Electric Underfloor Heating Guide — Mat, Loose Wire and BS 7671 Section 753

Heating Mat Systems

• Twin-conductor cable pre-attached to fibreglass mesh at a fixed spacing (typically 50mm to 100mm between passes). Standard output: 100W/m\u00b2 to 200W/m\u00b2. Suitable for installation in tile adhesive, self-levelling compound, or thin-bed mortar. Total system height: 3mm to 6mm.
• Best for: Rectangular rooms, quick installation, tiled floors. The mat is cut and folded to navigate the room shape — only the mesh is cut, never the cable itself.
• Limitations: Fixed spacing means fixed watt density — no ability to increase output in colder areas. Less suitable for irregular-shaped rooms or areas with many obstacles.

Loose Wire Systems

• Twin-conductor cable fixed to the floor substrate using clip rails or adhesive fixing clips. Spacing is set by the installer to achieve the required watt density. Typically embedded in a 50mm to 75mm sand-cement screed or self-levelling compound. Cold tail feeds back to thermostat.
• Best for: Irregular-shaped rooms, new-build screeded floors, whole-floor heating in living areas, variable watt density requirements (e.g. more output near external walls).
• Limitations: Slower to install, requires screeding (adds floor height), longer heat-up time due to screed thermal mass.

• Solid concrete slab: Loose wire in screed is the standard approach. Lay insulation board (typically 25mm to 50mm PIR) first to prevent heat loss downward. Fix heating cable clips to the insulation at the required spacing. Pour 50 to 75mm sand-cement screed or self-levelling compound over the cables. Allow 28 days full cure before commissioning.
• Suspended timber floor: UFH can be installed between joists (clipped to the underside of floorboards) using specialist between-joist heating elements, or above the subfloor using overlay panels. Standard mat systems in tile adhesive are not suitable directly on timber — the adhesive layer is too thin to protect the cable and the timber movement causes cracking.
• Tiled floor (renovation): Heating mat in tile adhesive is the standard system for retrofitting UFH beneath tiles. The mat adds minimal floor height (3 to 6mm). Decouple the mat from the subfloor using a decoupling membrane where the substrate is subject to minor movement.

Wiring a UFH Thermostat

• Supply: Live, neutral, and earth from the consumer unit or fused spur to the thermostat backplate. The heating element (cold tail) connects to the load terminals. Confirm the thermostat load rating exceeds the heating element wattage.
• Floor sensor: A two-core sensor cable (typically 3m to 5m) runs from the thermostat back-box through conduit into the floor. The sensor tip sits within a conduit loop in the screed or adhesive layer, positioned between two cable passes midway across the heated area. The conduit allows sensor replacement without lifting the floor.
• Temperature limiting: Set the floor temperature limit to no more than 35\u00b0C for most floor constructions per BS 7671 Regulation 753.423 (Protection against burns), or per the floor covering manufacturer specification where lower (e.g. 27\u00b0C for engineered timber). The floor limiter takes priority over the air temperature setpoint — if the floor reaches the limit, the system shuts off regardless of air temperature.

• Design current (Ib): Total wattage \u00f7 230V. A 1,500W bathroom heating mat: Ib = 1,500 \u00f7 230 = 6.5A. A 3,000W living room system: Ib = 3,000 \u00f7 230 = 13A.
• Overcurrent device (In): Select the RCBO rating above the design current. For 6.5A, use a 10A RCBO. For 13A, use a 16A RCBO.
• Cable size: 1.5mm\u00b2 twin and earth is suitable for most domestic UFH circuits up to 16A when run in standard conditions. For larger systems (20A+), use 2.5mm\u00b2. Apply installation method and grouping derating factors per BS 7671 Chapter 52.
• RCD protection: BS 7671 Regulation 753.415.1 requires that circuits supplying heating units shall have additional protection by RCDs having the characteristics of Regulation 415.1.1 (rated residual operating current not exceeding 30mA). An RCBO at the consumer unit provides both overcurrent and RCD protection in a single device.

• Regulation 753.2: Heating systems must be installed in accordance with the manufacturer instructions, which form part of the compliance evidence alongside the EIC.
• Regulation 753.423 — Protection against burns: In floor areas where contact with skin or footwear is possible, the surface temperature of the floor shall be limited. The NOTE to this regulation gives 35°C as the example maximum for floor heating systems.
• Regulation 753.424.201 — Overheating protection (A4:2026): For floor or ceiling heating systems in buildings, one or more of the following shall be applied within the zone where heating units are installed to limit the temperature to a maximum of 80°C: (a) appropriate design of the heating system, (b) appropriate installation, or (c) protective devices for temperature limiting. Note that 80°C is the upper limit for the heating unit zone — the separate 35°C limit in Regulation 753.423 governs the walkable floor surface.
• Regulation 753.424.201 — Inseparable cold tail connections: Heating units shall be inseparably connected to cold tails — for example, by a crimped connection. This A4:2026 requirement is checked on EICR; a heating element with a detachable or plug-in cold tail connection does not comply.
• Regulation 753.415.1 — RCD protection: Circuits supplying heating units shall have additional protection by RCDs with the characteristics of Regulation 415.1.1 (rated residual operating current not exceeding 30mA).
• Regulation 753.520.4 — Heating-free areas: Where room fittings (such as fixed kitchen units, bathroom vanity units, or built-in furniture) are to be attached adjacent to or above UFH elements, heating-free areas shall be provided so that the fittings do not prevent heat emission. This must be planned before cable layout is fixed. Manufacturer instructions typically specify the required clearances.

• Try it free for 7 days

  
  — record UFH circuit details, RCBO rating, floor sensor type, and all test results including pre- and post-installation IR readings.
• Try it free for 7 days

  
  — issue a minor works certificate when connecting UFH to an existing suitable circuit without consumer unit modifications.