Typical combi installation costs
The biggest factor in the price is how much the job changes your existing setup. These ranges include a new mid-range combi, fitting by a Gas Safe registered engineer, a system clean, a magnetic filter and basic controls.
| Job | Typical cost, supply and fit | Typical time |
|---|---|---|
| Combi to combi, same position | Typically £1,800 to £3,500 | One day |
| Combi to combi, moved to a new position | Typically £2,500 to £4,500 | One to two days |
| System boiler to combi | Typically £2,800 to £4,500 | Two days |
| Regular (heat-only) boiler with loft tanks to combi | Typically £3,000 to £5,000 | Two to three days |
For system and regular boiler prices, and grants, see our main new boiler cost guide.
Like-for-like combi swap
This is the simplest job. The old combi comes off the wall and a new one goes in the same place, using the existing pipes where they are suitable. It is usually done in one day. Even so, a like-for-like swap is rarely just a boiler change. The new boiler often needs a new flue, because old flues do not fit new models, and the pipe connections under the boiler may need adapting. The installer should clean the system and add inhibitor before handing over.
What pushes a swap to the top of the range is mostly the boiler itself (premium brands and higher outputs cost more), a long warranty package, a tricky position such as a loft, and London labour rates.
Converting from a system or regular boiler
A conversion is a bigger job because the whole hot water setup changes. It usually involves:
- removing the hot water cylinder, and on a regular system the cold water and feed and expansion tanks in the loft
- capping off redundant pipes and re-piping hot and cold water so the combi feeds your taps directly
- converting an open-vented heating system to a sealed, pressurised one
- fitting a new flue and condensate pipe if the boiler moves
- making good any holes and the old cylinder cupboard, which may be charged separately
Older systems sometimes develop leaks when they are pressurised for the first time, because joints and radiators that were fine at low pressure are now working harder. A good installer checks the system first and may test it under pressure before the new boiler goes in. If you already see drips, our guide to a boiler leaking water explains the common sources.
Extra work that adds to the price
| Extra | Why it may be needed | Typical cost |
|---|---|---|
| New horizontal flue | Old flues rarely fit new boilers | Often included, or typically £100 to £300 |
| Vertical or extended flue | Boiler moved inland or into a loft | Typically £300 to £700 |
| Gas pipe upgrade | Combis can need a larger pipe, often 22mm, from the meter | Typically £150 to £500 |
| Magnetic system filter | Protects the boiler, often needed for full warranty | Often included, or typically £100 to £250 |
| Smart or compliant controls | Time and temperature controls, plus an energy-saving measure on combi installs in England | Typically £150 to £350 |
| New or rerouted condensate pipe | Needed if the boiler moves or the old route freezes | Typically £100 to £250 |
| Powerflush | Dirty system with heavy sludge | Typically £350 to £800 |
The gas pipe
A modern combi can burn gas at a higher rate than the boiler it replaces, especially when heating hot water. If the gas pipe from the meter is too small or too long, the boiler may not get enough gas and can struggle to light or run properly. The engineer should check the pipe size and run, and test the gas pressure at the boiler once it is working.
The condensate pipe
Every condensing combi produces acidic condensate that drains through a plastic pipe. Where it has to run outside, the installer should keep the external run short, use a wider pipe and insulate it, which reduces the risk of a frozen condensate pipe in winter.
If the system water is very dirty, see our powerflush cost guide for when a full flush is worth paying for.
Is a combi right for your home?
A combi heats water only as you use it, so there is no stored hot water. That suits many homes, but not all.
- It works best in homes with one bathroom, or two where showers rarely run at the same time.
- Your mains water flow and pressure set the limit on hot water performance. The installer should measure both before recommending a combi.
- If you lose the cylinder, you also lose any immersion heater backup and the option of storing hot water from solar panels.
If two showers often run at once, a system boiler with an unvented cylinder may suit you better. A combi that is too small for your needs leads to the kind of complaints covered in our guide to hot water problems on a combi.
How to keep the installation cost down
- Keep the boiler where it is. Moving it is one of the biggest single extras, because of the new pipework, flue and condensate run.
- Pick the right output. A combi is sized mainly on hot water flow. A larger model costs more and will not give you more hot water if your mains supply cannot keep up.
- Compare like with like. One quote may include a filter, smart controls and a flush while another does not. Ask each installer to list what is included so the totals are comparable.
- Time it well. If the old boiler is still working, booking in late spring or summer often means more choice of dates, and sometimes a sharper price.
- Check the warranty terms. A longer warranty is only valuable if you keep up the annual service it requires, so factor that yearly cost into your comparison.
Get at least three written quotes, and be cautious of a price far below the others. It may leave out the flue, the system clean or the gas pipe work that the job actually needs.
What to check in your quote
- The exact boiler make, model and output, and the warranty length and conditions.
- Whether the flue, filter, controls, system clean and inhibitor are included.
- Whether a gas pipe upgrade or condensate reroute is needed, and the cost.
- Removal and disposal of the old boiler, tanks and cylinder.
- Registration with Building Control through Gas Safe and the completed Benchmark checklist.
- That the installer is Gas Safe registered. Ask to see the ID card on the day.
Only a Gas Safe registered engineer can legally install a gas boiler. If you smell gas at any time, call the National Gas Emergency Service on 0800 111 999.