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 GBP 2,300 to GBP 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 / May 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: GBP 1,800 to GBP 3,500 all-in. Add GBP 500 to GBP 2,000 when converting from a regular or system boiler.

The two scenarios

Scenario A

GBP 1,800 - GBP 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. Quotes can come in lower (GBP 1,650-GBP 1,800) on installer-direct packages from Heatable or BOXT for entry-level Ideal models.

Scenario B

GBP 2,300 - GBP 4,500

Conversion to combi from a regular or system boiler. Add GBP 500-GBP 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	GBP 1,800 - GBP 2,400
2-bed terrace	1	24-28 kW	GBP 1,900 - GBP 2,700
3-bed semi	1	28-30 kW	GBP 2,000 - GBP 3,000
3-4 bed	2	32-35 kW	GBP 2,400 - GBP 3,500
4-bed detached	2+	35 kW or switch to system	GBP 2,800 - GBP 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

Boiler unit prices below are RRPs cross-checked against Heatable and BOXT 2026 installer-direct pricing. Installed cost typically adds GBP 700-GBP 1,500 of labour on top depending on region and complexity.

Brand + model	kW range	Unit RRP	Warranty
Worcester Bosch Greenstar 4000	25, 30 kW	GBP 1,250 - GBP 1,450	10 years
Worcester Bosch Greenstar 8000 Life	30, 35, 40, 50 kW	GBP 1,800 - GBP 2,400	12 years
Vaillant ecoTEC Plus	25, 30, 35, 38 kW	GBP 1,300 - GBP 1,900	10 years
Ideal Logic Max	24, 30, 35 kW	GBP 900 - GBP 1,250	10 years
Ideal Vogue Max	26, 32, 40 kW	GBP 1,400 - GBP 1,750	12 years
Baxi 800 Combi	24, 30, 36 kW	GBP 950 - GBP 1,350	10 years
Viessmann Vitodens 100-W	25, 30, 35 kW	GBP 1,500 - GBP 1,950	10 years

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 (GBP 900-GBP 2,400)
Installation labour (GBP 500-GBP 1,200 for like-for-like; +GBP 300-GBP 800 for conversions)
Flue + fittings (GBP 100-GBP 300, standard horizontal vs vertical-through-roof)
Magnetic system filter like MagnaClean Atom or Sentinel Eliminator (GBP 80-GBP 150 fitted) - now required by most OEMs for warranty validation
Powerflush or chemical flush on systems older than 10 years (GBP 300-GBP 600)
System inhibitor top-up (GBP 30-GBP 50)
Building Control notification under Part L + Part J (Gas Safe Competent Persons Scheme, usually included GBP 100-GBP 200)
Smart thermostat upgrade if desired (Nest, Hive, Tado from GBP 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 GBP 1,800 to GBP 3,500 all-in. Budget Ideal Logic Max from GBP 1,800. Premium Worcester Bosch Greenstar 8000 Life GBP 3,200-GBP 3,500. Converting from regular or system adds GBP 500-GBP 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?

Worcester Bosch tops installer recommendation surveys (Which? Best Buy 2026, Trustpilot installer reviews). Vaillant a close second. Ideal Logic Max wins on value for money. See the brand quick-compare on /by-brand for warranties and price ranges.

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.