How Much Does It Cost to Replace a Combi Boiler?

Replacing a combi boiler with another combi in the same place typically costs between £1,050 and £2,250 for a 24-27kW model and between £2,000 and £2,800 for a 28-30kW model, based on UK industry averages. Those figures assume a straight swap with minimal pipework changes. Moving the boiler, rerouting the flue, upgrading the gas pipe or cleaning a dirty system all add to the price, which is why two quotes for the same house can be far apart.

How much does a like-for-like combi swap cost?

For a straightforward combi-for-combi replacement, national averages sit at roughly £1,050 to £2,250 for a 24-27kW boiler and £2,000 to £2,800 for a 28-30kW boiler. The price covers the boiler, fitting, commissioning and the paperwork, on the assumption that the new boiler goes where the old one was.

Treat these as guidance rather than a London price list. They are national averages, and they describe the simplest version of the job. Our guide to new boiler installation sets out what that simplest version includes and what it leaves out. Larger combis, and anything that changes the layout of the system, fall outside these ranges and are not reliably captured by any single published figure.

What is included in the price of a new combi boiler?

A complete quote should cover the boiler, the flue kit, removal of the old boiler, connection to the existing gas, water and heating pipework, commissioning, and registering the work under Building Regulations. If any of those lines is missing, the quote is not finished.

Most of the cost of a combi replacement is not the boiler itself. It is the labour, the flue, the controls and the work to leave the system in a state the manufacturer will warranty. In England, a new combi also has to meet the Boiler Plus rules: it needs a timer and a room thermostat, the system has to be cleaned and treated with inhibitor, and one extra energy-saving measure must be fitted, such as weather compensation, load compensation or a smart thermostat with automation and optimisation. A quote that leaves out the system clean or the controls is cheaper on paper because it is doing less than the rules require.

Ask each installer to write down:

• the exact boiler make, model and output, not just a brand name;
• the warranty length, and whether it depends on the installer's accreditation with the manufacturer;
• which controls are included;
• how the system will be cleaned and whether a magnetic filter is fitted;
• whether VAT is included in the total.

What makes a combi replacement cost more?

The price rises when the job stops being a straight swap. The most common reasons are moving the boiler, changing the flue route, upgrading the gas supply pipe and dealing with a heavily sludged system.

• Moving the boiler. Relocating it from a kitchen wall to a cupboard or loft means new gas, water, heating and condensate runs plus a new flue position, and usually turns a one-day job into two.
• The flue. A new boiler normally needs a new flue kit. If the old flue position no longer meets the current rules on distances from windows, doors and boundaries, the flue has to go somewhere else, sometimes with an extension or a plume kit, and a high flue in a terrace or flat can need access equipment.
• The gas pipe. Modern combis draw a lot of gas when heating hot water. If the pipe from the meter is too small for the new boiler, it has to be upsized so the boiler receives enough gas at full output.
• A dirty system. Black radiator water, cold spots and sludge mean a more thorough clean before the new boiler goes on. Skipping it shortens the life of the new boiler and can put the warranty at risk.
• Access and disposal. Top-floor flats without a lift, restricted parking and old boilers that are heavy or awkward to strip out all add labour.

Does a bigger combi cost more to replace?

Yes, but only modestly, and buying a larger combi than the house needs is money spent for nothing. The gap in the averages between a 24-27kW and a 28-30kW combi mostly reflects the boiler, not extra fitting work.

On a combi, the kW figure is sized around hot water flow, not the number of radiators. A one-bathroom flat rarely needs the output a larger house with an en suite does, and a combi cannot deliver more hot water than the incoming mains supply allows, whatever its rating. The right output comes from the heat loss of the property and its hot water demand, which our guide on how to size a boiler walks through. An installer who automatically quotes the largest model in a range is not necessarily giving you a better boiler.

Is changing from a system or regular boiler to a combi the same price?

No. Converting to a combi is a different and larger job than a combi-for-combi swap, and the averages above do not apply to it.

A conversion usually means removing the hot water cylinder and loft tanks, capping redundant pipework, reworking the hot water pipes to run from the mains, and often moving the boiler. That adds labour, materials and making-good. If that is your situation, changing to a combi boiler covers what the conversion involves, what drives its price and the checks to make before you decide.

Can you get help with the cost of a new combi boiler?

Only in limited cases. The ECO4 scheme can fund a replacement boiler for some households on qualifying benefits or low incomes, typically where the existing boiler is broken or very inefficient, and the scheme is delivered through energy suppliers and councils rather than chosen installers.

The Boiler Upgrade Scheme does not pay towards a gas boiler; it is for heat pumps and biomass. Most homeowners replacing a combi therefore pay for it themselves, either outright or through a finance product they arrange. Be wary of anyone promising a free boiler without first checking your eligibility against the scheme rules.

When is replacing a combi the wrong answer?

When the boiler is young and the fault is a single repairable part, or when a combi is the wrong type of boiler for the house. In either case, a new combi is money spent on the wrong fix.

A boiler under around ten years old with one failed component, such as a fan, pump or diverter valve, is usually worth repairing if parts are still available. Replacement makes more sense when the boiler is older, when repairs are becoming frequent, or when parts are no longer made. Separately, a combi struggles in homes where two showers regularly run at once or the mains flow is weak, and in those homes a system boiler with a cylinder can be the better buy even though it costs more to fit. The fixed price on a combi boiler replacement comes after a free survey, because the survey is what shows whether you are looking at a straight swap, a relocation or a different boiler altogether.