Summary

Electric underfloor heating (eUFH) is particularly popular in bathrooms and kitchens where it provides comfortable background heat and warm floors without visible radiators. It's typically used as a primary heat source in well-insulated spaces or as a supplementary comfort heater in rooms that already have wet central heating.

Unlike hydronic (wet) underfloor heating — which requires a heat source, pump, and manifold — electric UFH is a direct conversion of electricity to heat through resistance cable. It's significantly cheaper to install than wet UFH (no pipework, manifold, or pump) but more expensive to run, especially if used as a primary heat source in larger rooms.

The regulatory requirements for electric UFH overlap between electrical installation (BS 7671:2018+A2:2022) and thermal performance. The installation is notifiable work under Part P. A floor temperature-limiting sensor must be used with certain floor coverings (particularly wood and laminate) to prevent damage to the floor.

Key Facts

  • Heating mat — Resistance cable pre-spaced and fixed to fibreglass mesh; easy to unroll and position; widths of 0.5m, 1.0m, 0.5m standard
  • Loose cable — Individual cable on a reel; can be spaced to suit any shape; more flexible but more labour-intensive
  • Cable spacing — Standard mats have 75-100mm spacing; loose cable can be adjusted (wider spacing = less output per m²)
  • Typical output — 100-200W/m² (150W/m² is most common for bathrooms); 100W/m² for supplementary heating
  • Minimum cable spacing — 50mm between adjacent cable passes; never cross cables or allow touching
  • Thermostat requirement — A thermostat with both air temperature sensor AND floor temperature limiting sensor is required for most installations
  • Floor temperature limit — Wood and laminate floors: typically 27°C max; tiles: 40°C max; check floor manufacturer's guidance
  • Cable-to-floor contact — Cables must be fully embedded in adhesive or screed; air gaps reduce performance and may cause hot spots and cable damage
  • Insulation board — Installing a 6mm insulation board (Ditra-Heat, Warmup Insulation Board, etc.) beneath the mat reduces downward heat loss significantly and improves efficiency
  • Circuit protection — 30mA RCD protection required; typically 16A circuit for a bathroom mat (sufficient for up to 3.5kW)
  • Tiled floors — Most compatible with electric UFH; flexible cable works over cable before tiling
  • Part P — New circuit from consumer unit is notifiable work
  • Manufacturer guarantee — Most manufacturers (Warmup, Nuheat, Devi) offer 10-25 year warranties if installed correctly with a compatible thermostat

Quick Reference Table

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Area Type Recommended Output Suitable For Notes
Bathroom (primary heat) 150-200 W/m² All bathrooms Standard; run by programmable thermostat
Kitchen (supplementary) 100-150 W/m² Tiled or stone floors Supplementary to radiator
Living room (supplementary) 100 W/m² Various floor types Check floor type temperature limit
Conservatory (primary) 150-200 W/m² Well-insulated conservatory Poor insulation makes this expensive
Bathroom wet room 150 W/m² All floors IP-rated thermostat required if in wet zone
Floor Covering Max Floor Temperature Notes
Ceramic/porcelain tiles 40°C Most compatible; no restrictions
Natural stone 40°C Ensure no thermal shock risk for sealed stone
Laminate 27°C Floor sensor limiting essential; check manufacturer
Engineered hardwood 27°C Floor sensor limiting essential; check floor warranty
Solid hardwood Not recommended Risk of cracking; some manufacturers prohibit
Carpet Not recommended Very poor heat transfer; not practical
Vinyl/LVT 27°C Check specific product; some LVT not suitable

Detailed Guidance

Mat vs Loose Cable: When to Use Each

Heating mats:

  • Pre-spaced cable on a mesh carrier; available in standard sizes (0.5m, 1.0m wide; various lengths)
  • Simple to install: unroll, position, and tile or screed over
  • Best for regular rectangular areas
  • Not easily customised for L-shapes or rooms with islands/obstacles
  • Installation time: 30-60 minutes for a standard bathroom

Loose cable:

  • Single cable on a reel; installer determines the spacing
  • Can be arranged to suit any room shape: L-shapes, around toilet bases, alcoves
  • Requires calculating the correct cable length for the intended area and spacing
  • More time-consuming to install: 1-2 hours for a bathroom
  • More flexible for unusual spaces

Calculation for loose cable:

  • Determine heated area (exclude under fixed furniture: bath, toilet, vanity units, shower tray)
  • Decide on output level (W/m²) and cable wattage per metre
  • Cable spacing = (cable watt/m ÷ desired W/m²) × 1000mm
  • Example: 12W/m cable in a 5m² bathroom at 150W/m² = 750W total cable length = 62.5m → spacing = (12/150) × 1000 = 80mm

Thermostat Selection and Wiring

A dual-function thermostat with air sensor and floor sensor is standard for electric UFH:

Air sensor: Measures room temperature; controls comfort level (e.g., target air temperature of 21°C)

Floor sensor: A separate NTC thermistor probe laid in the screed or tile adhesive between heating cables; prevents floor exceeding the maximum temperature for the floor covering

Thermostat wiring (typical):

  • Line (L) — Permanent live from circuit
  • Neutral (N) — Neutral
  • Earth (E) — Earth
  • Load L — Switched live to heating mat/cable
  • Load N — Neutral to heating mat/cable
  • Floor sensor — Two-wire NTC probe (polarity insensitive; typically leads in conduit to position between cables)
  • Air sensor — Usually built into the thermostat body

Thermostat position: Outside of the Zone 2 IP boundary in bathrooms (see bathroom zones); typically on the wall adjacent to the bathroom door, not inside the bathroom zone.

IP rating in bathrooms: If the thermostat must be inside Zone 2, it requires IP44 or better. Consider installing it in an adjacent landing or hallway where possible.

Installation Step by Step

Preparation:

  1. Check the subfloor is clean, level, and dry
  2. Fit any insulation board (strongly recommended; reduces heat loss to subfloor by 50%+)
  3. Install a plastic conduit for the thermostat floor probe — run to the sensor location between cables; leave an adequate loop so the probe can be withdrawn for replacement if needed

Mat/cable installation: 4. Unroll the mat or lay loose cable starting from the thermostat position 5. Avoid cutting or crossing cables; mats can be turned by cutting the carrier mesh (NOT the cable) 6. Maintain minimum 50mm spacing between cable passes 7. Keep cables at least 50mm from walls 8. Position the thermostat probe conduit between two cable runs; the probe tip should be midway between cables

Testing: 9. Use a multimeter to check cable resistance before and after installation; compare against manufacturer specification (should be within ±10%) 10. Use a 500V insulation resistance tester to check cable insulation to earth (should be >1MΩ; most good cable reads >100MΩ) 11. Record resistance values in the commissioning certificate

Floor covering: 12. Apply tile adhesive or screed over the cable (depth per manufacturer instructions — typically 15-20mm in adhesive) 13. Allow full cure before connecting the circuit and running the system

Running Cost Calculation

Running cost per hour = (watts ÷ 1000) × electricity tariff (p/kWh)

Example — 5m² bathroom, 150W/m², 30p/kWh tariff:

  • Total output: 5m² × 150W/m² = 750W
  • Cost per hour at 100% on: 0.75kW × 30p = 22.5p per hour
  • Thermostat duty cycle: typically 30-50% on time for a bathroom used twice daily
  • Practical daily cost (used 4h, 40% duty cycle): 4h × 40% × 22.5p ≈ 36p per day (approximately £10/month)

For larger areas used as primary heat sources, running costs escalate significantly. A 20m² living room at 150W/m² running 8 hours daily would cost approximately £2-3 per day — heating the living room with electric UFH alone is expensive at standard electricity tariffs.

Making it cheaper:

  • Off-peak tariff (Economy 7, Octopus Go): run during cheap-rate hours to pre-warm the floor; the floor's thermal mass stores heat
  • Solar PV self-generated electricity reduces marginal cost significantly

Frequently Asked Questions

Do I need a new circuit for electric underfloor heating?

For anything more than a very small mat (under 1kW), yes — a dedicated circuit from the consumer unit is strongly recommended. This provides clean fault isolation, ensures correct circuit protection sizing, and is generally required for the warranty. For a typical bathroom mat (500-750W), a 16A circuit with 30mA RCBO is standard.

Can electric underfloor heating be fitted under laminate or engineered wood?

Yes, but carefully. A floor temperature sensor limiting thermostat is essential — the floor must not exceed 27°C (or the floor manufacturer's specified maximum, whichever is lower). Some wood floor manufacturers void their warranty if UFH is installed without their specific approval; check the floor warranty before installation. Never install under solid hardwood (too high expansion/contraction risk).

What's the minimum installation depth for a heating mat?

The cable should be fully embedded in adhesive or screed — typically 15-20mm above the cable. Insufficient depth can create hot spots (damaging the cable), poor heat distribution, and cracked tiles. Most manufacturers specify a minimum of 10mm of adhesive or mortar coverage above the cable.

Can I lay tiles over electric UFH straight away?

No. Tile adhesive and any levelling compound must be fully cured before the heating system is switched on. This typically means at least 7-28 days depending on the screed or adhesive type. Switching on too early while the screed is green can cause cracking. After the cure period, bring the temperature up gradually over 3-7 days to the full operating temperature.

Regulations & Standards

  • BS 7671:2018+A2:2022 — Wiring Regulations; Section 753 (Heating systems using resistive heating cable, embedded in floors, ceilings, or walls)

  • Part P of the Building Regulations — New electrical circuits are notifiable work

  • BS EN 60335-2-96 — Household and similar electrical appliances; safety for flexible sheet heating elements for room heating; relates to electric UFH product standards

  • Warmup Electric Underfloor Heating Installation Guide — Detailed installation guidance from a leading UK manufacturer

  • Devi Electric Underfloor Heating — Product and installation documentation

  • BEAMA Guidance on Electric Heating — UK electrical industry guidance

  • underfloor heating — Wet/hydronic underfloor heating comparison

  • bathroom zones — IP requirements for thermostat positioning in bathrooms

  • cable sizing — Circuit sizing for UFH circuits

  • part p notifications — Part P notification for new circuits