The Boiler Upgrade Scheme has made heat pumps financially accessible for more homeowners than at any point since the technology arrived in the UK. But the grant does not change the physics: a heat pump installed into a poorly insulated, draughty home will run at a low Coefficient of Performance, cost more to operate than the boiler it replaced, and leave the homeowner disappointed. The order of works matters as much as the technology itself.
This guide explains the fabric-first sequence, the radiator sizing question, BUS Grant eligibility in 2026, and — critically — how to time a heating upgrade to coincide with a house extension or refurbishment so that the disruption and cost of both projects overlap rather than stack.
The Fabric-First Principle: Why Insulation Comes Before the Heat Pump
A heat pump's running cost is determined by its CoP — the ratio of heat output to electrical energy consumed. A well-designed system in a well-insulated home achieves a seasonal CoP of 3.0 to 4.0, meaning every £1 of electricity produces £3 to £4 of heat. In a poorly insulated home, the same unit may achieve a seasonal CoP of 1.8 to 2.2 — barely better than direct electric heating and significantly more expensive to run than a modern gas boiler.
The fabric-first principle means addressing the building envelope before sizing or specifying a heat pump:
| Fabric measure | Priority | Typical cost (2026) | Heat-loss reduction |
|---|---|---|---|
| Loft insulation (top-up to 270mm) | 1st | £300–£600 | 15–25% |
| Cavity wall insulation | 2nd | £500–£1,500 | 10–20% |
| Solid wall insulation (EWI or IWI) | 3rd | £8,000–£22,000 | 25–40% |
| Double / triple glazing | 4th | £4,000–£12,000 | 10–15% |
| Draught-proofing & airtightness | 5th | £200–£800 | 5–10% |
Source: CIBSE Guide A (2024 edition); Checkatrade national data, 2026. Costs are indicative; obtain a site-specific quote.
The EPC requirement for BUS Grant eligibility reinforces this logic: if your EPC has outstanding recommendations for loft or cavity wall insulation, you must address them before the grant will be approved. In practice, this means the insulation work and the heat pump installation are sequenced — and a building project is the most cost-effective moment to do both.
Radiator Sizing: The Most Overlooked Part of a Heat Pump Retrofit
Gas boilers typically deliver water to radiators at 70–80°C. Heat pumps operate most efficiently at 45–55°C flow temperatures — and the lower the flow temperature, the higher the CoP. A radiator sized for a 70°C system will deliver noticeably less heat at 50°C, leaving rooms cold on the coldest days of the year.
The correct approach is a room-by-room heat loss calculation (to BS EN 12831) that determines the heat demand of each space at the design outdoor temperature (typically −3°C for Hertfordshire). The calculation then identifies whether each existing radiator has sufficient surface area to meet that demand at the heat pump's operating flow temperature.
In a typical 1970s semi-detached property in Hertfordshire, the outcome is usually:
- Living room and master bedroom radiators need upsizing (often doubled in area)
- Kitchen and bathroom radiators are frequently adequate due to smaller heat loss
- Hallway and landing radiators often need replacing with larger panels or underfloor heating
- Any room with a single-panel radiator almost certainly needs upgrading to a double-panel convector
When a house extension is being built, the new extension rooms can be designed from the outset for low-temperature heating — underfloor heating (UFH) is standard in new-build extensions and is the ideal emitter for a heat pump, achieving full output at flow temperatures as low as 35°C. This creates a hybrid system where the extension runs on UFH and the existing house runs on upgraded radiators, all served by a single heat pump.
The Boiler Upgrade Scheme (BUS) in 2026: What You Need to Know
| Technology | Grant value | Key eligibility conditions |
|---|---|---|
| Air source heat pump (ASHP) | £7,500 | Valid EPC; no outstanding loft/cavity insulation recommendations; MCS-certified installer |
| Ground source heat pump (GSHP) | £7,500 | As above, plus adequate land for ground loops or borehole |
| Biomass boiler | £5,000 | Off-gas-grid properties only; fuel storage requirements apply |
Source: DESNZ Boiler Upgrade Scheme guidance, updated April 2026. Grant values are subject to change; verify at gov.uk/boiler-upgrade-scheme before committing.
The grant is paid directly to the MCS-certified installer and deducted from your invoice — you never handle the grant money. The installer applies on your behalf after the system is commissioned. This means the grant is only available through MCS-certified contractors; non-certified installers cannot access it regardless of the quality of their work.
One common point of confusion: the BUS Grant is separate from the ECO4 scheme (which targets fuel-poor households) and the Great British Insulation Scheme (which funds insulation measures). A homeowner can, in principle, claim ECO4 or GBIS funding for insulation and then separately apply for the BUS Grant for the heat pump — but the sequencing must be correct, and the EPC must be updated after the insulation work before the BUS Grant application is submitted.
From Our Work: Timing a Heat Pump with a Rear Extension in Borehamwood
On a 1960s detached property in Borehamwood, the client wanted a single-storey rear kitchen extension and had separately been quoted for a heat pump by an energy company. The two projects were being planned independently, which would have meant two separate sets of disruption — groundworks, plastering, and decoration twice — and a heat pump sized for the existing house without accounting for the new extension's heat load.
By coordinating both scopes, TCM was able to: lay the UFH pipework in the extension slab before the screed was poured; route the heat pump pipework through the new cavity wall before plastering; upsize four radiators in the existing house as part of the first-fix programme; and upgrade the loft insulation during the scaffold phase. The heat pump was then sized for the combined heat load of the original house plus the extension — a single system, installed once, with no return visit required.
The client's EPC moved from a D to a B, the BUS Grant was secured, and the total project cost was lower than it would have been had the two scopes been executed separately. [SUGGESTED — verify exact EPC bands and cost saving with client before publication]
When to Time a Heating Upgrade: The Building Project Window
A heating upgrade is significantly cheaper and less disruptive when coordinated with a building project. The reasons are structural:
- Walls are open: pipework can be chased and routed without making good separately
- Floors are up: underfloor heating can be laid in screeds or between joists without lifting finished flooring
- Scaffold is already erected: external wall insulation or loft access is available without additional cost
- Decoration is pending: radiator replacements and pipe runs can be concealed before final plastering and painting
- Heat loss calculations can account for the new space: the heat pump is sized for the completed building, not the current one
The worst time to install a heat pump is immediately after a building project has been completed and decorated — every trade that should have been coordinated during the build now requires a return visit, making good, and additional decoration.
Upgrading Windows During an Extension: The Overlooked Opportunity
The topical map specifies "retrofit windows during extensions" as part of the EAV scope for this article, and it is worth addressing directly. When a house extension is being built, the building regulations require the new extension to meet current Part L thermal performance standards. This creates a natural moment to also upgrade the existing house's windows — particularly if they are single-glazed or early double-glazed units with failed seals.
Replacing windows during a building project has two practical advantages: the scaffold is already in place for upper-floor windows, and the decoration disruption is absorbed into the wider project programme. The thermal benefit is meaningful — upgrading from single glazing to modern double glazing (Uw ≤ 1.4 W/m²K) reduces window heat loss by approximately 50%, and upgrading to triple glazing (Uw ≤ 0.8 W/m²K) reduces it by approximately 70%.
For a heat pump system, lower window heat loss means a smaller required heat pump output and lower running costs — the window upgrade pays for itself through reduced heat pump sizing and improved seasonal CoP.
Frequently Asked Questions
Related Guides & Articles
Planning a Heating Upgrade Alongside Your Building Project?
TCM Building & Maintenance coordinates heating upgrades — including heat pump preparation, radiator replacements, and underfloor heating — as part of house extensions and refurbishments across Hertfordshire and North London. Get a single, coordinated quote.