Combi, system or regular boiler: which costs more to replace?
Boiler type, not brand, drives your replacement bill. We compare combi, system and regular boiler costs, plus what conversions add, using live UK prices.
A straight swap costs the least whatever type you have, but a combi replacement typically runs £2,200 – £3,650, a system boiler replacement £3,050, and a regular (heat-only) boiler £2,800. The real cost driver isn't the brand on the casing, it's which of these three types you're fitting, and whether you're keeping the same setup or converting to something else.
Why type matters more than brand
Most people shopping for a new boiler compare brands. In practice, the installer's quote is driven far more by how many pipes, flues and cylinders they have to touch. A combi heats water on demand and needs no tank or cylinder, so swapping one combi for another is largely a day's work: disconnect, refit, reconnect, test. A system boiler keeps a hot water cylinder but loses the cold-water tank in the loft, while a regular boiler keeps both a cylinder and a loft tank — more pipework, more access, more labour either way.
| Material | Typical | Usual range | Lifespan |
|---|---|---|---|
| Combi boiler (like-for-like) | £2,800 | £2,200 – £3,650 | 12-15 years |
| Regular (heat-only) boiler, tanks kept | £2,800 | £2,200 – £3,650 | 12-15 years |
| System boiler (existing cylinder kept) | £3,050 | £2,400 – £4,000 | 12-15 years |
| Convert regular or system to combi | £3,800 | £2,950 – £4,950 | 12-15 years |
The hidden cost: converting to combi
The quote that surprises people isn't the like-for-like swap, it's converting a regular or system boiler to a combi. That job costs around £3,800, more than a straight combi swap, because the installer has to remove the old cylinder and any loft tanks, cap off redundant pipework, and sometimes reroute the hot water supply to bathrooms that previously relied on gravity or pumped pressure. It's worth doing if you want the airing cupboard space back and don't need the hot water capacity a cylinder gives you, but it's rarely the cheapest route if your current system still works.
The reverse rarely makes financial sense: converting a combi to a system or regular boiler means installing a cylinder and tank from scratch, which is closer to a new-build hot water installation than a boiler swap.
What else changes the price within each type
- Controls. All new gas boilers need a timer and room thermostat under Boiler Plus rules. A combi installation must also add one of: flue gas heat recovery, weather or load compensation, or smart controls with automation — a smart thermostat with an app is the common route and adds a modest amount over the basic programmable thermostat.
- Flue position. Keeping the same flue position is cheapest. Fitting a new vertical flue through the roof, or relocating the boiler to another room altogether, both add scaffolding, pipework and time.
- Power flush. A standard chemical clean is included in most quotes; a full power flush, worth doing on an older system with sludge or cold spots on radiators, costs more but can protect the new boiler's warranty.
- Efficiency. New boilers must meet a minimum efficiency rating, which is standard across modern models and not something you need to shop around for.
If you're also replacing the cylinder
System and regular boiler owners often find their hot water cylinder is as old as the boiler. If it needs replacing at the same time — or if you're converting from a vented to an unvented cylinder for better pressure — budget separately for that work; see our Hot water cylinder guide for current prices. Installing an unvented cylinder is notifiable building work unless your installer is registered with a competent person scheme, in which case they self-certify and you get a certificate automatically.
Who can do the work, and what it must meet
Gas work in UK homes must be carried out by a Gas Safe registered engineer — it's illegal for anyone else to do it, so always check the engineer's Gas Safe ID card before work starts. There's currently no requirement for new boilers to be "hydrogen-ready"; the government has a decision on hydrogen's role in heating still pending, and any such requirement would only apply from 2030 if hydrogen gets a substantive role.
Our boiler figures include 20% VAT, since boiler replacements don't qualify for the 0% VAT relief that applies to heat pumps, insulation, solar panels and batteries. If you're weighing a heat pump against a like-for-like gas boiler swap as your current system reaches the end of its life, our Air source heat pump guide has current costs and grant details for that comparison.
How we calculate this
We build these figures from labour hours by job type (straight swap, conversion, flue relocation) multiplied by current UK trade rates, plus materials, then check the result against market pricing and real installer quotes. If you've already got a quote in hand, you can run it through our "is my quote fair" tool to see how it compares with what the same job costs in your area before you sign off on it.