Underfloor Heating in London Renovations: Hydronic vs Electric, and How to Specify It

Underfloor heating (UFH) is one of the most transformative specifications in a London renovation. Radiators are eliminated or reduced to a discreet supplement, the floor surface becomes warm rather than cold, and the room heats evenly from the ground up rather than stratifying with warm air at ceiling height. In a high-specification renovation, UFH is essentially standard in ground floor zones and bathrooms. Understanding the system types and the installation constraints determines whether it is feasible for a given project and how it is specified.

Hydronic vs electric: the core decision

The fundamental choice in UFH is between a water-based (hydronic) system and an electric resistance system. They differ in operating cost, installation cost, floor build-up depth, and appropriate application.

Hydronic UFH

Hydronic systems circulate warm water through pipework embedded in or beneath the floor screed. The water is heated by the building's boiler or heat pump. Heat output is controlled by a manifold system with individual zone valves and programmable thermostats.

Advantages: - Low operating cost — the water is heated by the main heating system, which can be a high-efficiency heat pump or condensing boiler - Appropriate for whole-house or large-area heating (it is the primary heat emitter, not a supplement) - Long service life — pipework buried in screed is maintenance-free for decades if installed correctly

Disadvantages: - Higher installation cost (manifold, pipework, screed) - Requires a wet trade screed — adds 65–100mm to floor build-up - Slow response time — a 75mm screed takes 2–4 hours to warm up from cold

Appropriate applications: ground floor zones in a whole-house renovation where the heating system is being replaced or upgraded. Any zone where the floor is being taken back to subfloor level and a new screed is being laid.

Electric UFH

Electric systems use resistance heating cables or mats laid under the floor finish, controlled by a programmable thermostat. No wet trade is required — the mats are laid directly on a thin bed of adhesive or self-levelling compound under a tile or stone finish.

Advantages: - Low installation cost and minimal floor build-up (6–12mm additional depth) - Rapid response — an electric mat under tiles heats to temperature in 20–30 minutes - Appropriate for bathroom and kitchen retrofits where screed is not viable

Disadvantages: - High operating cost — resistance heating from the grid is significantly more expensive per kWh than heat-pump-heated water, particularly as electricity prices remain elevated - Not suitable as a primary heat source for a whole room — it is a comfort supplement (warm floor) rather than a space heating system

Appropriate applications: bathrooms, en suites, cloakrooms, and kitchen floor areas where a screed is not being laid, or where the zone is supplementary to a primary heat emitter.

Floor build-up and structural implications

The floor build-up determines feasibility. In London's Victorian and Edwardian housing stock, the ground floor is often at a level constrained by:

• —The threshold between inside and external paving (lowering inside means water ingress risk)
• —The height of existing stair nosings and door frames (any significant screed depth requires joinery adjustments)
• —The depth of the existing subfloor structure (a suspended timber ground floor requires an entirely different approach to a solid concrete slab)

Over a concrete slab

The typical build-up for a hydronic system over a concrete slab:

1. 1.Concrete slab (existing)
2. 2.DPM (damp proof membrane) if required
3. 3.50–75mm insulation (PIR board or EPS — critical to prevent heat loss downward)
4. 4.UFH pipework clipped to insulation
5. 5.65–75mm liquid screed or sand/cement screed
6. 6.Floor finish (stone, tile, LVT, engineered timber)

Total added floor depth: approximately 130–160mm over the original slab. This is the number that must be coordinated with architect and structural engineer before committing to UFH.

Over a suspended timber floor (ground floor)

Retrofitting hydronic UFH to a suspended timber floor is possible but more complex. Pipework is typically run between joists in aluminium heat-spreader plates, and insulation is installed between joists from below. There is no screed — the floor finish sits on the existing boarding over the UFH system. Heat output is lower than a screed system, and response time is faster.

This approach is appropriate where floor level cannot be raised and the existing structure is accessible from below (e.g. via a cellar or sufficient underfloor void).

Bathrooms (electric, thin bed)

Electric mat systems under tiles: 1. Existing floor structure 2. Tile adhesive 3. Electric heating mat (6mm) 4. Tile adhesive 5. Floor tile

Additional floor depth: 6–15mm. This is generally feasible in any bathroom renovation.

Controls and zoning

A well-designed UFH installation divides the building into zones — each controlled independently with its own thermostat and timer. Minimum zoning:

• —Ground floor open-plan living/kitchen: one or two zones
• —Each bathroom: independent zone
• —Upper floors (if heated): per-floor or per-room zoning

Smart thermostats (Heatmiser, Honeywell, Nest) integrate with UFH manifolds and allow remote control and scheduling. Specify smart controls from the outset — retrofitting them to a basic system is possible but adds cost.

Commissioning and screed curing

A hydronic UFH system in screed must be commissioned (slowly heated from cold to operating temperature) before the building is occupied. The process:

1. 1.Screed cure: sand/cement screed requires 28 days of natural curing before any heat is applied. Liquid anhydrite screed requires a minimum of 7 days.
2. 2.Initial heat-up: raise the flow temperature to 25°C and hold for 3 days, then raise to maximum operating temperature (typically 45–55°C flow) for 4 days.
3. 3.Floor covering installation: only after commissioning is complete. Installing floor coverings over an uncured or uncommissioned screed is a leading cause of tile and grout cracking.

The commissioning log must be documented and provided to the client. Without it, the UFH manufacturer's warranty may be void, and a future insurance claim for floor damage may be disputed.

Costs

Approximate supply and install costs in London (2025–26):

• —Hydronic UFH to ground floor of a 3-bed terrace (approx. 50–60m²): £6,000–£12,000 including manifold, pipework, and screed
• —Electric UFH mat to a single bathroom (approx. 4–6m²): £400–£900 supply; £300–£600 installation
• —Smart thermostat per zone: £150–£350 installed

Insulation below the screed is not optional — a hydronic system without adequate downward insulation heats the slab rather than the room and performs poorly. Budget £15–£25/m² for 50mm PIR board.