Choosing the Right Heating System When Refurbishing a London Home

The gas boiler has been the dominant residential heating system in the UK for 50 years, and it remains the most straightforward and cost-effective solution for most London refurbishments in 2025. However, the regulatory direction of travel is clear — and understanding what is and isn't changing affects how you specify a boiler during a refurbishment.

• **Current position (2025)**:
• •New gas boilers can still be installed in existing properties — the 2025 ban applies only to *new-build* homes, which must now have low-carbon heating systems
• •Gas boilers in existing properties are not banned and will not be banned in the near future — existing boilers can be replaced like-for-like
• •The Government's Future Homes Standard for existing homes, and the formal regulation of gas boiler replacement, is expected to evolve through the late 2020s — but no specific ban date for existing-home replacements has been set as of 2025

• **Specifying a gas boiler during a refurbishment in 2025**:
• If replacing a gas boiler in an existing London property, specify:
• •A modern, high-efficiency condensing combination boiler (combi) or system boiler — all modern gas boilers are condensing and achieve SEDBUK efficiency ratings of 92–94%
• •A correctly sized boiler: oversizing is the most common error — a 3-bedroom terrace typically needs 24–30kW; a 4–5 bedroom house 30–35kW; a larger detached 35–40kW
• •A magnetic filter (e.g., Magnaclean) to protect the heat exchanger from magnetite sludge — a small cost that significantly extends boiler life
• •Thermostatic radiator valves (TRVs) on all radiators except the bathroom towel rail — required by Part L for all new heating system installations
• •A room thermostat and programmer or smart thermostat (e.g., Nest, Hive, Drayton Wiser) — also required by Part L

• **Cost of gas boiler replacement during a refurbishment (London 2025)**:
• •Standard combi boiler replacement (like-for-like, no major pipe changes): £1,800–£3,500 supply and install
• •Boiler replacement with significant pipework changes (e.g., moving the boiler, adding new radiators): £2,500–£5,000
• •Full heating system overhaul (new boiler, all new radiators, full pipework, underfloor heating to one zone): £6,000–£14,000

**Future-proofing note**: If there is a possibility that the property will switch to a heat pump within 10 years, it is worth specifying slightly larger radiators now — a heat pump requires lower flow temperatures (45–55°C vs 70–80°C for a gas boiler) and needs larger emitter surface areas to deliver the same heat output.

An air source heat pump (ASHP) extracts heat from the outside air and uses a refrigerant cycle to deliver it to the heating and hot water system at higher temperature — even at air temperatures as low as -15°C. A heat pump is not a heating element: it does not generate heat from electricity directly, but moves heat from the ambient air into the building.

**How efficient are heat pumps?** Heat pump efficiency is expressed as Coefficient of Performance (COP) or Seasonal Coefficient of Performance (SCOP). A SCOP of 2.5–3.5 means the heat pump delivers 2.5–3.5kW of heat for every 1kW of electricity consumed. At current UK electricity prices (approximately 25p/kWh) vs gas prices (approximately 6p/kWh), the maths requires a SCOP of approximately 4.0 to be cost-neutral against gas — achievable in well-insulated, low-temperature systems, but not in older properties running high-temperature radiator systems.

• **When heat pumps work well in London properties**:
• A heat pump works most effectively when:
• •The property is well-insulated (EPC C or better, or heading there)
• •The heat distribution system runs at low flow temperature (45–55°C) — which means underfloor heating throughout, or significantly oversized radiators
• •The heating demand is moderate — the outside air temperature rarely goes below -5°C in London
• •The property is not converting from a very low gas tariff

• **When heat pumps are harder to justify in London**:
• •Victorian terraces with solid walls and limited insulation — the heat loss is too high for the heat pump to be cost-effective at current electricity prices
• •Properties that cannot accommodate underfloor heating and cannot easily upsize all radiators
• •Properties with high occupancy and high domestic hot water (DHW) demand — heat pumps producing DHW are less efficient than those producing space heating, and large DHW volumes require a correctly sized hot water cylinder (typically 200–300L)

**Boiler Upgrade Scheme (BUS)**: The Government's Boiler Upgrade Scheme offers a £7,500 grant toward the installation of an air source heat pump (as of 2025 — value was increased from £5,000 in October 2023). The grant is available for properties in England and Wales. It applies to new ASHP installations, not replacements. The installer must be MCS-certified.

• **Cost of ASHP installation in London (2025, including BUS grant)**:
• •5–8kW ASHP with hot water cylinder, basic installation: £8,000–£14,000 (after £7,500 grant: effectively £500–£6,500)
• •8–12kW ASHP with hot water cylinder, full pipework and radiator upgrade: £12,000–£22,000 (after grant: £4,500–£14,500)

**Hybrid heat pump systems**: A hybrid system combines an ASHP with a gas boiler. The heat pump handles the majority of space heating; the gas boiler tops up on the coldest days when the heat pump is less efficient. Hybrids are often the most practical solution for London Victorian terraces that cannot be insulated to the level required for a pure heat pump — the gas boiler removes the efficiency concern in cold weather. Hybrids do not currently qualify for the BUS grant.

Underfloor heating (UFH) delivers heat through a network of pipes (wet UFH) or electric cables (electric UFH) embedded in or under the floor. Heat is delivered at low surface temperature (typically 25–29°C floor surface) over a large area, rather than at high temperature from a small radiator.

• **Wet UFH vs electric UFH**:
• •Wet UFH (hydronic): connected to the boiler or heat pump via a manifold. Higher installation cost, lower running cost. Suitable as the primary heating system throughout the property.
• •Electric UFH: lower installation cost, higher running cost (direct electrical heating at approximately 25p/kWh). Best used as a supplement in single rooms (bathroom floor, kitchen floor) rather than as the primary heating system.

• **When to specify wet UFH during a refurbishment**:
• A refurbishment is the best time to install wet UFH — especially when the floors are being replaced. UFH can be installed in:
• •Screed (65–70mm depth above joists or concrete slab): most effective system, most thermal mass, slowest response time (heat up from cold takes 2–4 hours)
• •Low-profile systems (Warmup Sticky Mat + self-levelling compound, 15–25mm total depth): useful where floor height is constrained
• •Above-board clip systems (between battens): can be used on existing timber floors with insulation below

**Zoning**: Wet UFH should be zoned — typically one zone per floor or per section. Each zone has its own manifold connections and thermostat. At minimum: ground floor and first floor are separate zones; kitchen, living, and bathroom are ideally separate.

**Part L compliance for UFH**: When wet UFH is installed as the primary system, the flow temperature from the heat source must be capable of delivering the UFH at maximum 45°C (so that the system is compatible with a future heat pump upgrade — Part L requirement for new and replacement heating systems).

**UFH and heat pumps**: Wet UFH is the optimal distribution system for air source heat pumps because of its low flow temperature requirement. If there is any possibility of installing a heat pump in the future, specifying UFH during the refurbishment is the most practical future-proofing measure available.

• **Costs for wet UFH during a refurbishment (London 2025)**:
• •Ground floor only (screed or low-profile), single zone: £2,500–£5,500
• •Full property, two floors, two zones: £5,000–£12,000
• •Full property with manifold, wiring centre, and smart controls: £7,000–£16,000
• (These are in addition to the boiler or heat pump cost.)