Last Updated on November 10, 2026
Boiler advice · Combi conversions · Prices checked 10 November 2026
Swapping a system or regular boiler for a combi costs more than a like-for-like swap, because the cylinder and tanks come out and the pipework changes. Here’s what it costs with Eco Happy, how long it takes and when a combi isn’t the right choice.
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Contents
Quick answer
Quick answer
Converting a system or regular (conventional) boiler to a combi costs from about £2,629 fitted including VAT with Eco Happy. That’s our cheapest combi (from £1,928.99) plus a £700 conversion. A back boiler to combi costs from about £3,539 (combi + £1,610). A like-for-like combi swap, with no tanks or cylinder to remove, starts from £1,928.99. Prices are from our online quote, checked 10 November 2026. Most conversions take around two days for a system boiler, or up to three days for a regular boiler.
What a combi conversion actually is
A combi heats water straight from the mains when you turn on a tap, so it doesn’t need a hot water cylinder or cold water tanks. Converting means taking those out and re-piping your system for the new boiler.
A system boiler already has its pump and expansion parts built in and usually runs on a sealed system, so converting it is normally the simpler job: the cylinder comes out. A regular (conventional or heat-only) boiler also has tanks in the loft, so there’s more to remove and more pipework to change. That’s why it can take an extra day.
When a combi isn’t the right choice
A combi suits most homes with one main bathroom. If you have two or more bathrooms in heavy use at the same time, or very low mains water pressure, a system boiler with a cylinder may serve you better. Our system vs combi guide and what type of boiler do I need explain the trade-offs.
Combi conversion cost in 2026
| Type of job | Eco Happy fixed price (inc. VAT) | Typical time |
|---|---|---|
| Like-for-like combi swap | From £1,928.99 (range £1,928.99–£3,140) | 4–6 hours, done in a day |
| System or regular boiler → combi | From about £2,629 (your combi + £700) | Around 2 days (system), up to 3 days (regular) |
| Back boiler → combi | From about £3,539 (your combi + £1,610) | Longer than a standard swap. See our back boiler guide |
| Moving the boiler (added to any of the above) | Kitchen to airing cupboard £0 · same room £400 · another room £650 · cellar £650 · loft £950 | Adds pipework time |
For a system or regular boiler conversion, the engineer removes the old cylinder and tanks and re-pipes for the new boiler. Conversions also come with a bigger gas pipe allowance: up to 10m of 22mm gas pipe is included, against 5m on a like-for-like swap. Beyond that, extra gas pipe is £40 per metre.
Other extras that can change the price: a vertical flue through the roof (+£400 for the kit), extra flue parts (extensions £100 per metre, bends £75 each, plume kit £100), smart controls (Google Nest or Hive £219, Salus £129, T6 £199) and Saturday installation (+£50). Not included: asbestos removal, scaffolding and specialist access equipment.
Found a cheaper like-for-like quote? We’ll match it. Like-for-like quotes only: same boiler model, output and warranty, including installation.
What’s included in the boiler price
- Removal, safe disposal and recycling of your old boiler
- Your new combi boiler and the flue it needs
- Pipework alterations and fixings to Building Regulations
- Chemical flush of your pipework and radiators
- Chemical inhibitor and a magnetic system filter
- Fixed materials pack
- Wireless heating controls (upgrade to smart at checkout)
- Rebalancing of your whole heating system
- New or upgraded condensate pipe if needed
- Gas pipe upgrade if needed, within the included allowance
- Any required electrical work
- Required external brickwork, including bricking up
- Installation by a Gas Safe registered engineer
- CP1 certificate, Building Control notification and warranty registration
- Manufacturer warranty plus our 12-month workmanship warranty
- Access to our customer support line
This is the standard package listed on our combi installation page, checked 10 November 2026. For system and regular boiler prices, see our new boiler cost guide.
Which boilers we fit for a conversion
We fit three brands: Viessmann, Worcester Bosch and Alpha. Read more about Viessmann boilers, Worcester Bosch prices and Alpha boilers, or compare every model on our combi boiler installation page.
How the job runs
- Get your price onlineAnswer a few questions about your home and your current boiler at our online quote. Tell us it’s a system, regular or back boiler so the conversion is priced in.
- Choose your boilerCompare Viessmann, Worcester Bosch and Alpha combis side by side, each with a fixed price including VAT.
- Pick your installation datesAllow around two days for a system boiler conversion, or up to three days for a regular boiler, because the cylinder, tanks and pipework all change.
- HandoverYour engineer commissions the boiler, shows you the controls and leaves you with the CP1 certificate. Building Control notification and warranty registration follow.
Do you need a survey first? No survey needed: your conversion is priced online, and that’s your fixed price.
Paying monthly
0% APR over 12 or 24 months is available on selected boilers only (for example the Greenstar 4000, not the Vitodens 025), so don’t assume 0% on the cheapest conversion price. See every option in our boilers on finance guide.
Representative 11.9% APR
120 monthly payments of £34.51 on a £2,474.96 boiler, £0 deposit
Spread the cost over 120 months at 11.9% APR (see the representative example below), or choose 0% APR over 12 or 24 months on selected boilers. Finance is arranged through a panel of lenders, with £0 deposit, and the minimum you can borrow is £1,000.
Representative example: cash price £2,474.96 (Worcester Bosch Greenstar 4000 25kW). Deposit £0. Amount of credit £2,474.96. 120 monthly payments of £34.51. Total amount payable £4,141.20. Total charge for credit £1,666.24. Representative 11.9% APR. Early settlement fee £29.
Eco Happy Installs Limited (FRN 1051640) is a credit broker, not a lender, working with a panel of lenders. Credit is subject to status, affordability and lender approval.
What our customers say
Rated Excellent, 4.6 on Trustpilot from over 650 reviews (Trustpilot, November 2026).
★★★★★
“My boiler was installed by two very helpful, pleasant young men who were a credit to their firm. They explained how to manage the boiler and checked all the existing radiators before leaving.”
Rosalind P. · Trustpilot, 2 November 2026
★★★★★
“Competitive price, good communication, installer was on time, removed old boiler installed new one and you would never know he had been, unless you looked at the new boiler”
Trevor C. · Trustpilot, 20 July 2026
Reviews from Eco Happy boiler customers across the UK, quoted as written on Trustpilot. These aren’t necessarily conversion jobs.
Combi conversion FAQs
How much does it cost to convert a conventional boiler to a combi?
With Eco Happy, a system or regular boiler to a combi costs from about £2,629 fitted including VAT (our cheapest combi, £1,928.99, plus £700). Your online quote is the fixed price for your home.
How much is a back boiler to combi conversion?
From about £3,539 fitted including VAT (our cheapest combi plus £1,610). Our back boiler guide explains the process.
How long does a combi conversion take?
Around two days for a system boiler to combi, and up to three days for a regular or conventional boiler, according to our combi installation page (checked 10 November 2026).
What happens to my cylinder and loft tanks?
On a system or regular boiler conversion, the engineer removes the old cylinder and tanks and re-pipes for the new combi.
Do I have to have a combi?
No. If you have two or more bathrooms in heavy use, or poor mains pressure, a system boiler may be the better fit. See our system vs combi guide.
Do you need a survey for a combi conversion?
No survey needed. Your conversion is priced online, and that’s your fixed price.
Can I spread the cost?
Yes. 0% APR over 12 or 24 months on selected boilers, or 120 months at 11.9% representative APR, through a panel of lenders, subject to status. See our boilers on finance guide.
- Combi boiler prices
- New boiler cost
- Combi boiler installation
- Combi boiler cost
- Back boilers
- Boilers on finance
Get your fixed conversion price
Tell us about your current boiler, pick a Viessmann, Worcester Bosch or Alpha combi and see your fixed fitted price, including VAT, in minutes.
Prefer to talk? Call us on 0114 387 7066.
Eco Happy Installs Limited (FRN 1051640) is a credit broker working with a panel of lenders. We are authorised and regulated by the Financial Conduct Authority as a credit broker, not a lender. The availability of finance and the terms offered will depend on your individual circumstances. Credit is subject to status, affordability and lender approval. Terms and conditions apply.
