Is it worth converting from a regular boiler to a combi?

Often yes if your home is 1-3 bedrooms with one bathroom and decent mains pressure. The conversion typically costs £2,300 to £4,000 (combi unit + install + pipework changes + loft tank decommission + cylinder removal). Savings come from reclaimed cupboard / loft space and 5-10 percent efficiency gain over an older regular boiler. The decision flips at 4+ beds or 2+ bathrooms - a system boiler typically wins on flow rate to multiple taps.

UK 2026 / June 2026

Combi boiler replacement cost

Q: What size combi boiler do I need?

Combi sizing is driven by domestic hot water demand, not radiator output. A 1-bed flat needs 24 kW. A 2-3 bed home with one bathroom needs 28-30 kW. A 4-bed home with two bathrooms needs 32-35 kW (and at this size a system boiler often makes more sense than a combi). Energy Saving Trust publishes a sizing rule-of-thumb that tracks well against this. Always have a Gas Safe engineer confirm sizing on-site; oversizing wastes money and undersizing produces weak hot water.

UK 2026 combi-to-combi swap: £1,800 to £3,500 all-in. Add £500 to £2,000 when converting from a regular or system boiler.

The two scenarios

Scenario A

£1,800 - £3,500

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 £1,699 for an Ideal Logic Combi 24kW (checked October 2026).

Scenario B

£2,300 - £4,500

Conversion to combi from a regular or system boiler. Add £500-£2,000 of pipework + cylinder removal + loft tank decommission + flue work. 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. Rule of thumb published by Energy Saving Trust:

Property	Bathrooms	Recommended kW	Typical price band
1-bed flat	1	24 kW	£1,800 - £2,400
2-bed terrace	1	24-28 kW	£1,900 - £2,700
3-bed semi	1	28-30 kW	£2,000 - £3,000
3-4 bed	2	32-35 kW	£2,400 - £3,500
4-bed detached	2+	35 kW or switch to system	£2,800 - £3,800

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 October 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
Ideal Logic Combi 24kW	£1,699	Check at quote
Worcester Bosch Greenstar 4000	£2,299	10 years as listed by BOXT
Vaillant ecoFIT pure	£2,519	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	£3,350	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 (£500-£1,200 for like-for-like; +£300-£800 for conversions)
Flue + fittings (£100-£300, standard horizontal vs vertical-through-roof)
Magnetic system filter like MagnaClean Atom or Sentinel Eliminator (£80-£150 fitted) - now required by most OEMs for warranty validation
Powerflush or chemical flush on systems older than 10 years (£300-£600)
System inhibitor top-up (£30-£50)
Building Control notification under Part L + Part J (Gas Safe Competent Persons Scheme, usually included £100-£200)
Smart thermostat upgrade if desired (Nest, Hive, Tado from £150 installed)

Frequently asked questions

How much does a new combi boiler cost installed in the UK in 2026?

A like-for-like combi-to-combi swap typically costs £1,800 to £3,500 all-in. BOXT's published fixed installed prices run from £1,699 (Ideal Logic Combi 24kW) to £3,350 (Worcester Bosch Greenstar 8000), checked October 2026. Converting from regular or system adds £500-£2,000.

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 + cupboard space, 5-10pp efficiency gain. 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.