time after the mat switches off.

What fuse rating for an underfloor heating thermostat FCU?

For a mat drawing under 3 kW (13 A or less), a 13 A switched fused connection unit is appropriate. For mats between 13 A and 16 A, use a 20 A double-pole switch fed from a dedicated radial. Most domestic bathroom or kitchen mats in normal-sized rooms will be under 1.5 kW and a 13 A FCU is perfectly adequate.

How to Wire an Electric Underfloor Heating Mat (UK)

Helpful video reference. A UK electrician's tutorial "Electric Underfloor Heating Installation UK - Back at the Studio" (July 2020) shows a full mat installation in a UK oak-frame studio, including laying the mat, running the sensor conduit and wiring the thermostat. Worth watching to see how the mat rolls out, how much depth the cable adds under tiles, and how the conduit for the sensor probe is routed into the wall.

Before you start. Measure mat resistance with a calibrated multimeter and record it on paper now. Most mats have a resistance label sewn into the mesh. Keep this baseline reading -- you will need it to prove the cable was undamaged before, during and after tiling. A mat with a broken cable cannot be repaired under tiles without lifting them. Catching damage before the tiles go down costs nothing; catching it after costs everything.

1. Calculate the total wattage and choose the circuit

Most UK cable mats are rated at 150 W per m2 or 200 W per m2. Multiply the heated area (not the full room area -- avoid mats under fixed furniture or a toilet) by the watt rating:

3 m2 at 150 W/m2 = 450 W (under 2 A) -- spur from the bathroom lighting circuit
5 m2 at 150 W/m2 = 750 W (about 3.3 A) -- spur from a bedroom or hallway socket circuit
12 m2 at 200 W/m2 = 2,400 W (10.4 A) -- dedicated 16 A radial from the consumer unit
Over 3 kW total -- dedicated circuit, not a spur

For most bathrooms the mat is small enough to spur from the lighting circuit. For a kitchen or open-plan area with a larger mat, a dedicated radial is the right answer.

2. Choose the thermostat position

The thermostat goes on the wall at a convenient height -- usually at socket height or slightly higher so the display is easy to read. In a bathroom it must be outside Zone 2 (at least 0.6 m from the edge of the bath or shower), or on the landing wall just outside the bathroom door.

Mark the thermostat back box position, then mark the cable route down the wall to floor level. You will also need to chase a short channel across the floor (in the tile adhesive screed layer) to house the sensor probe conduit.

3. Chase the wall and floor for the cable and sensor conduit

Chase a narrow channel down the wall from the thermostat position to floor level to carry the supply cable and the sensor conduit. At floor level, continue the chase about 300 mm into the area that will be covered by the mat.

Lay a length of 6 mm or 10 mm corrugated conduit in the floor channel. Leave one end open at the thermostat back box -- this is where the sensor probe enters -- and the other end sealed or closed with a conduit end-cap so tile adhesive cannot fill the conduit. The sensor probe must be replaceable without lifting tiles; the conduit makes that possible.

4. Lay the heating mat

Roll out the mat over the prepared subfloor (clean, flat, primed concrete or cement board). Keep the mesh flat and do not cross the heating cable over itself. Cut the mesh (never the cable) to turn corners. Leave the cold tail -- the non-heated section of flex that runs from the mat edge up the wall to the thermostat -- routed in the wall chase.

Thread the floor sensor probe into the conduit from the thermostat position so the sensor tip sits between two cable runs, about 300 mm from the mat edge -- towards the coolest part of the heated area rather than near the perimeter.

At this point, measure mat resistance again and compare to the baseline. Any significant deviation means the cable has been damaged while laying. Investigate now.

5. Wire the thermostat

Most UK underfloor heating thermostats have six terminals. The exact labelling varies by manufacturer, but the connections are:

Supply L, N, E -- from the FCU or radial circuit
Load L -- the switched live going to the mat's brown (live) conductor
Load N -- joined to the mat's blue (neutral) conductor (or connected to the thermostat's neutral bus)
Sensor terminals (two) -- for the two-core sensor cable from the probe

Earth the thermostat back box, earth the mat tail's earth conductor, and earth the sensor conduit if it is metal.

Set the floor temperature limit in the thermostat settings before you tile. Most flexible tile adhesives specify a maximum floor temperature of 27 degrees C to prevent the adhesive becoming brittle. Hard-set adhesives may allow a higher limit -- check the adhesive datasheet.

6. Test before tiling

With the mat laid and sensor probe in place but no adhesive down yet:

Mat resistance -- compare to the data label on the mat. Within plus or minus 10% is acceptable.
Insulation resistance between mat conductors and earth screen -- should be well above 1 MOhm at 500 V DC.
Polarity at the thermostat terminals -- confirm brown is arriving at the Live-in terminal and blue at the Neutral terminal.

Record all values. Now tile. The adhesive can take 24 to 72 hours to cure fully; do not energise the mat during the cure period.

7. Commission and certify

After full adhesive cure, energise the circuit and run the thermostat on manual override. With a contact thermometer on an uncovered tile surface, confirm the mat is producing heat evenly after 30 minutes.

Verify earth fault loop impedance (Zs) at the thermostat supply terminals, confirm the 30 mA RCD or RCBO trips in under 40 ms at 5 times rated current, and check insulation resistance one final time with everything energised (a screed crack or tile adhesive fault can show up on the final IR test that was invisible before tiling).

Issue a Minor Works Certificate (spur off existing circuit) or a full Electrical Installation Certificate (new dedicated radial). Give the homeowner a copy, along with the mat data sheet and the recorded resistance readings, for the property records and any future EICR.

Stop and call a qualified electrician if: the mat resistance reading after laying differs by more than 10% from the data label value, the floor sensor probe cannot be seated between cable runs (the sensor must not sit directly on a heating cable or it will overheat), the bathroom circuit has no RCD protection, or the mat area calculated requires a load above 3 kW -- at that point a dedicated circuit from the consumer unit is needed and the work is Part P notifiable.

When to call us

Richard installs and certifies electric underfloor heating mat systems across Sandwich and east Kent -- bathroom, kitchen, hallway or utility. The job includes the thermostat, the FCU, the circuit tests and a full resistance-test record so the property file is in order for any future EICR or sale.

Fitting underfloor heating in Sandwich or east Kent?

Richard wires and certifies the electrical supply so your tiler can tile the same day. Part P registered, fully insured.

Contact Richard

How to wire an electric underfloor heating mat