Electric Underfloor Heating — Cable Systems, Heating Mats, and Thermostat Wiring for UK Electricians

Electric underfloor heating (UFH) is a popular choice for bathrooms, kitchens, and single-room retrofits where wet systems are impractical. Unlike wet UFH, electric systems require no pipework or boiler connection — just a dedicated circuit, the right cable or mat, and a suitable thermostat. This guide covers system selection, circuit protection, installation steps, and commissioning to current BS 7671:2018 (18th Edition) requirements.

How Electric UFH Works

Electric UFH converts electrical energy directly to heat via a resistance element embedded in the floor construction. The element heats the screed or tile adhesive bed, which radiates warmth upward. Systems are controlled by a floor-sensing or air-sensing thermostat that switches the circuit on and off to maintain the setpoint temperature.

Operating temperatures are typically 25–40°C at the floor surface — comfortable underfoot and safe for most floor coverings. Maximum floor surface temperature is usually limited to 27°C under seated areas and 35°C in bathrooms by thermostat setpoint.

System Types

Loose Resistance Cable

A single-conductor or twin-conductor cable is laid in a serpentine pattern across the floor, stapled to an insulation board or secured with fixing strips. Spacing is calculated based on required wattage per square metre — typically 100–200 W/m² for primary heating, 75–100 W/m² for comfort heating under tiles.

Loose cable offers flexibility: spacing can be adjusted around obstacles, and outputs can be matched precisely to the area. It requires more time to install than a mat but suits irregular spaces and screed beds.

Heating Mats

Resistance cable is factory-woven onto a fibreglass mesh at a fixed spacing, typically 150 mm centres. Mats are rolled out on the subfloor and can be cut through the mesh (not the cable) to change direction. They are designed for installation directly into tile adhesive — no screed required — making them ideal for bathroom renovations.

Standard outputs: 100 W/m² or 150 W/m². Mats are sized to the room area and available in standard widths (0.5 m, 1.0 m) and various lengths.

Electric UFH Film

Carbon-ink heating film panels are installed under floating floors (laminate, engineered timber). They are not embedded in adhesive or screed. Output is lower — around 60–80 W/m² — making them suitable only for comfort heating, not primary heat sources. Wiring is surface-run or routed through the floating floor substrate.

Calculating Load and Circuit Requirements

To size the circuit:

1. Determine heated area (m²) — exclude areas under fixed furniture, cabinets, and WC pans.
2. Select output W/m² based on use: 100–150 W/m² for primary heating; 75–100 W/m² for comfort heating.
3. Calculate total load: e.g. 6 m² × 150 W/m² = 900 W.
4. Calculate design current: I = P / V = 900 / 230 = 3.9 A.
5. Apply diversity if required — most thermostats duty-cycle continuously so full load applies.

For most domestic bathroom installs (up to 3 kW), a 20 A radial circuit is standard. Larger areas or commercial installs may require 32 A circuits or multiple zones on separate circuits.

Circuit Protection Requirements (BS 7671)

RCD Protection

All electric UFH circuits require 30 mA RCD protection under BS 7671 Regulation 701 (bathrooms) and Regulation 422 (floor heating in general). An RCBO combining overcurrent and RCD protection in a single device is the most convenient solution for consumer units with limited ways.

Supplementary Equipotential Bonding

In bathroom zones, supplementary bonding connects exposed metalwork to the protective conductor. Metal sheaths of UFH cables, if present, must be bonded. Most modern UFH cables have a plastic outer sheath with no metallic screen requiring bonding.

ADS and Earth Fault Loop Impedance

Verify that Zs is within the tabulated maximum for the protective device. For a 20 A B-curve MCB or RCBO: Zs ≤ 1.92 Ω. Measure with a loop impedance tester at the furthest point of the cable run (the cold-tail connection in the floor junction box).

AFDD (Arc Fault Detection Devices)

From Amendment 2 (A2:2022) to the 18th Edition, AFDDs are recommended for circuits serving sleeping accommodation and certain kitchen circuits. While not mandated for all UFH circuits, they are increasingly specified for bedrooms with electric UFH to detect cable damage under flooring.

Thermostat Selection and Wiring

Thermostat Types

Electric UFH thermostats control the heating element by switching the live supply. Types include:

• Floor sensor only: A NTC thermistor probe embedded in the floor monitors screed/tile temperature. Prevents overheating of sensitive floor coverings. Simple and reliable.
• Air sensor only: Responds to room air temperature. Overshoots possible if the floor has high thermal mass.
• Dual sensor (floor + air): Air sensor controls comfort setpoint; floor sensor limits maximum floor temperature. Recommended for wood and laminate floors where overheating damages the covering.
• Smart thermostats: WiFi or ZigBee connected, programmable via app. Compatible with systems such as Google Home and Amazon Alexa. Require neutral at the thermostat backbox (check compatibility).

Thermostat Wiring

Thermostats are surface-mounted or flush-mounted in a standard 25 mm deep backbox. A typical wiring arrangement:

• Live (L) and Neutral (N) from consumer unit to thermostat supply terminals
• Switched live output (L out) from thermostat to UFH element live
• Neutral from thermostat to UFH element neutral
• Earth continuity throughout
• Floor probe wire connected to thermostat sensor terminals (low voltage signal — not mains)

Install the floor probe in a conduit or tube before the screed or adhesive is applied so it can be replaced without lifting the floor if it fails.

Location Rules (Bathroom Zones)

Under BS 7671 Regulation 701, thermostats must be located outside Zone 1 (above the bath or shower tray) and Zone 2 (within 0.6 m horizontally from the bath rim). Minimum IPX4 rating is required for thermostats within Zone 2; IPX5 for areas subject to water jets. Most thermostats are rated IP21 and must be mounted outside these zones — typically outside the bathroom or high on the wall above the door.

Installation Procedure

1. Prepare the Substrate

The subfloor must be clean, dry, and level. Fix any cracks or hollows with floor levelling compound. Install rigid insulation board (minimum 6 mm, ideally 10 mm PIR or XPS) to reduce downward heat loss. Insulation is critical — without it, much of the heat warms the structural floor rather than the room above.

2. Plan the Layout

Mark the area to be heated (excluding zones under fixed furniture). Calculate cable spacing: spacing (mm) = (area in m² / cable length in m) × 1000. Fix the cold tail (the mains cable section) at the wall closest to the thermostat position and route it up to the backbox.

3. Lay the Cable or Mat

For loose cable, secure to fixing strips or insulation board every 250–300 mm. Maintain consistent spacing throughout. For mats, unroll in the required pattern, cutting the mesh only to change direction. Keep a minimum 50 mm clearance from walls and obstacles.

Perform a resistance check before embedding. Record the expected resistance from the manufacturer's data (typically printed on the element). Measure actual resistance with a multimeter — it should be within ±10% of the rated value. Also perform an insulation resistance test: IR ≥ 1 MΩ at 500 V DC (BS 7671 Regulation 643.3).

4. Embed and Cure

For mats: apply tile adhesive bed (minimum 6–8 mm deep) over the mat, working carefully to avoid displacing the element. Do not energise the heating until the adhesive has fully cured — typically 28 days for cement-based adhesives. Premature energising causes differential expansion and cracking.

For loose cable in screed: pour or pump screed over the cable, ensuring full encapsulation. Cure time for screed is typically 21–28 days before energising.

5. Commission

After curing:

1. Check resistance again — compare with pre-installation reading. Any significant change indicates cable damage.
2. Perform IR test at 500 V DC: IR ≥ 1 MΩ.
3. Set thermostat to minimum and energise. Verify the element warms up (check with surface thermometer after 30 minutes).
4. Conduct loop impedance test (Zs) at the thermostat backbox.
5. Test RCD: press test button — RCD must trip. Use a loop tester's RCD function to verify trip time ≤ 40 ms at I∆n.
6. Complete an Electrical Installation Certificate (EIC) for new circuits or a Minor Works Certificate (MEIWC) for additions to an existing circuit.

Floor Covering Compatibility

Not all floor coverings are suitable for electric UFH. Check the manufacturer's maximum operating temperature:

Common Faults and Troubleshooting

• No heat, thermostat display on: Check switching output with a multimeter. Verify the element resistance — open circuit (infinite resistance) means a broken cable. Element damage during installation is the most common cause.
• Tripping RCD: Insulation failure — moisture ingress during screed curing, or mechanical damage. IR test will confirm. Allow full curing time before energising.
• Uneven heating: Cold spots indicate cable damage or voids in adhesive/screed. Thermal imaging can identify problem areas without lifting the floor.
• Thermostat probe failure: Replace via the conduit tube without disturbing the floor. Always install probe in conduit.
• High running costs: Inadequate insulation below the element is the most common cause. Heating the structural slab rather than the room wastes energy.

Notifiable Work

Electric UFH circuits are notifiable under Part P of the Building Regulations when installed in dwellings. Notification can be made via a registered competent person scheme (e.g. NICEIC, NAPIT) or by submitting a Building Regulations application to the local authority. The EIC must be provided to the homeowner and a copy forwarded to the certification body.

Recommended Products

When quoting or installing electric UFH, these products are commonly required: