Heat pumps are the centrepiece of the UK's home heating decarbonisation strategy. The Climate Change Act 2008 (as amended in 2019) commits the UK to net zero by 2050, and the Future Homes Standard — expected to apply to new build from 2025 — effectively mandates low-carbon heating. For existing homes, the Boiler Upgrade Scheme (BUS) offers a £7,500 grant toward installation costs, with the grant level set by the UK Government through Ofgem.
But a heat pump is not a straightforward boiler swap. It requires a well-prepared building fabric, a correctly specified distribution system, and an MCS-certified installer. Getting any of these wrong results in a system that is both inefficient and expensive to run.
How a Heat Pump Works (Without the Jargon)
A heat pump moves heat from one place to another rather than generating it by burning fuel. A refrigerant fluid circulates through a loop: it absorbs heat from the outside air (or the ground), is compressed to raise its temperature further, then releases that heat through a heat exchanger into your central heating system. The refrigerant then expands and cools, ready to absorb heat again.
The key metric is the Coefficient of Performance (COP): for every 1 kW of electricity input, a well-specified heat pump delivers 2.5–4.0 kW of heat output. A gas boiler is typically 90–95% efficient — it converts fuel to heat. A heat pump running at a COP of 3.0 is 300% "efficient" relative to its electricity input. This is why heat pumps work even when it is cold outside — there is thermal energy in outdoor air down to around -15°C.
Air Source Heat Pump (ASHP) vs Ground Source Heat Pump (GSHP)
ASHP
An ASHP unit sits outside the building (similar in size to an air conditioning condenser unit), drawing heat from ambient outdoor air. Most modern ASHP units operate efficiently down to -20°C, though efficiency (COP) falls as outdoor temperature drops.
Typical COP: 2.5–3.5 at UK winter conditions (0–7°C outdoor temperature).
Space required: An external area sufficient for the unit (typically 1m × 0.5m footprint) with clearance for airflow on three sides. Most suburban gardens are sufficient.
Best for: The majority of UK residential properties — houses, bungalows, larger flats with external space.
GSHP
A GSHP extracts heat from the ground via a loop of pipework buried either horizontally (ground collector) or vertically (borehole). Ground temperatures at 1–2m depth in the UK remain stable at around 10–12°C year-round, which means GSHP efficiency is more consistent than ASHP and typically slightly higher.
Typical COP: 3.0–4.5.
Space required: A horizontal ground loop requires approximately 300–600m² of garden (you cannot build over it). A vertical borehole system requires much less surface area but needs professional drilling to 80–150m depth — this is where the significant additional cost arises.
When GSHP is worth the additional cost: Where you have the land for a horizontal collector (the cheaper option), where your electricity tariff is high and maximum efficiency is a priority, or where ASHP noise is a planning constraint. For most urban and suburban properties, ASHP delivers excellent results at lower capital cost.
Planning Rules for ASHP Under Part 14 GPDO
ASHP installation is permitted development (PD) in England under Class A, Part 14 of the Town and Country Planning (General Permitted Development) (England) Order 2015, subject to conditions:
- Only one unit per property
- Not installed on a wall or roof facing a highway if there is an alternative siting
- The unit must be at least 1m from the edge of the property
- Noise levels must not exceed 42 dB(A) at 1m from a neighbour's window or door (measured to MCS planning guidance)
- Must be removed when no longer needed
- Not permitted on listed buildings — listed building consent is required
- Conservation areas: Installation on a wall or roof facing a highway is not PD; rear siting on a non-highway elevation usually remains PD
Always check whether an Article 4 Direction has removed PD rights in your area before proceeding. An MCS-certified installer should carry out the noise assessment as part of the design process.
MCS Certification: Why It Is Mandatory for the BUS Grant
The Microgeneration Certification Scheme (MCS) is the industry standard for small-scale renewable energy and low-carbon heat installations in the UK. An MCS certification covers both the product (the heat pump unit) and the installer (the company carrying out the work).
If you want to claim the Boiler Upgrade Scheme grant, your installer must be MCS-certified, and the installation must comply with MCS 020 (the heat pump installation standard). The grant application is made by the installer on your behalf through the BUS portal, administered by Ofgem. You do not apply directly.
The Boiler Upgrade Scheme: £7,500 Grant
The BUS provides a voucher of £7,500 toward the cost of an ASHP or GSHP. The voucher is applied at point of invoice — you pay the net amount and the installer claims the difference from Ofgem.
Eligibility requirements:
- Property in England or Wales
- Existing heating system must be replaced (not added to)
- Property must have an EPC dated within the last 10 years with no outstanding recommendations for loft or cavity wall insulation (if applicable) — a critical requirement that catches out many applicants
- The installer must be MCS-certified
- Property must not have received a previous BUS voucher for the same system type
Note: biomass boilers were previously eligible (£5,000 grant) and remain so under the scheme, though Heat Pumps are the primary target technology.
Costs
| System | Typical Installed Cost | After £7,500 BUS Grant |
|---|---|---|
| ASHP (3–4 bedroom house) | £8,000–£15,000 | £500–£7,500 |
| GSHP (horizontal collector) | £15,000–£25,000 | £7,500–£17,500 |
| GSHP (vertical borehole) | £25,000–£35,000+ | £17,500–£27,500+ |
| Underfloor heating (new zone) | £1,500–£5,000 | — |
| Radiator upsizing (whole house) | £2,000–£6,000 | — |
| Hot water cylinder (if replacing) | £800–£1,500 | — |
The Fabric-First Prerequisite
A heat pump must not be installed before the building fabric is adequately prepared. This is not a recommendation — it is a condition of sensible design. An MCS 020-compliant heat loss calculation (typically using a BS EN 12831 method) must be carried out before specification. If the calculated heat loss for your home is too high, the pump will be oversized or run at high flow temperatures, destroying its efficiency advantage.
Fabric-first steps before installation:
- Loft insulation to 270mm mineral wool (if applicable)
- Cavity wall insulation (if cavities are present and dry)
- Draught-proofing around doors, windows, letterboxes, and loft hatches
- Solid wall insulation (EWI or IWI) if the property has solid walls — the single biggest intervention
Low-Temperature vs High-Temperature Systems
Standard ASHP systems operate most efficiently at flow temperatures of 35–55°C. Most UK homes designed for a gas boiler operating at 70–80°C will require either:
- Radiator upsizing (larger radiators emit more heat at lower flow temperatures — typically 20–50% larger surface area), or
- Underfloor heating (UFH naturally operates at 30–40°C flow temperature, ideal for heat pump compatibility)
High-temperature ASHP systems (capable of 65–80°C flow) are available for homes where radiator replacement is impractical, but their COP is significantly lower. A proper heat loss survey and hydraulic system design by the installer should determine which approach is appropriate for your property.
SAP and EPC Impact
Installing an ASHP or GSHP will improve your EPC rating, sometimes significantly. SAP 10.2 (the current calculation methodology) assigns heat pumps a high efficiency factor. A property moving from a gas boiler to an ASHP typically gains 5–20 SAP points, depending on current EPC baseline and system specification. This can shift a Band D property to Band C — relevant for landlords facing MEES compliance.
Related Guides
- EPC Improvement: Full Room-by-Room Action Plan — the correct sequencing of insulation and heating measures for maximum SAP points before installing a heat pump
- Retrofit Insulation: EWI vs IWI — why insulating solid walls before installing a heat pump is essential, and how PAS 2035 governs the sequence
- Boiler Replacement Costs UK 2025 — how heat pump costs compare to a modern boiler replacement and the long-term running cost case
- Heat Pump Costs UK 2025 — detailed cost breakdown for ASHP and GSHP including running costs and the Boiler Upgrade Scheme
- Building Regs Completion Certificate UK — the Building Regulations notification requirements for heat pump installations under Part L
- EPC Rating: How to Improve Your Energy Performance Certificate — how the BUS grant's outstanding EPC recommendations requirement interacts with the fabric-first insulation sequence
- EPC Guide for Buyers and Sellers — how EPCs work in property transactions, the MEES obligations landlords face, and how a heat pump installation changes your EPC rating and green mortgage eligibility
- Boiler Upgrade Scheme (BUS) Grant for Heat Pumps — the £7,500 BUS grant in detail: who qualifies, how the MCS installer applies, EPC compliance requirement, and ASHP costs before and after grant
- Heat Pump Installation Guide UK: ASHP, GSHP, MCS, and the BUS Grant — a homeowner-focused guide to ASHP vs GSHP, the suitability checklist, running cost comparisons at 2025 energy prices, and the BUS application process