UK 2026 / Prices checked 14 November 2026
Combi boiler replacement cost
New combi boilers are installed from £1,890 at BOXT's published fixed prices (Navien NCB 300 30kW, checked 14 November 2026), with branded models from £2,110 to £3,400. The Energy Saving Trust average for a boiler replacement is £3,500. Converting from a regular or system boiler is priced by survey.
The two scenarios
Scenario A
From £1,890
Like-for-like combi swap. Existing combi has failed; replace with a new combi in the same location. Cheapest UK replacement path. 1-day install. BOXT publishes a fixed installed price from £1,890 for a Navien NCB 300 30kW (checked 14 November 2026); model prices are in the table below.
Scenario B
Quote after survey
Conversion to combi from a regular or system boiler. Adds pipework, cylinder removal, loft tank decommissioning and flue work, so no fixed price is published. 1.5-2 day install.
Sizing by house
Combi kW output drives hot water flow rate (the cold-water mains feeds straight through the boiler). Undersized combis produce weak showers; oversized combis waste money. Common starting points, which your engineer should confirm on site:
| Property | Bathrooms | Recommended kW |
|---|---|---|
| 1-bed flat | 1 | 24 kW |
| 2-bed terrace | 1 | 24-28 kW |
| 3-bed semi | 1 | 28-30 kW |
| 3-4 bed | 2 | 32-35 kW |
| 4-bed detached | 2+ | 35 kW or switch to system |
Above 4 bedrooms with two or more bathrooms, a system boiler with a hot water cylinder usually beats a combi on flow rate. See system boiler cost.
Top combi models by brand
Installed prices are BOXT's published fixed "from" prices (checked 14 November 2026), the floor for the smallest output in a like-for-like swap. Warranty terms are from each manufacturer's own UK warranty pages; most depend on registration, an accredited installer or a filter fitted at install. A dash means no published price.
| Brand + model | Installed from (BOXT) | Warranty |
|---|---|---|
| Navien NCB 300 30kW | £1,890 | Check at quote |
| Ideal Logic Combi 24kW | £2,110 | Check at quote |
| Worcester Bosch Greenstar 1000 30kW | £2,249 | Check at quote |
| Worcester Bosch Greenstar 4000 | £2,349 | 10 years as listed by BOXT |
| Vaillant ecoFIT pure | £2,569 | Up to 10 years with Vaillant's Advance kit |
| Vaillant ecoTEC plus | £2,849 | Up to 10 years with Vaillant's Advance kit |
| Worcester Bosch Greenstar 8000 | £3,400 | Up to 12 years (8000 Life, accredited installer) |
| Ideal Vogue Max | - | Up to 12 years |
| Baxi 800 Combi 2 | - | 10 years with the supplied Adey filter |
| Viessmann Vitodens 100-W | - | 7 years on registration, 10 or 12 for a fee |
What should be in a combi replacement quote
A Gas Safe registered installer should itemise these lines. If the quote is a single all-in number with no breakdown, ask for one - reputable installers will provide it without resistance.
- Boiler unit + warranty registration (ask for the model and output by name, and the warranty term the installer will register)
- Installation labour (ask how many engineer-days the price assumes; a conversion takes longer than a swap)
- Flue + fittings (standard horizontal vs vertical-through-roof; longer or vertical runs cost more)
- Magnetic system filter like MagnaClean Atom or Sentinel Eliminator, now required by most OEMs for warranty validation
- Powerflush or chemical flush on older systems (check whether it is included or extra)
- System inhibitor top-up
- Building Control notification under Part L + Part J (self-certified through the Gas Safe Competent Persons Scheme; should be included)
- Smart thermostat upgrade if desired (Nest, Hive, Tado; ask for the model and price as a separate line)
Frequently asked questions
How much does a new combi boiler cost installed in the UK in 2026?
BOXT, a fixed-price online installer, publishes combi prices supplied and fitted from £1,890 (Navien NCB 300 30kW), with branded models from £2,110 (Ideal Logic Combi 24kW) to £3,400 (Worcester Bosch Greenstar 8000), checked 14 November 2026. The Energy Saving Trust puts the average cost of replacing a G-rated gas boiler with an A-rated one, with radiator valves, at £3,500 in Great Britain. A conversion from a regular or system boiler adds pipework, cylinder and tank removal that installers price by survey.
What size combi boiler do I need?
24 kW for a 1-bed flat. 28-30 kW for a 2-3 bed with one bathroom. 32-35 kW for 3-4 bed with two bathrooms. Above that, consider a system boiler instead.
Is converting from regular to combi worth it?
Often yes for 1-3 bed homes with one bathroom and decent mains pressure. Reclaims loft and cupboard space. Not worth it for 4+ bed homes with multiple bathrooms (system boiler wins).
How long does a combi installation take?
Like-for-like combi swap: 6-8 hours. Conversion: 1.5-2 days. All work under the Gas Safety (Installation and Use) Regulations 1998 by a Gas Safe registered engineer.
What is the most reliable combi boiler brand in the UK?
There is no free, independent reliability ranking we can cite with figures. What is published is each maker's warranty: up to 12 years on Ideal Vogue Max and Worcester Bosch Greenstar 8000 Life, up to 10 on Vaillant ecoTEC plus and Baxi 800, each with conditions. See /by-brand for the terms side by side.
Combi not right? See system boiler cost for 3+ bed homes with multiple bathrooms, or back boiler conversion cost if you have an old back boiler behind a gas fire. For a deeper combi-specific guide see combiboilercost.com.