Air Source Heat Pumps in London 2025: Costs, BUS Grant, and What Victorian Terraces Need

**The Boiler Upgrade Scheme (BUS) — what the £7,500 grant covers**:

The Boiler Upgrade Scheme (BUS), administered by Ofgem, provides a one-off grant of **£7,500** (as of 2025) towards the cost of replacing a fossil fuel heating system (gas boiler; oil boiler; LPG boiler; electric storage heater) with an **air source heat pump (ASHP)**. Key rules:

• •**Eligible properties**: homes and small non-domestic buildings in England and Wales. The property must have an existing heating system being replaced — new-build properties with no existing heating are not eligible
• •**Who can claim**: the grant is paid directly to the MCS-certified installer (not to the homeowner). The installer deducts the £7,500 from the installation cost and claims it from Ofgem. This means you pay the net cost upfront (total installation cost minus £7,500) — you do not pay the full cost then wait for a government payment
• •**MCS certification requirement**: the heat pump and the installer must both be certified by the Microgeneration Certification Scheme (MCS). Only MCS-certified equipment installed by an MCS-certified installer qualifies for the BUS grant. A valid MCS certificate is also required for Smart Export Guarantee (SEG) payments for heat pump systems with battery storage
• •**EPC requirement**: as of 2025, there is no minimum EPC rating required to claim the BUS grant (this requirement was removed to make the scheme more accessible) — however, the heat pump must be correctly sized for the property's actual heat demand, and poorly insulated properties will experience higher running costs
• •**Previous grant history**: if a BUS grant has already been claimed for the same property, a second claim is not eligible. If the property has an active RHI (Renewable Heat Incentive) tariff, it is not eligible for BUS
• •**Grant validity period**: BUS funding is subject to government allocation — check the current Ofgem BUS page for current grant availability and any policy changes

**MCS certification and heat pump installers in London**:

**Always use an MCS-certified installer for a heat pump in London.** This is non-negotiable for two reasons: (1) MCS certification is required to access the £7,500 BUS grant; and (2) MCS-certified installers are trained and assessed to size and install heat pump systems to the MCS Installation Standard (MCS 020), which includes heat loss calculations, system design, and DHW (domestic hot water) specification requirements that are not standard training for general plumbing and heating engineers.

The MCS Installer Database is publicly searchable at mcscertified.com — you can search for heat pump installers by postcode to find certified installers operating in your area.

**Planning permission for air source heat pumps in London — Permitted Development rules**:

ASHPs benefit from Permitted Development rights under Class G of Part 14 of Schedule 2 to the GPDO 2015. The conditions for ASHP installation within PD are:

• *The unit must meet these conditions to be Permitted Development*:
• •**One unit per property** — only a single ASHP is permitted under PD rights (second unit requires full planning permission)
• •**Siting — not on a wall or roof facing a highway** (road or public footpath used by the public) — ASHPs installed on a roof or wall facing a highway require planning permission
• •**Noise limit** — the unit must not exceed **42 dB(A)** measured at 1m from any neighbour's window or door. This is a critical condition — a unit that exceeds 42 dB(A) at this measurement point is not within PD and requires planning permission, and may also constitute a statutory noise nuisance. Modern quality ASHPs (Mitsubishi Ecodan; Daikin Altherma; Vaillant aroTHERM; Samsung EHS) typically achieve 42–48 dB(A) at 1m from the unit; measurement at the neighbour's window (which may be considerably further away) will usually be below the 42 dB(A) limit
• •**The unit must not be installed on a site that is a listed building or within the curtilage of a listed building**
• •**The unit must not be installed on a site in a World Heritage Site, SSSI, or National Park**

*Conservation areas and ASHPs — the most common London planning question*:

ASHPs installed in a **conservation area** are within Permitted Development **provided** the unit is not installed on a wall or roof visible from a highway AND the conditions above are met. An ASHP installed in the rear garden or on the rear wall of a conservation area property, not visible from the street, is Permitted Development in a conservation area. An ASHP on a front elevation in a conservation area (visible from the highway) requires planning permission — and is generally likely to be refused on character grounds.

• For most London Victorian terraces in conservation areas, the ASHP is installed either:
• •Against the rear wall of the property or extension
• •In the rear garden, adjacent to the rear boundary (reducing the visual impact on the historic street scene)
• •In a covered side passage, where the unit faces the side passage rather than the street

*Prior approval for ASHPs*:

Before installing an ASHP under Permitted Development, the installer must apply for **prior approval** from the local planning authority (Class G of Part 14 requires prior approval for an ASHP even when it is otherwise within PD). Prior approval applications are submitted to the council; the council has 28 days to respond; refusal is only permitted on specific grounds (siting; noise impact on neighbours). Most prior approval applications for rear-garden or rear-elevation ASHPs in London are approved without condition.

The MCS-certified installer should handle the prior approval application as part of the installation process — confirm this with your installer before signing a contract.

**The fundamental difference between an ASHP and a gas boiler**:

A gas boiler delivers heat to a home by burning gas to produce water at 65–80°C, which circulates through the radiator system. A heat pump works differently — it extracts heat energy from outside air (even at -15°C) and concentrates it to produce hot water, but at a lower flow temperature: typically 35–55°C for a well-designed system.

This lower flow temperature has critical implications for an existing London property:

1. **Existing radiators sized for 65–80°C flow temperature are significantly undersized at 45°C flow temperature**: a standard steel panel radiator that delivers 1,000W of heat output at 65/55°C (flow/return) delivers only approximately 400–500W at 45/40°C — less than half the heat output. If the existing radiator system is not replaced or augmented, the heat pump will struggle to heat the property effectively, will run at higher flow temperatures than designed (reducing efficiency), or will run for extended periods without achieving the desired room temperature.

2. **The SCOP (Seasonal Coefficient of Performance) is the key efficiency metric**: a heat pump's COP (Coefficient of Performance) varies with the difference between the outside air temperature and the system flow temperature. The lower the flow temperature, the higher the COP (more heat output per unit of electricity consumed). A system designed for 35°C flow temperature achieves a COP of approximately 4.0–5.0 (producing 4–5 kWh of heat for every 1 kWh of electricity). A system forced to run at 55–65°C (because existing radiators are undersized) achieves a COP of approximately 2.0–2.5 — significantly reducing the efficiency advantage over a modern gas boiler (which achieves an efficiency of approximately 0.90).

3. **The heat pump must be sized to the actual heat demand of the property**: an oversized heat pump (a system designed for a 6kW heat demand installed in a 4kW property) 'short-cycles' — it turns on; reaches temperature quickly; turns off; and cycles rapidly. Short-cycling reduces efficiency and lifespan. An undersized heat pump runs continuously and cannot meet the design temperature on the coldest days. The MCS 020 installation standard requires a full heat loss calculation (to BS EN 12831 or equivalent) before specifying the heat pump capacity.

**Is a London Victorian terrace suitable for an ASHP?**

This is the most common question for London homeowners considering heat pumps, and the honest answer is: *it depends entirely on the property's insulation standard and the willingness to replace or upgrade the radiator system*.

• *Victorian terrace heat loss characteristics*:
• •**Solid brick walls (215mm or 327mm, U-value 1.8–2.2 W/m²K)**: the largest source of heat loss in an uninsulated Victorian terrace. External wall insulation (EWI) or internal wall insulation (IWI) is required to bring the U-value below 0.45 W/m²K for the heat pump to perform efficiently
• •**Suspended timber floors (U-value 0.5–1.5 W/m²K depending on whether the void is sealed)**: significant heat loss route; floor insulation (rigid PIR board between joists) is achievable and cost-effective
• •**Single-glazed original sash windows**: U-value 4.5–5.5 W/m²K vs. modern double-glazed Rw 1.4–1.6 W/m²K. Replacement with secondary glazing (1.8–2.4 W/m²K) or slim-profile double glazing is a major improvement for a Victorian terrace considering a heat pump
• •**Poor loft insulation**: 270–400mm mineral wool (>0.14 W/m²K) is achievable and cost-effective

A typical inner London uninsulated Victorian terrace (3-bed; 80–100m² GIA) has a design heat loss of approximately 8–14 kW at -3°C design outside temperature. This requires a large (8–12 kW) ASHP and a significantly redesigned radiator system — often larger radiators throughout the house — to achieve comfortable temperatures at an efficient flow temperature.

A Victorian terrace that has been thermally upgraded (IWI or EWI to the main walls; floor insulation; loft insulation to 300mm+; replacement double glazing) typically has a design heat loss of approximately 4–7 kW — suitable for a 5–8 kW ASHP with lower running costs and better COP performance.

**Insulation first — the recommended sequence for London Victorian terraces**:

For most London Victorian terraces, the recommended sequence for a heat pump project is: 1. Heat loss calculation to establish the current heat demand 2. Insulation improvements to reduce the heat demand to a range suitable for a modestly sized heat pump (5–8 kW) 3. Radiator replacement or augmentation to enable lower flow temperatures (45–50°C) 4. Heat pump installation and commissioning 5. Underfloor heating (where a new screed is being laid as part of an extension or ground floor refurbishment) as the ideal emitter system for low-temperature heat pump operation

**Hot water (DHW) considerations for ASHPs in London**:

• An ASHP requires a **hot water storage cylinder** (typically 200–300 litres for a 3–4 person household) — the ASHP heats a cylinder of stored water, which provides the domestic hot water supply. This is different from a combi boiler (which provides hot water on demand without storage). Implications for London properties:
• •Space for the hot water cylinder must be found — typically in the airing cupboard, utility room, or (in a loft conversion) a dedicated cylinder cupboard. A minimum of approximately 600×600mm footprint and 1,600–1,900mm height is required for a standard 200–250 litre cylinder
• •The cylinder is heated once or twice daily (depending on the control strategy) — this is more energy-efficient than on-demand heating but requires planning around peak hot water demand periods

**The key variable — electricity vs gas unit costs**:

• In the UK in 2025, typical unit energy costs under the Energy Price Cap are approximately:
• •**Electricity**: approximately 24–28p/kWh (Ofgem cap; variable by tariff)
• •**Gas**: approximately 5.5–7p/kWh (Ofgem cap; variable by tariff)

This means that electricity costs approximately 4× as much per unit as gas. For a heat pump to be cost-competitive with a gas boiler, it must deliver approximately 4× as much heat output for every unit of electricity consumed — i.e., it must achieve a Seasonal COP (SCOP) of approximately 3.5–4.0 or above.

*Modern ASHPs in well-insulated properties can achieve SCOP 3.5–4.5 in UK climate conditions* — which means they are broadly cost-neutral or marginally cheaper to run than an efficient gas boiler in a well-insulated property.

*ASHPs in poorly insulated London Victorian terraces may achieve SCOP 2.0–2.8* — which means they are significantly more expensive to run than a modern gas condensing boiler. This is the primary reason that insulation improvements should precede heat pump installation.

**Indicative running cost comparison for a London 3-bed Victorian terrace (semi-detached or mid-terrace; 80–100m² GIA)**:

| Scenario | Annual heat demand | Heating system | Annual running cost estimate | |---|---|---|---| | Uninsulated Victorian terrace | ~18,000–22,000 kWh/yr | Modern gas condensing boiler (90% eff.) | ~£1,100–£1,700/yr | | Uninsulated Victorian terrace | ~18,000–22,000 kWh/yr | ASHP (SCOP 2.5) | ~£1,700–£2,500/yr | | Well-insulated Victorian terrace | ~8,000–12,000 kWh/yr | Modern gas condensing boiler (90% eff.) | ~£490–£930/yr | | Well-insulated Victorian terrace | ~8,000–12,000 kWh/yr | ASHP (SCOP 3.8) | ~£500–£880/yr |

*Note: These estimates use 2025 Ofgem Energy Price Cap rates. Actual costs depend on the property's specific heat demand; tariff; thermostat settings; and occupancy. Time-of-use tariffs (such as Octopus Agile or Go) that offer cheap overnight electricity can improve the ASHP cost case — the heat pump charges the hot water cylinder at night when electricity is cheap.*

**ASHP installation costs in London 2025 (after £7,500 BUS grant)**:

| Property type | ASHP size | Estimated total cost (inc. installation; exc. grant) | After BUS grant (£7,500) | |---|---|---|---| | Well-insulated 2-bed flat or maisonette | 5 kW | £8,000–£13,000 | £500–£5,500 | | 3-bed Victorian terrace (post-insulation upgrade) | 7–8 kW | £10,000–£18,000 | £2,500–£10,500 | | 4-bed Victorian terrace (post-insulation upgrade) | 10 kW | £14,000–£22,000 | £6,500–£14,500 | | Costs if radiator replacement required (whole house) | Add | £3,000–£8,000 | — | | Costs if DHW cylinder required (200–250L) | Add | £600–£1,500 | — |

*Note: For properties that also require wall insulation and radiator replacement, the total project cost (insulation + ASHP + radiators) may be £25,000–£45,000 before the grant. This is a major project and should be approached as a whole-house thermal upgrade rather than a like-for-like boiler replacement.*

**Summary — who should consider an ASHP in London in 2025?**

• *Well-suited*:
• •Properties that have already been insulated to a reasonable standard (or where insulation works are planned as part of a wider refurbishment)
• •New extensions with underfloor heating (the ideal emitter for ASHP; UFH operates efficiently at 35–45°C flow)
• •Properties planning loft conversions or full refurbishments where the heating system will be replaced in any case
• •Homeowners with a long time horizon (10–15+ years in the property to realise the lifecycle cost benefit)
• •Properties with larger gardens or south-facing rear aspects where the external unit placement is straightforward

• *Less well-suited (at this stage)*:
• •Uninsulated Victorian terraces where the wall fabric will not be improved — running costs will be higher than gas
• •Properties where there is no space for a hot water cylinder
• •Properties where planning restrictions make external unit siting difficult (very narrow rear access; front-only elevation; conservation area restrictions)
• •Homeowners planning to move in the next 3–5 years (the upfront investment is unlikely to be fully recovered in this timeframe)