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.

Common Faults and Troubleshooting

No Heat — Element Open Circuit

Measure element resistance at the thermostat terminals. An infinity reading indicates a break in the heating cable — typically caused by the mat being cut during subsequent floor work (drilling into the slab, chasing a wall). Locate the break using a time-domain reflectometer (TDR) or by running the system briefly and using a thermal imaging camera to spot the cold zone. Unfortunately, this almost always requires lifting tiles to reach the break point.

No Heat — Thermostat Fault

Confirm mains supply voltage is present at the thermostat line terminals. Verify the thermostat is calling for heat — set point must be above the current floor or air temperature. Test the switched output terminal with a multimeter while the thermostat calls for heat. If the output switches correctly but the element does not warm, retest element resistance. If the thermostat does not switch, the unit is faulty and requires replacement.

RCD Tripping on the UFH Circuit

A 30mA RCD trip on a UFH circuit almost always points to reduced insulation resistance on the heating element — moisture ingress following grouting, a pinhole in the cable insulation, or contamination at the cold tail connection point. Disconnect the element at the thermostat and retest insulation resistance at 500V DC. Allow the floor to dry for several weeks before retesting. If IR remains below 1MΩ, the element has failed and replacement is the only option.

Uneven Floor Temperature

Localised hot spots with surrounding cool areas usually result from adhesive voids under or over the heating cable. The cable overheats in the void, creating a hot patch at the tile surface while the surrounding adhesive-embedded cable dissipates heat normally. Prevention is always preferable — use a notched trowel, ensure full cable embedment, and check for hollow spots by tapping after tiling. Thermal imaging under load is the most effective diagnostic tool if uneven heating is reported after commissioning.

High Running Costs

Electric UFH running costs are a frequent customer complaint. At a 2026 unit rate of approximately 25p/kWh, a 160W/m² mat over 2m² draws around 320W at full load — costing roughly 8p per hour. With a well-programmed thermostat running 4–6 hours per day, this amounts to £0.32–£0.48/day. The floor sensor limit should be set to 27–28°C for LVT or 35°C for tile; higher limits cause the thermostat to run the element for longer than necessary. Set accurate expectations at point of sale, particularly on uninsulated ground-floor concrete slabs where a significant proportion of heat is lost downward.

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.