Underfloor Heating in London Extensions and Refurbishments: Wet vs. Electric, Design, and Installation

**Wet UFH system components**:

A wet UFH system consists of:

• *1. Heating pipes*:
• •Material: cross-linked polyethylene (PEX-a or PEX-b) or polybutylene (PB) — flexible, UV-resistant, durable; rated for continuous service at 70°C water temperature
• •Diameter: typically 15mm or 16mm OD; 12mm OD in some high-density installations
• •Installation: pipes are laid in the floor in a continuous serpentine (snaking) pattern before the concrete screed or slab is poured over them
• •Two pipe patterns are used: serpentine (bifilar winding — the pipe goes back and forth across the zone in parallel rows; creates a slight temperature gradient from the hotter supply end to the cooler return end) and spiral (the pipe spirals inward from the perimeter to the centre; more even temperature distribution but more complex to lay)
• •Pipe spacing: 150mm, 200mm, or 250mm centres — closer spacing gives higher heat output; wider spacing reduces heat output but lowers material cost. Standard domestic specification: 200mm centres throughout; 150mm centres in colder zones (bathrooms; adjacent to external walls)

• *2. Manifold*:
• The UFH manifold is the central distribution point for the system. It receives the hot water from the boiler and distributes it to the individual pipe circuits (one circuit per room or zone). Key manifold components:
• •Flow and return headers: the main manifold bar (typically stainless steel or brass) with individual circuit ports
• •Actuators: electrically-operated valves on each circuit port, controlled by the zone thermostats — when a thermostat calls for heat, the actuator on that circuit opens, allowing water to circulate through the floor of that zone
• •Balancing valves: manual flow-control valves on each return port, adjusted at commissioning to balance the flow across circuits
• •Flowmeters: sight-glass flowmeters on each circuit port, showing the water flow rate (l/min) in each circuit — used to verify balanced flow after commissioning
• •Pump and blending valve: the UFH manifold typically includes a small circulating pump (or a connection to the boiler's pump) and a thermostatic blending valve (TBV) that blends the high-temperature boiler water with cooler return water to achieve the low UFH flow temperature (35–45°C) — even if the boiler is running at 65–75°C, the blending valve steps down the temperature to the correct UFH level

• *3. Zone thermostats and controls*:
• •Each zone (room or area) has a wall-mounted thermostat that monitors the air temperature in that zone
• •When the temperature drops below the set point, the thermostat signals the manifold actuator to open, allowing water to circulate in that zone's floor loop
• •The thermostat also signals the boiler (via a wiring centre or smart controller) to fire up if any zone is calling for heat
• •Smart thermostat compatibility: Heatmiser, Honeywell T6R, Nest (with compatible relay wiring), Tado — all work with standard wet UFH manifold systems

**Pipe circuit lengths and manifold position**:

• Each UFH pipe circuit is a single continuous length of pipe from the manifold to the zone and back. Design rules for circuit lengths:
• •Maximum circuit length: approximately 100–120m for 15/16mm pipe at standard flow rates — above this length, the pressure drop across the circuit becomes too high for the pump to overcome, resulting in inadequate flow and poor heat distribution
• •For a 25m² extension with 200mm pipe spacing: circuit length = (25m² / 0.2m spacing) × 2 (return) ≈ 250m — this exceeds the single circuit maximum, so two circuits (each approximately 125m) should be used
• •Manifold position: the manifold should be positioned centrally within the UFH zone (to minimise the length of the pipe run from the manifold to the zone, known as the 'flow and return header run' — this section of pipe is not actively heating and should be insulated or minimised)

**Floor construction with wet UFH**:

*On a concrete ground-bearing slab in a London extension*: 1. Concrete slab poured (C25/30, 100–150mm thick) 2. Perimeter insulation installed (50mm PIR strip around all edges) 3. 100mm PIR insulation laid over the slab 4. Pipe clips fixed to the insulation (proprietary UFH staple track or pipe clip rail system) 5. Pipes laid to the designed layout 6. Pipes pressure-tested (at least 4 bar; 24 hours before screed pour) 7. Concrete screed poured over the pipes (75–100mm depth; minimum 50mm cover above the pipe) 8. Floor finish laid once screed is cured and dry (tiles; engineered wood; LVT)

• *On a beam and block floor in a London extension*:
• •Standard wet UFH in screed is more difficult on a suspended B&B floor — the screed weight adds load to the structure (a 75mm sand-cement screed weighs approximately 135 kg/m² — a structural engineer must confirm the beam and block system can carry this)
• •Alternative for B&B: low-profile UFH panels (Warmup Foil System; Nu-Heat FastKlip; Schlüter Bekotec-Therm) with thin-bed finishes — maximum overlay floor build-up approximately 18–25mm; lighter than screed; faster response time

**Screed types and their compatibility with wet UFH in London extensions**:

• *Sand-cement screed (traditional)*:
• •Mix: 1:3–1:4 cement to sharp sand; w/c ratio 0.5–0.55
• •Depth: minimum 65mm above the pipe crown (if unbound floating screed) — typically 75–100mm total screed depth over the pipes with 50mm cover above the pipe
• •Drying: approximately 1mm per day per 25mm depth (i.e., a 75mm screed takes approximately 75 days to dry naturally in UK conditions) — this is a significant programme impact; calcium silicate hydration is slow
• •Advantages: well-established; low cost; compatible with virtually all floor finishes
• •Disadvantages: very slow drying time; can crack if shrinkage is not managed (polypropylene fibres added to control shrinkage cracking; movement joints at doorways and zone changes)
• •UFH commissioning: must wait for screed to be dry before commissioning UFH — if UFH is switched on too early, the heat drives moisture to the surface, causing tile adhesive failure and tile delamination. Wait until the screed moisture content is below 75% RH (measured with a hygrometer probe) before fixing tiles

• *Liquid anhydrite screed (calcium sulphate-based)*:
• Products: Gyvlon (Saint-Gobain); Flo-screed; Cemfloor
• •Depth: only 35–40mm above the pipe crown required — the self-levelling flow of liquid anhydrite ensures complete encapsulation of the pipe without air voids, and the 40mm total screed depth is possible because the anhydrite is stronger in thin section than sand-cement screed
• •Drying: approximately 1mm per day for the first 40mm, then 0.5mm per day — a 40mm liquid anhydrite screed typically dries in 28–35 days (significantly faster than sand-cement)
• •Advantages: thinner section; faster drying; better thermal conductivity than sand-cement screed (improves UFH efficiency); superior encapsulation of UFH pipe (no air voids)
• •CRITICAL RESTRICTIONS on liquid anhydrite screed:
• - NOT for use in wet rooms or bathrooms (anhydrite reacts with persistent moisture and degrades over time; not suitable where water may penetrate through the floor covering)
• - NOT compatible with aluminium UFH manifolds or pipe fittings in direct contact with the screed (the anhydrite's alkalinity attacks aluminium; use stainless steel or plastic manifolds and fittings within the screed zone)
• - Anhydrite screed must be sealed (with the specific primer recommended by the screed manufacturer) before any cement-based tile adhesive is applied — failure to prime results in adhesive incompatibility and tile delamination

**Screed drying and UFH commissioning — the correct sequence**:

1. Screed pour 2. Allow screed to cure naturally (no heating) for a minimum of 7 days (sand-cement) or 3 days (anhydrite) 3. Begin the UFH commissioning programme: bring the flow temperature up gradually over 5–7 days — starting at 25°C for 3 days; increasing to 35°C for 2 days; increasing to the maximum design flow temperature for the final 2 days — this slow warm-up dries the screed gradually without cracking 4. Allow screed to cool and check moisture content with a hygrometer probe: below 75% RH for sand-cement; below 65% RH for anhydrite 5. Fix tiles or floor finish once moisture readings are within the adhesive manufacturer's stated tolerance

Failing to follow this commissioning sequence is one of the most common causes of tile delamination and floor covering failures in London extensions with wet UFH.

**Floor finish compatibility with wet UFH**:

• •*Porcelain and ceramic tiles*: the best floor finish for UFH — low thermal resistance; heats up quickly; retains heat well. Always use flexible polymer-modified adhesive (C2 S1 minimum) for tiles over UFH — standard tile adhesive will crack and fail under thermal cycling. Large-format tiles (600×600mm+): require movement joints at 4–5m centres and at all perimeter walls
• •*Engineered wood*: compatible with UFH if the product is specifically rated for UFH use and the UFH flow temperature does not exceed 55°C (most UK manufacturers specify max 50°C UFH). Run UFH commissioning sequence before installation; the moisture content of the subfloor must be below 65% RH before laying. Maximum 22mm total thickness for rapid thermal response
• •*LVT (Luxury Vinyl Tile) and LVP (Luxury Vinyl Plank)*: highly compatible with UFH; low thermal resistance; flexible (accommodates UFH thermal movement without cracking or gapping). Most LVT products have max floor surface temperature ratings of 27–30°C — check manufacturer specification
• •*Solid hardwood*: NOT recommended over wet UFH — solid wood is dimensionally unstable under the humidity changes caused by UFH cycling; gaps form in winter; cupping occurs in summer
• •*Carpet*: possible over UFH if the total thermal resistance (tog rating) of carpet plus underlay does not exceed 1.5 tog (UFH standard). Most carpet + standard underlay combinations exceed this — high-density felt underlay specifically rated for UFH is required. UFH efficiency is reduced by carpet insulation effect; UFH response time is longer

**Electric underfloor heating — when it is the right choice**:

Electric UFH (resistance heating cable or mat embedded below floor tiles; powered directly from the electrical supply) is a fundamentally different system from wet UFH. It does not connect to the boiler — it generates heat electrically, directly in the floor.

• *When electric UFH makes sense in a London extension or refurbishment*:
• •Small areas (bathroom floor; kitchen floor in a limited zone) where the cost and complexity of wet UFH manifold installation and pipe circuit is not justified — electric UFH is simpler and cheaper to install for areas under approximately 10–12m²
• •Retrofitting into an existing room where the floor is not being reconstructed (the electric heating mat can be installed under a thin-bed adhesive and tile layer with minimum floor build-up — often only 10–15mm total increase in floor level)
• •Where a boiler connection is not available or not practical
• •As a secondary or supplementary heat source (main heating by existing radiators; electric UFH in the extension or bathroom as a comfort top-up)

• *Running cost comparison (London, 2025)*:
• •Electric UFH running cost: approximately £0.245–£0.30/kWh (UK retail electricity tariff, 2025, average); a 25m² electric UFH mat consuming 100W/m² = 2.5kW; running 8 hours/day in winter ≈ 20kWh/day × £0.275 = £5.50/day of direct heating cost
• •Wet UFH running cost: the same 25m² zone heated via the boiler at 90% efficiency; gas tariff approximately £0.055/kWh (2025 average retail); equivalent heat delivery ≈ 20kWh/day ÷ 0.90 = 22.2kWh gas × £0.055 = £1.22/day
• •Electric UFH costs approximately 4.5× more per unit of heat than gas-fired wet UFH at 2025 tariffs — this is the primary reason electric UFH is used only for small areas or supplementary heating, not as a whole-extension primary heating source

**UFH zoning for London extension projects**:

Effective zoning is essential for comfortable and efficient UFH operation in a London open-plan extension:

*Kitchen zone*: The kitchen zone under the kitchen units and cooking area gains significant incidental heat from cooking — the UFH thermostat in this zone will therefore reach setpoint quickly and the system will switch off for long periods during kitchen use. This is normal and efficient — the UFH provides background warmth; cooking supplements it.

*Dining and living zone*: The open-plan dining/living area receives less incidental heat — the thermostat should be set at a consistent background temperature (18–20°C) and can be programmed to reduce during unoccupied periods.

*Extension zones vs. existing house zones*: The extension UFH system should be independently zoned from the existing house heating — the extension warms up and cools down at different rates from the rest of the house (more glazing; often better insulation; different occupancy patterns). A separate manifold for the extension with its own thermostats allows the extension zone to be programmed independently.

**Costs for wet UFH in a London extension (2025, all-in supply and install)**:

| Zone area | Wet UFH (concrete slab; manifold; pipe; actuators; thermostat; commissioning) | |---|---| | 15m² (small extension) | £900–£1,600 | | 25m² (typical rear extension) | £1,500–£2,800 | | 40m² (large open-plan) | £2,500–£4,500 | | Additional zone thermostat | £100–£250 per zone | | Anhydrite screed upgrade (over sand-cement) | £3–£6/m² additional cost |

Electric UFH mat (supply and install, tiles not included): approximately £60–£120/m² including mat, thermostat, wiring, and fixing. A 5m² bathroom: approximately £300–£600.