DATA-BACKED GUIDE · UPDATED JULY 2026
Combi boiler replacement cost: swap vs conversion
If your combi has finally given up, a like-for-like replacement is usually the simplest heating job going. The price shifts a lot, though, depending on whether you are swapping one combi for another or converting from an old system with a tank and cylinder. The real prices on this page are drawn from genuine jobs so you can gauge what your own might run to, and see how far the swap and conversion figures sit apart.
The quick version
- A like-for-like combi boiler swap is the cheapest route because the pipework, flue run and gas supply are largely already in place.
- Converting from a system or conventional setup to a combi costs more, as tanks and cylinders come out, pipework changes and the loft or airing cupboard gets reworked.
- The boiler make, output size and warranty length all move the final figure, even on a straight swap.
- Relocating the boiler to a new wall or room adds flue and pipe runs, so keeping it where it already sits keeps costs down.
- A national brand installer usually quotes above a local independent engineer for the same job, sometimes by a wide margin.
- Whether a power flush is included or charged as an extra can change the quote as much as the boiler itself.
What people actually paid
Real prices, in people's own words
- £2,000“a 30kw one at a cost of £2k January 2021”
- £2,000“£2k”
- £2,400“I recently had a Worcester boiler installed (no other work, just a new boiler) and that was £2400 on it's own (including VAT and labour).”
- £2,500“just paid 2500 for a new combi (similar cost boiler)”
- £2,500“just paid 2500 for a new combi”
- £3,500“We paid £3500 to have a new boiler fitted and the old back boiler removed for my mum”
Genuine amounts posted publicly. We publish the price and the quote, never the person.
Why the price varies so much
The biggest single factor is whether it is a swap or a conversion. Replacing an old combi with a new combi in the same place is quick, so labour is lower: the flue is already through the wall, the gas supply is already sized and the pipework mostly stays as it is. Going from a conventional boiler with a hot water cylinder and cold water tank over to a combi means removing that kit, capping off redundant pipes, and reworking the system, which is a bigger day or two of work rather than a few hours. On top of that, the brand and output you choose changes the price, since a higher-output boiler for a bigger house with more radiators and bathrooms costs more than a smaller one for a flat. The condition of your existing radiators and pipework matters too: old, sludged-up pipework often needs a power flush before a new boiler goes in, and that is extra labour and materials. Who owns the installer also plays a part. A national brand carries higher overheads, call centre costs and marketing spend than a local independent, and that tends to show up in the quote for identical work. Region matters as well, with labour rates in and around London and the South East generally running above the rest of the country.
How to pay less
- Ask installers to price a like-for-like combi boiler swap first before you even mention relocation, then get a separate price if you do want it moved.
- Get at least three Gas Safe quotes and check each one lists the same boiler model, output size and length of warranty, so you are comparing like with like.
- Question whether a power flush is genuinely needed for your system or whether a cheaper chemical clean will do the job.
- Compare independent local engineers against national brands for the identical spec, since the local quote is often lower for the same boiler.
- Ask what is included in the warranty length quoted, since a longer manufacturer warranty sometimes costs more upfront but saves on breakdown call-outs later.
- If converting from a tank-and-cylinder system, ask whether the old tank, cylinder and pipework removal is priced separately or bundled in, so there are no surprises on the day.
Common questions
Is replacing a combi cheaper than converting to one?
Yes. A like-for-like combi boiler replacement reuses most of the existing pipework and flue run, so it is faster and cheaper. Converting from a tank-and-cylinder system to a combi involves stripping out the old kit, capping pipes and reworking the system, which adds a day or more of labour. The real prices on this page show both, and the gap between them is significant.
What size combi boiler do I need?
It depends on the number of radiators and bathrooms and how much hot water you use at once. A larger output suits a busy family home with more than one bathroom, while a smaller flat needs less. A Gas Safe engineer will size it properly rather than guessing, which avoids paying for more boiler than you actually need.
Can I replace a combi boiler myself?
No. It is illegal to fit or replace a gas boiler unless you are Gas Safe registered. Beyond the legal side, incorrect work risks gas leaks and carbon monoxide, and it will void the manufacturer warranty. Always use a registered engineer and keep the certificate for your records.
Does converting to a combi boiler add value or save money long term?
It can free up space by removing the loft tank and airing cupboard cylinder, which some buyers see as a plus. Whether it saves money depends on your household's hot water use, since a combi heats water on demand rather than storing it. The upfront cost of conversion is higher than a swap, so weigh that against how long you plan to stay in the property.
Why do quotes for what sounds like the same job vary so much?
Quotes differ because installers are not always pricing the same boiler, the same warranty length or the same extras. One quote might include a power flush and a new filter as standard, another might charge those separately or skip them. Always ask for the boiler model, output, warranty length and what is included before comparing prices side by side.
Is a longer warranty worth paying more for?
Often, yes, since a boiler is expected to run for many years and call-out charges after the warranty ends add up. A longer manufacturer warranty tends to push the upfront price up a little, but it can save you money if anything goes wrong in year five or six rather than year one or two. Check whether the warranty is tied to using the same installer for annual servicing, as some manufacturers require that to keep it valid.