Air source heat pump (ASHP) — the default London choice
ASHP extracts heat from outside air via a refrigerant cycle and upgrades it to flow temperatures of 35–55°C suitable for radiators (oversized) or underfloor heating. Install cost £8,500–£14,500 for a typical 8–14kW unit serving a 3–4 bed London house; Boiler Upgrade Scheme (BUS) grant £7,500 reduces net cost to £1,000–£7,000. Footprint: external unit 1.0m × 0.4m × 1.2m mounted on ground-level concrete pad or wall bracket; needs 1m clearance to front, 200mm sides, and 2.5m to nearest habitable window of neighbouring property (MCS standard). Permitted development applies in most London boroughs subject to noise (42dB at 1m from neighbour window per MCS 020). SCOP (seasonal coefficient of performance) typically 3.2–4.1 — i.e. 1kWh electricity produces 3.2–4.1kWh heat. Running cost vs gas: at 2026 prices (28p/kWh electricity, 7p/kWh gas) ASHP at SCOP 3.5 costs 8p/kWh of heat vs gas combi at 91% efficiency costs 7.7p/kWh — broadly equivalent. Real benefit: EPC uplift (D→B typical) and 75% carbon reduction.
Ground source heat pump (GSHP) — when it wins
GSHP extracts heat from the ground via a horizontal slinky collector (requires 300–600m² of garden) or vertical borehole (100–150m depth, drilled by specialist rig). Total install £18,000–£35,000 — typically 2–3× ASHP. BUS grant £7,500 same as ASHP. Higher SCOP 3.8–4.6 because ground temperature is more stable (8–12°C year-round) than air (–5 to +25°C). Lifespan: collector loop 50+ years; heat pump unit 20–25 years (vs ASHP unit 15–20 years). Wins on: very large detached houses (>250m² floor area), rural outer-London plots, projects targeting Passivhaus or off-grid energy positive, clients with sufficient garden for horizontal collectors. Loses on: small London plots, side-return terraces, gardens with mature trees (roots foul collectors), basements (no garden access). Borehole option works almost anywhere but adds £8,000–£18,000 drilling cost and requires Environment Agency abstraction notification.
Sizing, radiators and flow temperature
Heat pumps deliver lower flow temperatures (35–55°C) than gas boilers (typically 65–75°C in older systems). Lower flow means radiators must be oversized to deliver equivalent heat — typical retrofit upsizes 60–80% of existing radiators (1.5–2× surface area). Cost £180–£420 per radiator including labour. Alternative: pair heat pump with wet underfloor heating which works optimally at 35–40°C flow — no radiator upsizing needed but UFH retrofit costs £75–£140/m² for screed buildup or £55–£95/m² for spreader plate between joists. Hot water: heat pumps fill a buffer cylinder (180–250L for 3–4 bed house) at 50°C with weekly Legionella sterilisation cycle to 60°C — typical cylinder install £1,400–£2,400. Heat loss survey is mandatory pre-install (MCS requirement) £350–£650; specifies kW capacity, radiator schedule, flow temperatures and predicted SCOP.
BUS grant, MCS and consent route
Boiler Upgrade Scheme grant £7,500 (ASHP, GSHP, biomass) — claimed by MCS-certified installer on behalf of homeowner; paid as direct deduction from quoted price. Eligibility: property must have a valid EPC (less than 10 years old) with no outstanding loft or cavity wall insulation recommendations; property must have been heated by fossil fuel or electric heating; new builds excluded. Installer must be MCS-certified — non-MCS installs do not qualify. Planning: ASHP is permitted development outside conservation areas and listed buildings subject to MCS 020 noise compliance; conservation areas often require full planning to assess external visual impact. Listed buildings require listed building consent for external unit and any visible pipework. GSHP collector trenches typically PD even in conservation areas (subsurface, invisible) but boreholes require Environment Agency notification. Lead time: MCS heat loss survey + design 2–4 weeks; install 3–7 days (ASHP) or 2–4 weeks (GSHP including drilling). Total project 6–12 weeks (ASHP) or 12–20 weeks (GSHP).
