Replacing a gas boiler with a heat pump is one of the bigger decisions a UK homeowner can make. Done well, it can substantially cut your carbon footprint, protect you from future gas price rises, and — with the £7,500 Boiler Upgrade Scheme grant — be affordable upfront. This guide walks you through the process from first assessment to the day your new system switches on.
Step 1: Assess Your Home’s Suitability
Not every home is an ideal candidate for a heat pump, but most UK homes can accommodate one with the right preparation. The key factors are:
- Insulation: Loft insulation (100mm+) and cavity or solid wall insulation significantly reduce the heat pump size needed and improve running costs. A poorly-insulated Victorian terrace will have higher running costs than a 1990s semi.
- Heat loss calculation: Your MCS installer must carry out a room-by-room heat loss calculation (BS EN 12831). This determines the correct heat pump size — typically 5–12 kW for a UK home.
- Outdoor space: Air source heat pumps need 1–1.5m clearance around the outdoor unit, away from boundaries and ideally not directly under a bedroom window.
- Hot water: Most heat pump installations include a new hot water cylinder (typically 200–300 litres). Check you have space — an airing cupboard or utility room is ideal.
Before committing, it’s worth getting 2–3 quotes from MCS-certified installers. BoilerHelper has detailed guides on boiler assessments and replacement costs if you want to understand what your existing system is worth and what a full replacement involves.
Step 2: Understand What Happens to Your Existing Pipework
A common concern is whether existing pipework needs replacing. The good news: in most cases, it doesn’t. The existing radiator circuit pipework (typically 15mm or 22mm copper) is usually suitable for a heat pump — the water flow rates are similar, just at a lower temperature.
What often does change:
- Radiators: Many homes need larger radiators to compensate for lower flow temperatures. Your installer will calculate which rooms need upgrades — typically the ones with the smallest existing radiators.
- Hot water cylinder: A new cylinder optimised for heat pump use (with a large heat exchanger coil) is almost always required.
- Buffer tank: Some installations require a small buffer tank to ensure minimum flow through the heat pump — your installer will advise.
Step 3: Radiator Upgrades
Heat pumps work best at flow temperatures of 35–45°C, compared to 65–70°C for a gas boiler. A radiator sized for 70°C will give out significantly less heat at 45°C — sometimes less than half. Your heat loss calculation identifies which radiators need upgrading.
Typical upgrade costs run £150–400 per radiator including labour. In a 3-bedroom house, you might need to upgrade 3–6 radiators at a total cost of £500–1,500 — this is often included or partially included in the overall heat pump installation quote.
Step 4: Choosing an MCS-Certified Installer
To access the Boiler Upgrade Scheme grant, your installer must be MCS-certified. MCS (Microgeneration Certification Scheme) sets standards for heat pump installation quality. You can search for local installers at the MCS website.
When getting quotes, ask each installer:
- Will they carry out a full BS EN 12831 heat loss calculation?
- What heat pump brand and model do they recommend, and why?
- What radiator upgrades are needed and are they included?
- Do they handle the Boiler Upgrade Scheme application?
- What is the warranty period and who services it?
Step 5: Applying for the Boiler Upgrade Scheme Grant
The Boiler Upgrade Scheme (BUS) provides a £7,500 voucher towards the cost of a heat pump. The process works as follows:
- Your MCS installer applies for the voucher on your behalf before installation
- The voucher is issued and deducted from your installation invoice — you never handle the money directly
- After installation, the installer redeems the voucher with the scheme
Eligibility requirements: the property must have a valid EPC (Energy Performance Certificate) with no outstanding recommendations for loft or cavity wall insulation. If yours has those recommendations, you’ll need to address them (or get an updated EPC) before your installer can apply. See our detailed guide to heat pump grants in the UK for full eligibility details.
Step 6: Timelines
From initial quote to installation, expect:
- Quotes and heat loss surveys: 2–4 weeks
- BUS voucher application and approval: 2–4 weeks
- Equipment lead time: 2–6 weeks depending on brand
- Installation: 2–3 days for a typical semi-detached home
Total time from first enquiry to live system: typically 6–12 weeks. Plan to have interim heating cover if your boiler is on its last legs.
What to Expect on Installation Day
Installation usually takes 2–3 days. Day one typically involves decommissioning the old boiler, fitting the outdoor unit, and beginning pipework modifications. Day two covers cylinder installation, electrical work, and radiator upgrades. Day three is commissioning, controls setup, and handover.
Your installer should walk you through the controls in detail — how to set schedules, adjust flow temperatures, and read your energy data. Ask questions: understanding your system is the single best thing you can do for your running costs.
For more on what installation involves, see our full guide to heat pump installation in the UK.