The short answer
Converting from a regular or system boiler to a combi typically adds £300–£700 to the base replacement cost, bringing the total to roughly £2,000–£4,000 depending on the boiler chosen. The extra work covers removing the hot-water cylinder, capping or removing the loft tank (for regular-to-combi conversions), and re-routing the hot-water pipework. Not every home is a good candidate — see our comparison of boiler types first.
Many homeowners consider converting to a combi when their regular or system boiler fails, attracted by the simpler setup, freed-up cylinder space and the elimination of the loft tank. The conversion is often straightforward, but the costs and practicalities vary depending on how much pipework runs between the existing cylinder and the new boiler location, and whether your home’s hot-water demand genuinely suits a combi.
Boiler conversion costs at a glance
- Conversion extra cost £300–£700 on top of boiler price
- Total installed (typical) £2,000–£4,000
- What’s extra Remove cylinder, cap loft tanks, re-route pipes
- Time 1–2 days (vs 1 day for like-for-like)
- Not ideal for 3+ bathrooms, simultaneous high demand
- Decision driver Hot-water demand and available space
What the conversion involves
Converting from a regular or system boiler to a combi means the existing hot-water cylinder becomes redundant and must be removed. For a regular-to-combi conversion, the cold-water feed-and-expansion tanks in the loft are also decommissioned and either removed or capped. The hot-water pipework that previously ran from the cylinder to the taps must be rerouted to come from the combi directly. The gas and central-heating pipework may largely remain in place. In a straightforward layout where the cylinder is close to the new boiler location, this is a day’s additional work on top of the standard installation. In older properties with complex pipework, or where the cylinder is a long distance from the boiler, the labour and materials cost is higher.
Typical costs broken down
The table below shows typical additional costs on top of a standard combi supply-and-fit price for a like-for-like replacement:
| Conversion task | Typical cost |
|---|---|
| Remove hot-water cylinder and cap off | £100–£250 |
| Remove or cap loft tanks (regular only) | £100–£200 |
| Re-route hot-water pipework | £100–£300+ |
| Magnetic filter (if not included) | £80–£150 |
| Total conversion addition | £300–£700 |
When conversion is a good idea
Conversion makes most sense when the existing cylinder is taking up valuable airing-cupboard or bathroom space that you want back, when the loft tanks are old and a leak risk, or when the property is smaller and hot-water demand is modest — one bathroom, one or two people, rarely running simultaneous hot outlets. It also makes sense if the existing cylinder is in poor condition and would need replacing anyway, because the combined cost of a new cylinder and a system boiler installation can approach or exceed the cost of a combi conversion.
When conversion may not suit your home
Conversion to a combi is less suitable for homes with three or more bathrooms and high simultaneous hot-water demand. A family of four or five where two showers might run at once will often find a combi’s on-demand flow rate inadequate. Homes with existing solar thermal panels feeding the hot-water cylinder cannot straightforwardly use a combi without losing the solar input. And older properties with a large number of radiators — where a high-output combi is needed and the pipework is large-bore — may need additional work to balance the system. In those cases, replacing like-for-like with a system boiler and a new cylinder is often the better option. See combi vs system vs regular and the full installation cost breakdown.
What to confirm with your engineer
Before committing to a conversion, confirm: the mains pressure is adequate; the new combi’s hot-water output suits your peak demand; the pipework runs are manageable; and what happens to the cylinder space. Some homeowners also ask about immersion heater backup — a combi eliminates this, so there is no fallback if the boiler breaks down. That is worth factoring in if reliability matters. This is general guidance; a Gas Safe registered engineer should assess your specific property before any gas work is undertaken.
Get a combi conversion quote
Conversion costs vary with your property’s layout. Compare quotes from Gas Safe registered engineers to understand the full cost for your home.
Frequently asked questions
Is it worth converting to a combi boiler?
It depends on your home and usage. For smaller homes with one bathroom and modest hot-water demand, a combi is a practical choice and the conversion can free up cylinder space. For larger homes with multiple bathrooms, a system boiler with a cylinder may be more practical.
How long does a boiler conversion take?
A conversion from a regular or system boiler to a combi typically takes 1–2 days, compared to roughly one day for a like-for-like swap. The extra time covers removing the cylinder, capping tanks and re-routing hot-water pipework.
Do I lose my hot water if I convert to a combi?
You lose the stored hot-water cylinder, so there is no backup immersion heater if the boiler fails. The combi heats water on demand, so hot water is available whenever the boiler is running, but if the boiler breaks down you have no fallback source of hot water.
Can I keep my cylinder and still install a combi?
No. A combi heats hot water on demand and does not use a cylinder. If you want to retain stored hot water, a system boiler with a cylinder is the correct choice.
Sources & further reading
- Gas Safe Register — boiler conversion guidance and installer registration
- Energy Saving Trust — choosing the right boiler type for your home
- HHIC — Heating and Hotwater Industry Council, conversion and installation guidance
- GOV.UK / Building Regulations Approved Document G — hot-water supply and safety requirements
This is general information, not advice for your specific property or installation. Costs, timescales and outcomes vary with your home, system condition and chosen engineer. All gas boiler work must be carried out by a Gas Safe registered engineer.