When an ASHP makes sense on a London renovation
Air source heat pumps deliver heat at a Coefficient of Performance (COP) of typically 2.8–4.0 — meaning 1 kWh of electricity produces 2.8–4.0 kWh of heat. This makes them substantially more efficient than gas (efficiency ~92 percent on a condensing boiler) and cheaper to run on time-of-use tariffs. But ASHPs work best at low flow temperatures (35–45°C) which require either underfloor heating throughout or oversized radiators. A solid-wall Victorian terrace without retrofit insulation has heat losses that demand higher flow temperatures (55–65°C) where the ASHP COP drops below 2.5 and running cost approaches or exceeds gas. So ASHPs only make sense if the renovation includes: wall and loft insulation upgrade (internal wall insulation, mineral wool to loft floor and rafters), double or triple glazing, draught proofing, and a wet UFH ground floor or radiator sizing exercise. Without these the ASHP underperforms and pays back slowly or not at all.
Sizing, installation and MCS certification
An MCS-certified installer must perform a heat loss calculation under BS EN 12831 for the property in its post-renovation state — this drives the heat pump size. A typical 3-bed Victorian terrace in London with full retrofit insulation needs an 8–11kW heat pump; a 4-bed detached needs 11–14kW. The installation includes: the outdoor unit (typically wall- or ground-mounted on a paving slab or anti-vibration mount), the indoor cylinder unit (180–250L hot water cylinder with immersion backup), all primary pipework, a buffer tank if zoned, the new wiring back to a dedicated 32A circuit, the heating circulator and zone valves, weather compensation controls, and full commissioning to MCS standards. The MCS certificate is what unlocks the Boiler Upgrade Scheme grant — without it the grant is forfeited.
The Boiler Upgrade Scheme grant in 2026
The Boiler Upgrade Scheme (BUS) currently offers £7,500 toward an air source heat pump installation in England and Wales — confirmed extended through March 2028 at this rate. The grant is paid directly to the MCS-certified installer who deducts it from the homeowner's invoice. Eligibility requires: the property must have a valid EPC issued in the last 10 years with no outstanding loft or cavity wall insulation recommendations (or the homeowner must complete those before BUS payment), the existing fossil-fuel heating must be replaced (not added to), the new system must be MCS-certified and registered, and the property must not have received a previous BUS grant. Most London ASHP retrofits qualify — the EPC pre-condition catches some clients who must add loft insulation first.
