Electric Underfloor Heating Mats: Sizing, Installation, and Part P Compliance

Electric underfloor heating is one of the most requested bathroom and kitchen upgrades in the trade right now. No wet system, no manifolds, no pressurised pipework — just a mat, a thermostat, and a spur. Done right, it takes a good sparks a few hours. Done wrong, you’re cutting up tiles you just laid. This guide covers sizing, installation sequence, thermostat selection, and what Building Regs actually require.

Electric vs Wet UFH: Picking the Right System

Wet underfloor heating circulates warm water through buried pipework connected to the boiler. It’s the right choice for whole-house retrofits and new builds where the efficiency gain over the full floor area justifies the cost and floor depth. For a single bathroom, a kitchen extension, or a ground-floor WC, it’s overkill.

Electric mat systems sit in the tile adhesive or directly under floating floors. Total system depth is typically 3–4mm including the mat and adhesive build-up. You’re not raising floor height by 80mm. You’re not commissioning a manifold. The running cost comparison is a real consideration — electricity costs more per kWh than gas — but in a bathroom used for 30–60 minutes a day with a programmable thermostat, the total energy consumed is small. Most customers don’t think twice.

160W/m² vs 200W/m²: Which Output Rating?

The output rating (W/m²) tells you how much heat the mat produces per square metre of coverage.

160W/m² is the standard spec for primary heating in bathrooms, kitchens, and hallways. It will bring a cold tiled floor up to comfortable temperature within 20–30 minutes and maintain it without cycling heavily. This is the rating used across the eZheat range APM stocks.

200W/m² is used where you need faster warm-up times, where the room is poorly insulated, or where the mat is being laid over a solid concrete slab with no insulation board underneath (which wastes a significant proportion of the heat downward into the slab). If you’re laying over uninsulated concrete, either add 6mm insulation board first or step up to 200W/m² output.

Calculating Coverage: The m² You’re Actually Heating

This is where most installation errors start. The mat area is not the room area. You do not run heating mat under fixed furniture — baths, shower trays, vanity units, toilet pedestals, kitchen base units. Heat cannot escape upward through these, so you’re wasting energy and risking overheating the mat.

Calculate the free floor area: room length x room width, then subtract fixed furniture footprints. A typical en-suite shower room (1.8m x 2.4m = 4.32m² total) with a 900mm shower tray, toilet, and pedestal basin might yield 2.5m² of free floor. A 3m² mat is the right size — not 4m².

APM stocks the eZheat 160W range across all standard bathroom and kitchen sizes:

• eZheat 160W 1.5m² Mat — compact en-suites, cloakrooms
• eZheat 160W 2m² Mat — standard bathroom or kitchen section
• eZheat 800W 5m² Mat — larger bathroom or open-plan kitchen
• eZheat 160W 7m² Mat — master bathroom, hallway
• eZheat 160W 10m² Mat — large open-plan kitchen floor or multiple zones

Installation Sequence: Mat Before Tiles

Get the sequence right and the job is straightforward. Get it wrong and you’re cutting tiles you’ve already laid, or worse, damaging the mat under adhesive you can’t remove without replacing.

1. Substrate prep. The floor must be level, clean, and structurally sound. On timber subfloors, sheet with 18mm WBP ply before laying the mat — flex in the subfloor will crack both the adhesive and eventually the tiles. On concrete, check for damp. Add a DPM if needed before the mat goes down.

2. Dry lay the mat. Unroll and position the mat across the free floor area. The mat can be cut along the mesh (never through the cable) to change direction and fill the floor shape. The cold tail — the unheated lead cable — routes to the thermostat position on the wall. Standard cold tail length is 3m; check this reaches your spur/fused connection unit before committing.

3. Fix the mat. Self-adhesive backing holds the mat in place during tiling. On timber floors, staple the mat perimeter through the mesh to the ply before tiling.

4. Test before covering. Measure the mat resistance with a multimeter before laying any adhesive. Record the reading. It should match the figure on the product label (typically within ±10%). If it doesn’t, stop — there’s a fault. You do not want to discover a broken mat after you’ve tiled over it.

5. Lay the tile adhesive. Use a flexible tile adhesive rated for underfloor heating (look for “S1” or “S2” deformability class on the bag). Apply with a notched trowel, ensuring the mat is fully embedded — no air pockets under or over the cable. Let the adhesive cure fully (typically 24–48 hours depending on product and conditions) before energising the system.

6. Test again after tiling. Repeat the resistance check once the adhesive has cured. If the reading has changed significantly, investigate before grouting — you can still find the fault if you act now.

7. Connect and commission. Wire the cold tail into the thermostat. Commission the thermostat with a 21-day curing programme — most thermostats have this built in. Running at low temperature for the first three weeks cures the adhesive fully without thermal shock. Don’t run at full temperature on day one.

Thermostat Selection

Every electric mat system needs a dedicated thermostat — you cannot simply put the mat on a standard lighting circuit or a switched spur without temperature control. Thermostats for electric UFH use both a floor probe (a sensor cable embedded in the adhesive during installation) and optionally an air probe. The floor probe is essential — it limits the floor surface temperature and protects the mat and floor finish from overheating.

Key thermostat specifications to check:

• Current rating — most domestic electric UFH thermostats are rated 16A, which covers mats up to 3,500W (approximately 22m² at 160W/m²). Above this, you need two circuits or a higher-rated controller.
• Floor probe included — confirm the floor sensor is in the box. Some are sold separately.
• Programmability — 7-day programming allows different schedules for weekdays and weekends. Essential for keeping running costs down.
• WiFi/app control — useful for rental properties and tech-conscious clients. Not essential for basic installations.

The 16A Electric Underfloor Heating Thermostat APM stocks is a 16A unit with floor probe, suitable for all standard bathroom and kitchen installations in the eZheat range.

Part P: What’s Notifiable, What’s Not

Electric underfloor heating in a bathroom is notifiable work under Part P of the Building Regulations in England and Wales. A bathroom is a “special location” under BS 7671 (the IET Wiring Regulations), which means additional requirements apply regardless of notification status.

Key electrical requirements for bathroom UFH:

• RCD protection — 30mA RCD required for all circuits in a bathroom. If the property has an older consumer unit without RCD coverage on the bathroom circuit, this must be addressed as part of the installation — either by adding an RCBO or replacing the consumer unit.
• Zone rules — no switches or spur outlets within Zone 1 (inside the bath/shower) or Zone 2 (600mm from bath/shower edge horizontally, up to 2.25m high). The thermostat must be located outside these zones. In practice, this usually means above the towel rail position or on an adjacent wall.
• FELV/SELV separation — the mat is a mains-voltage product. Treat it as you would any mains circuit in a bathroom.
• Bonding — if there are exposed metallic parts in the bathroom, supplementary bonding requirements under BS 7671 Section 701 must be assessed.

If you’re a registered electrician under a competent person scheme (NICEIC, NAPIT, ELECSA), you self-certify. If not, a Building Control application is required before the work starts. The thermostat, mat, and associated wiring all form part of the notifiable installation.

Stock. Pick Up. Install Today.

APM Electricals in Acton stocks the full eZheat electric underfloor heating mat range — 1.5m² through 12m² — alongside thermostats and electrical accessories. Trade counter at 24 Western Avenue, Acton W3 7TZ. Same-day collection, Monday to Friday. Call ahead: 020 8702 8080.