Why Air Source Heat Pumps Make Sense in London — and When They Don't

Heat pumps attract evangelists and cynics, and both get the engineering wrong. A heat pump is neither a moral statement nor a boiler with a fashionable badge — it is a low-temperature heat source whose success depends almost entirely on the system designed around it. This guide sets out the case for air source heat pumps (ASHPs) in London property as we see it from design and installation experience: where they excel, what they genuinely cost, and the projects where we advise clients against them.

The physics that makes the case

A gas boiler converts fuel to heat at roughly 90% efficiency at best. A heat pump moves heat rather than making it: for each kilowatt-hour of electricity it consumes, a well-designed system delivers 3–4 kWh of heat across the season — a seasonal coefficient of performance (SCOP) of 3.0–4.0. That multiplier is the entire economic argument, and protecting it is the entire design challenge, because SCOP collapses when the system is forced to produce high water temperatures. Design flow temperature is the single number that decides whether a heat pump project succeeds: at 35°C (underfloor heating) you may see SCOP above 4; at 55°C (undersized radiators) you may see 2.5, and the running-cost case evaporates.

Running costs: the honest arithmetic

With electricity at roughly four times the unit price of gas, a heat pump needs a real-world SCOP above ~3.6 to beat a 90% boiler on pure running cost at standard tariffs — which well-designed low-temperature systems achieve and poorly designed ones do not. The picture improves materially with time-of-use tariffs designed for heat pumps and EVs, which reward load-shifting, and with any on-site solar generation. The engineering translation: the tariff strategy and the mechanical design are one conversation, not two.

The £7,500 Boiler Upgrade Scheme

The Boiler Upgrade Scheme currently provides a £7,500 grant towards an air source heat pump in England and Wales, claimed by the MCS-certified installer on the client's behalf. Conditions that matter in practice: the property needs a valid EPC; the installation must be MCS certified with capacity limits (45kW ceiling, far above domestic need); and the grant covers heat pump conversion, not hybrid gas-retention setups. On a typical high-quality domestic conversion — £15,000–£25,000 including cylinder and emitter work — the grant removes a third to a half of the premium over a boiler swap.

Electrical demand: check before you promise

A domestic ASHP typically draws 3–7kW electrical at full output — manageable on most 100A single-phase London supplies, but not automatically alongside an EV charger, electric shower and induction range. We run a maximum-demand assessment on every project, apply for a DNO connection or fuse upgrade where needed (lead times can be months — apply early), and increasingly specify load-management so the heat pump, EV charging and the rest of the house share the supply intelligently rather than tripping it.

Noise and planning in dense London streets

Most domestic ASHPs can be installed under permitted development if they meet the MCS 020 planning standard — the practical test being 42 dB(A) at one metre from the nearest neighbour's habitable window, with one unit per dwelling and siting restrictions. In conservation areas and on listed buildings, assume a planning application. Engineering the noise case is usually straightforward if it is designed rather than hoped for: unit selection (modern inverter units are dramatically quieter at part load), night-mode setback, anti-vibration mounts, discharge direction, and acoustic screening that does not choke the airflow — a blocked heat pump is a loud, inefficient heat pump.

Heat loss first, hardware second

Every credible heat pump design starts with a room-by-room heat loss calculation to BS EN 12831 — not a boiler-sizing rule of thumb. Oversizing is the classic failure: the unit short-cycles, efficiency and compressor life suffer, and the owner concludes heat pumps don't work. The calculation also drives fabric decisions — on refurbishments it is often cheaper to spend £3,000 on airtightness and insulation than £3,000 on a bigger heat pump that then costs more to run every winter for twenty years. This is the fabric-first logic embedded in our wider approach to MEP design on high-end residential projects.

Emitters: underfloor, radiators, or both

Underfloor heating is the natural partner — large emitting area, 35–40°C flow, invisible, and comfortable. On refurbishments where UFH is impractical upstairs, the standard pattern is UFH to ground floor and re-sized radiators above: as a rule of thumb, a radiator sized for 70°C boiler flow delivers roughly half its output at 45°C, so many need one size up rather than wholesale replacement — a calculation, not a guess, room by room. Fan-assisted radiators and trench convectors solve the awkward rooms. Hot water moves to a heat-pump-ready cylinder with a large coil; expect 55–60°C storage with a periodic legionella cycle.

Hybrids and the honest exceptions

There are projects where we advise against a full conversion today: homes with severely constrained external space where the noise case cannot be made; very leaky, hard-to-treat fabric where the flow temperatures required would destroy efficiency; and phased refurbishments where the fabric work comes later. A hybrid — heat pump carrying the base load, boiler covering the coldest peaks — can be a rational bridge (though it forfeits the BUS grant), and sometimes the right advice is simply: do the fabric first, then the heat pump. An engineering consultancy should be willing to say so.

Payback, properly framed

On pure fuel savings at today's tariffs, a well-designed conversion after the £7,500 grant typically returns its premium in eight to fifteen years — honest numbers, not brochure numbers. But the frame is wider than fuel: a heat pump removes gas standing charges and combustion from the house, adds cooling capability where reversible units are selected, future-proofs against tightening regulation, and increasingly reads as a value and compliance asset in high-end property transactions. For most London refurbishment clients the question has quietly shifted from whether to convert, to whether to convert now or at the next plant replacement — and the answer usually turns on how much fabric and emitter work the current project already includes.