A combi boiler replacement swaps an old combi for a new one, usually in the same position: the old boiler is isolated and removed, the heating system is cleaned, the new boiler, flue and filter are fitted, and the installation is commissioned and registered. It is normally the simplest boiler replacement because there are no tanks or cylinders to remove, but the flue, gas supply pipe and condensate route may still need changing to meet current rules.
The survey decides which of those apply. Here is what it looks at and what the day itself involves.
How a combi swap usually runs
- 1
Protect and isolate
Floors and worktops are covered, then gas, water and power to the old boiler are turned off.
- 2
Drain and remove
The heating is drained and the old boiler and flue are taken off the wall.
- 3
Clean the system
A chemical or power flush removes sludge so the new boiler is not fitted to a dirty system.
- 4
Fit the new boiler
New bracket, boiler, flue and a magnetic filter on the return pipe; pipework adapted to the new connections.
- 5
Fill, test and commission
The system is refilled with inhibitor, gas tightness and combustion are tested, and controls are set up.
- 6
Register and hand over
The installation is registered with Building Control through Gas Safe and the manufacturer's guarantee is registered.
What can make a like-for-like swap bigger
| Item | Why it may change | What it means |
|---|---|---|
| Flue | Older flue positions may not meet current clearances from windows or boundaries | A new route or a flue kit extension |
| Gas pipe | Modern combis can need a larger supply pipe to run at full output | Upsizing part of the gas run |
| Condensate | Every condensing boiler needs a correctly routed drain, protected from freezing | A new internal route or insulated external pipe |
| Water flow | A combi's hot water relies on mains flow | Flow checked before choosing the output size |
| Position | Moving the boiler to a cupboard or loft | Extra pipework and flue work |
Choosing the replacement
Output should be matched to the house and, for a combi, to the mains flow. A bigger boiler does not give more hot water if the mains cannot supply it, and an oversized boiler cycles on and off inefficiently. The survey should measure rather than guess.
If your household often runs two showers at once, it is worth asking whether a combi is still the right answer or whether stored hot water would suit you better. For most flats and smaller houses, though, a new combi in the old position is the straightforward choice.
As a Worcester Bosch accredited installer we can offer guarantees of up to 12 years on qualifying Worcester boilers, and 0% finance is available on boiler installations.
Preparing for the day
- Clear the cupboard or wall around the boiler and the route to the front door.
- Plan for no heating or hot water for most of the day.
- Make sure someone can let the engineer into the loft or other rooms if radiators need checking.
- Keep pets and children away from the work area while pipework is open.
Questions people ask
Can a new combi go in the same place as the old one?
Usually yes, provided the flue position meets current rules and the gas supply and condensate route can be brought up to standard.
Do I need a system flush when replacing a combi?
Cleaning the system is standard practice and most manufacturers expect it for their guarantee. Old sludge shortens the life of a new boiler.
Will I get better hot water from a new combi?
A new combi can heat water more efficiently, but the flow rate is limited by your mains supply. If flow is low, a bigger boiler will not help.
Is finance available on a combi replacement?
Yes. 0% finance is available on boiler installations, alongside an interest-bearing option at 7.9% APR. See our finance page for the details.
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Look at where the flue leaves the wall and where the small plastic condensate pipe goes. If both look straightforward, your swap probably will be too.
Thinking of replacing your combi?
Call 07891 410 960 to book a survey and get a written price.