Methodology / Sources reviewed June 2026
Methodology + sources
How every cost range, grant detail, and regulatory statement on this site is sourced and calibrated. Below: the 19 primary sources, how often each is refreshed, what we take from each, and what is explicitly out of scope.
Primary sources
| Source | Refresh | What we take |
|---|---|---|
| BOXT boxt.co.uk | Each review | Published fixed installed prices (from) per brand and model, checked on the date shown on each page |
| Energy Saving Trust energysavingtrust.org.uk/advice/boilers | On update | Average cost of replacing a G-rated gas boiler with an A-rated one (£3,500, GB); annual savings by old boiler band and home type, at the fuel prices EST states |
| Worcester Bosch UK worcester-bosch.co.uk | Each review | Greenstar guarantee lengths and accredited-installer conditions |
| Vaillant UK professional.vaillant.co.uk | Each review | Out-of-the-box and Advance (10-year) guarantee terms |
| Ideal Heating idealheating.com/boiler-warranties-guide | Each review | Logic Max and Vogue Max warranty lengths and conditions |
| Baxi baxi.co.uk/support/warranty | Each review | 600 and 800 range warranty lengths and filter conditions |
| Viessmann UK viessmann.co.uk/en/support/viessmann-product-warranties.html | Each review | Vitodens standard, registered and paid-extension warranty terms |
| gov.uk Boiler Upgrade Scheme gov.uk/apply-boiler-upgrade-scheme | On scheme update | BUS grant amounts (£7,500 ASHP / GSHP, £2,500 air-to-air, £5,000 biomass), eligibility, application flow |
| Ofgem BUS and ECO ofgem.gov.uk | On scheme update | Off-gas BUS uplift to 31 March 2027; ECO4 eligibility, boiler rules, contributions and end date (31 December 2026) |
| MHCLG Building Circular 01/2026 gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026 | On policy update | Future Homes and Buildings Standards: published 24 March 2026, in force in England 24 March 2027 |
| DESNZ Warm Homes Plan gov.uk/government/publications/warm-homes-plan | On policy update | BUS funding to 2029-30, ECO4 end date, successor Warm Homes Fund |
| Boiler Plus 2018 (BEIS guidance) gov.uk/government/publications/boiler-plus | On policy update | Control requirements for new boiler installs in England |
| Building Regulations Part L + Part J planningportal.co.uk/buildingregulations | On Approved Document update | Condensing boiler requirement, flue rules, Building Control notification via Competent Persons Scheme |
| Gas Safe Register gassaferegister.co.uk | Ongoing | Engineer registration checks and ID card protocol |
| MCS Certified Installer database mcscertified.com | Ongoing | Heat pump installer directory required for a BUS grant |
In scope
- UK domestic gas boiler replacement cost ranges (combi, system, regular, back-boiler conversion)
- Published fixed installed prices (BOXT) for named brands and models
- UK 2026 typical installed cost ranges including labour + flue + filter + flush + Building Control
- Brand-specific warranty terms and accredited-installer routes
- kW sizing rules of thumb
- Regional cost variation across UK regions
- ECO4 + BUS + Warm Home Discount grant frameworks and eligibility
- 0% finance, personal loan, boiler subscription, and credit card options
- Gas Safe + Building Regs Part L + Part J + Boiler Plus 2018 regulatory requirements
- Heat pump vs gas boiler cost + suitability comparison
- ErP A-G efficiency band framework and annual gas saving from upgrading
- UK domestic context only - not commercial or industrial boilers
Out of scope
- Specific installer / regional / property quotes - always get 2-3 quotes from Gas Safe registered engineers
- Diagnostic of specific boiler faults - call a Gas Safe engineer for individual diagnosis
- Commercial or industrial boiler pricing
- Off-grid LPG / oil boiler pricing beyond rural BUS / biomass context
- Detailed heat pump sizing and design (different professional discipline; see MCS certified installer directory)
- Country-specific pricing outside UK (US, Australian, EU pricing differs substantially)
- Substitution for an in-person inspection by a Gas Safe registered engineer
- Financial, legal, or engineering advice on specific installations
Calculation framework
Named prices
Brand and model prices are BOXT's published fixed installed prices, read off boxt.co.uk on the date shown. A from-price is the floor for the smallest output in a like-for-like swap. Manufacturers do not publish consumer prices, so where BOXT lists none (Baxi, Viessmann) we state none.
Published averages
The Energy Saving Trust puts the average cost of replacing a G-rated gas boiler with an A-rated one, with TRVs, at £3,500 in Great Britain. Its savings table gives annual savings by old boiler band and home type at the fuel prices it states.
Wider cost bands
The type, house-size, region, repair and line-item bands are editorial ranges, not taken from a published survey. Use them as a rough guide; written quotes for the same model and output are the figure that counts.
Grant amounts
From gov.uk and Ofgem published terms. BUS £7,500 ASHP/GSHP, £2,500 air-to-air, £5,000 biomass; £9,000 for eligible off-gas homes until 31 March 2027. ECO4 funding levels are set by each supplier, and households may be asked to contribute.
Refresh cadence
Single source of truth: LAST_VERIFIED_DATE and LAST_VERIFIED_LABEL in src/lib/schema.ts. Rolling either forward updates every freshness indicator on the site automatically (footer stamp, hero badge, Article schema dateModified, About + Methodology page badges). Refresh triggers:
- Each review: BOXT published prices and manufacturer warranty terms re-read; the check date on each brand page moves only when they are
- On publication: Energy Saving Trust cost and savings figures
- On policy update: ECO4 + BUS + Warm Home Discount + devolved grant changes
- On flagged correction: user-reported factual error escalated to next refresh cycle
Limitations
- Regional cost variation: London +15-25% can be wider in central postcodes; rural and island installs can be higher than urban regional averages
- Published prices move: BOXT's fixed prices change with promotions and demand; each page states the date they were checked
- Grant eligibility complexity: ECO4 has several qualifying benefits with sub-criteria; this site simplifies for reference, and the energy supplier or council makes the final eligibility decision
- Brand coverage: five brands have their own pages; smaller brands (Glow-worm, Alpha, Potterton, Ferroli) are not covered in depth
Corrections process
Spot a factual error or price that has drifted? Email [email protected] with:
- The page URL where you saw the error
- The specific text that needs correcting
- The correct figure / detail with a citation
We acknowledge corrections within 5 business days and update inline within the next monthly refresh cycle. Out-of-cycle update for material consumer protection issues (e.g. wrong grant figure that would mislead applicants).
Do not email for boiler emergencies. Call 0800 111 999 if you smell gas. For installation or service questions, find a Gas Safe registered engineer at gassaferegister.co.uk.
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