Combi Boiler Installation Cost Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

A combi boiler costs to installed for a like-for-like swap in 2026, with the market median around and premium-brand or conversion jobs reaching . The combi’s appeal is practical: no cylinder or tanks, hot water on demand for as long as you like, one compact unit, and the simplest installation of any configuration, which is why it dominates UK homes with one bathroom.

Boiler output

Combi output matters mostly for hot water flow: a 24 to 27 kW unit serves a flat or small house with one bathroom, 28 to 32 kW is the standard three bedroom fit, and 35 to 42 kW units push stronger flow for larger homes. What no combi can do is serve two showers at once well, which is the boundary where a system boiler with a cylinder becomes the better machine.

Job type

A like-for-like swap in the same spot is a one-day job and the cheapest route. Moving the boiler, to a kitchen cupboard, garage or loft, adds flue and pipe runs worth to . Converting from a tanked or system setup means removing the cylinder and tanks and rerouting hot water pipework, typically adding to and a second day, though it frees an airing cupboard and a loft in return.

Boiler brand tier

Brand moves a quote by several hundred pounds either way: budget boilers do the same job with shorter warranties of around 2 to 5 years, mid-range brands balance price against 5 to 10 year cover, and the premium names, led by Worcester Bosch and Vaillant, cost 15 to 25 percent more for industry-leading reliability records and warranties up to 12 years. Over a 10 to 15 year boiler life, a long parts-and-labour warranty is worth real money, which is why premium brands hold their price.

Where you live

Heating labour is not priced uniformly across the UK, with published guides putting London and South East installation quotes roughly 15 to 25 percent above the national average. The hardware costs the same everywhere, so the difference sits in engineer day rates, access and parking.

What else affects the figures

The combi’s limits are flow and simultaneity, since all its hot water passes through the unit in real time: mains pressure caps the flow, and a second open tap steals from the first, which is why homes with two busy bathrooms usually do better with a system boiler and cylinder. Mains pressure and flow rate are worth measuring before choosing a large combi, as a 40 kW unit cannot conjure flow the incoming main does not supply. On the quote, check the warranty length, the magnetic filter, a system flush and Boiler Plus compliant controls are all included, and remember every gas combi is legally Gas Safe work with building regulations notification, which registered installers self-certify as part of the job.

Certification, planning and grid connection

Every gas boiler installation, repair or removal is legally restricted to Gas Safe registered engineers, and working on gas without registration is a criminal offence, so checking the engineer’s Gas Safe ID card and registration number is the non-negotiable first step. The installation is notifiable under Building Regulations, which registered installers self-certify, and Boiler Plus rules require time and temperature controls plus an efficiency measure on new combi installations. Flue positions and condensate drainage follow set standards worth confirming in the quote, an annual service protects both safety and the manufacturer warranty, and a carbon monoxide alarm near the boiler is cheap, sensible and in rented properties legally required.