Heating System Specification in London Renovations: Boilers, Heat Pumps, and Radiators

The heating system is among the least visible and most consequential specifications in a London renovation. A poorly sized boiler, an inadequately designed radiator circuit, or a heating system that cannot integrate with a smart control platform will cause daily frustration for the life of the building. Getting it right requires decisions at design stage — before walls are closed and before floor screeds are poured.

This guide covers the principal heating system options available in London renovations, how to specify them correctly, and the decisions that cannot be deferred.

Heat source options

Gas condensing boiler

The gas condensing combi or system boiler remains the most common heat source in London residential renovation. Gas supply is available throughout most of inner London, installation and maintenance costs are well understood, and the infrastructure (flue, gas pipe, condensate drain) is straightforward to accommodate in most properties.

Combi boiler: heats domestic hot water on demand without a storage cylinder. Appropriate for smaller households (1–2 bathrooms) where simultaneous hot water demand is limited. A combi cannot supply multiple hot water outlets simultaneously at full flow rate — specifying a combi in a 4-bathroom house is a common mistake that produces weak showers when demand is concurrent.

System boiler with unvented cylinder: the system boiler heats a pressurised hot water cylinder (typically 150–300 litres) that stores hot water at mains pressure. This configuration delivers consistent mains-pressure hot water to multiple outlets simultaneously and is the correct specification for any property with more than two bathrooms. The cylinder requires a dedicated cupboard (minimum 600mm × 600mm floor area, 1,800mm height for a standard 200-litre cylinder).

Efficiency: all modern condensing boilers operate at approximately 89–94% efficiency (ErP A-rated). The brand and model have less impact on efficiency than correct sizing, good controls, and regular servicing. Over-specified boilers (too large for the heat load) cycle frequently and operate below their rated efficiency — always commission a heat loss calculation before specifying boiler output.

Air source heat pump (ASHP)

An air source heat pump extracts heat from the outside air and transfers it into the building's heating and hot water system. At its rated coefficient of performance (COP), it delivers 2.5–4 units of heat energy for every unit of electrical energy consumed — significantly more efficient than any direct electric or gas heating system, and eligible for the UK government's Boiler Upgrade Scheme (BUS) grant (currently £7,500 for ASHPs).

When ASHP is the right specification: - The building is well-insulated and can achieve acceptable heat loss at lower flow temperatures (ASHP efficiency falls sharply above 55°C flow temperature) - The property has outdoor space for the external unit (minimum 600mm clearance around the unit; permitted development in most cases) - The electrical supply can accommodate the additional load (typically a 40–60A dedicated circuit) - Underfloor heating is specified throughout or oversized radiators are used (lower flow temperature requires larger heat emitters)

When ASHP is not appropriate: poorly insulated Victorian stock with high heat loss and a heating system designed for 70–80°C flow temperatures. An ASHP running at high flow temperature to compensate for inadequate insulation performs no better than a gas boiler in efficiency terms and at higher fuel cost (electricity costs approximately 4× gas per kWh). The insulation must be upgraded before ASHP specification makes sense.

Planning: ASHPs fall under permitted development in England for most properties (subject to conditions: not on a listed building, not in a conservation area with visible elevation impact, meets noise requirements — 42dB at 1m from neighbour's window). In conservation areas or listed buildings, a planning application may be required.

Ground source heat pump (GSHP)

More efficient than ASHP (COP 3.5–5.0) but requires ground collectors — either horizontal trenches (requires significant garden area) or vertical boreholes (200–150m deep). Borehole drilling costs £8,000–£15,000 per borehole and requires specialist contractors. The BUS grant covers GSHPs at a higher rate (currently £7,500). Appropriate for large properties with suitable ground conditions where the higher capital cost is justified by long-term running cost reduction.

Rarely specified in standard London terraces due to the ground access requirement.

Distribution: radiators vs underfloor heating

Radiators: the existing distribution infrastructure in most London properties. Radiators can be retained and upgraded (larger or double-panel radiators to accommodate lower flow temperatures if an ASHP is being specified) or replaced as part of a renovation. Designer radiators (low-profile horizontal, vertical column types) are appropriate in principal rooms where visual minimalism is a priority.

Underfloor heating (UFH): see the separate UFH specification guide. In combination with a gas system boiler, UFH is specified at ground floor level with a blending valve (UFH requires 35–50°C flow temperature; boiler flow temperature is typically 60–70°C). The blending valve reduces the boiler flow to the appropriate UFH temperature.

In a heat pump installation, the lower flow temperature of the heat pump is well-matched to UFH — this is the optimal pairing for a low-carbon renovation.

Mixed systems: many London renovations specify UFH at ground floor and radiators on upper floors. This is entirely viable and the correct approach where full UFH throughout is not practical (suspended timber upper floors, inadequate ceiling height for screed build-up).

Hot water

Unvented cylinder sizing: the cylinder must be sized for peak demand. As a guide: - 2 bathrooms, 4 occupants: 150–180 litres - 3–4 bathrooms, 5–6 occupants: 200–300 litres - Properties with multiple simultaneous showers: 250–300 litres minimum, or consider a thermal store

Legionella: unvented cylinders must be set to heat to 60°C periodically (daily thermal cycling) to prevent Legionella bacteria growth. This is a regulatory requirement under L8 guidance, not optional. Smart cylinder controls (Heatmiser, Sunamp) can automate this cycle.

Heat pump hot water: ASHPs produce hot water less efficiently than space heating — the COP falls at the higher temperatures required for hot water (55–60°C). A heat pump with an immersion heater backup or a dedicated heat pump cylinder (designed for the lower flow temperature of the heat pump, with an immersion heater for the pasteurisation cycle) is the correct configuration.

Controls

A heating system without good controls operates inefficiently regardless of the efficiency of the heat source. Minimum specification for a quality renovation:

Smart thermostat with multi-zone control: Heatmiser neoStat, Honeywell T6R, or equivalent. Individual room thermostats (or zone thermostats for groups of rooms) with programmable schedules and remote app control. A single thermostat for the entire building is not appropriate in a multi-storey property — the ground floor heats differently from upper floors and the heating system should respond accordingly.

Weather compensation: available on most modern boilers and heat pumps. The flow temperature is adjusted automatically based on outside temperature — higher on cold days, lower on mild days. This maintains comfort while maximising efficiency. Specify weather compensation from the outset; retrofitting requires additional sensors and wiring.

Hydraulic separation (where required): in a system where a heat pump feeds both UFH and radiators at different flow temperatures, a hydraulic separator (low-loss header or buffer vessel) may be required to prevent interference between circuits. Specify in conjunction with the heating engineer.

Heat loss calculation and system sizing

A heating system specification without a heat loss calculation is guesswork. The heat loss calculation establishes:

• —The total heating load of the building (kW) based on insulation levels, glazing area, infiltration rate, and design outside temperature
• —The required output of each radiator or UFH zone
• —The appropriate boiler or heat pump output

In London, a typical well-insulated 3-bedroom Victorian terrace has a whole-house heat loss of 8–14kW. A poorly insulated equivalent may be 16–22kW. Specifying a 28kW combi for a 10kW heat loss building means the boiler is severely oversized — it short-cycles, wears faster, and operates at below-rated efficiency.

Commission a heat loss calculation from a heating engineer or qualified energy assessor before specifying any heat source or distribution system.