Install
Combi boiler installation cost & timeline
The labour line on a quote is set by the type of swap, not the brand. Here is what each install type actually costs, every line item that can appear on the bill, and the day-by-day process so you know what to expect on fit day.
Labour cost
By install type
The biggest single driver of fitter cost. Like-for-like is cheapest. Back-boiler conversion is the most expensive because it usually involves builder's work too.
| Install type | Indicative labour | Typical duration | What is involved |
|---|---|---|---|
| Like-for-like combi swap | £800 to £1,500 | 1 day | Drop the old, fit the new in the same spot. Reuse flue if compliant. |
| Relocating the boiler | £1,200 to £2,000 | 1 to 2 days | New gas, water, flue runs. Old position is capped and made good. |
| Regular to combi conversion | £1,500 to £2,500 | 2 days | Cylinder out, loft tanks out, gas pipe upgrade, pipework rerouted, full powerflush. |
| Back boiler to combi | £2,000 to £3,000 | 2 to 3 days | Back boiler removed from behind fireplace. Builder's work, hearth made good. |
London and the South East typically adds 15 to 25 percent to these labour figures.
Line items
Every extra cost itemised
Not all of these apply to every install. Use the list to sense-check the line items on a written quote.
| Item | Indicative cost | When it applies |
|---|---|---|
| New flue (different position) | £100 to £300 | Boiler moves to a new wall or different room |
| Relocating the boiler | £300 to £500 | Different wall, different room, or kitchen refit |
| Powerflush | £300 to £500 | System over 10 years old, sludge in radiators |
| Magnetic filter (MagnaClean) | £100 to £150 | Recommended on every install, often required by warranty |
| Removing cylinder & tanks | £200 to £400 | Regular or system to combi conversion |
| New condensate pipe | £50 to £150 | Existing pipe not compliant or too short |
| Gas pipe upgrade to 22 mm | £100 to £200 | Older homes with 15 mm gas supply |
| Smart thermostat fitted | £150 to £300 | Optional, pays back via gas savings |
| Vertical flue kit | £80 to £150 | Roof-exit flue (flat or internal positioning) |
| Asbestos flue removal | £200 to £500 | Pre-1980s properties, specialist contractor |
| Chemical flush | £100 to £200 | Lighter than a powerflush, cheaper alternative |
The day
Step by step on installation day
A like-for-like swap follows this rhythm. Conversions add a day for tank removal and pipework, but the structure is the same.
- 01
Drain the system
The fitter drains the central heating and isolates the gas. From this point you have no heating or hot water.
- 02
Strip out the old
Old boiler off the wall, gas, water, flue disconnected. Set aside for disposal at the end of the job.
- 03
Prep the new position
If relocating, new gas, water, and flue runs are installed. Wall bracket fitted and levelled.
- 04
Hang the new boiler
New combi mounted on the wall, gas valve and flow / return pipes connected. Cold mains feed terminated.
- 05
Install the flue
Flue routed through the wall (or vertically) to comply with Building Regulations Part J distance rules.
- 06
Fill and pressurise
Refilled to 1 to 1.5 bar. Powerflush or chemical flush done at this stage if it was specified.
- 07
Commission and test
Boiler fired, every radiator checked for heat, hot-water flow rate verified, flue gas analysis recorded.
- 08
Handover
Walk-through of the controls, Benchmark certificate completed, warranty registered with the manufacturer.
Hiring
Choosing the fitter
A well-fitted budget boiler outlasts a badly-fitted premium one. Spend more time on the fitter than the brand.
Gas Safe registration
Legal requirement. Verify the engineer's ID at gasaferegister.co.uk before any work begins. Card has a unique number and a photo.
Brand accreditation
For warranties of 10 to 12 years, the fitter must be on the brand's accredited installer list (Worcester Accredited, Vaillant Advance, Ideal Max).
Itemised written quote
Boiler model and kW, labour, flue, disposal, commissioning, extras, warranty, and VAT, all on one page. Bundled lump-sum quotes hide too much.
Red flags
Cash only, no invoice. Reluctance to show ID. Pressure to decide today. Suggesting you skip the powerflush on a clearly old system.
Compliance
Building Regulations
Benchmark certificate. Filled in by the fitter at handover. Required for warranty claims and resale paperwork.
Local authority notification. Gas Safe fitters self-certify under Part J (Combustion Appliances). The certificate arrives by post within a few weeks.
Condensate drainage. Modern combis produce acidic condensate that must drain into an internal soil pipe or external drain. External pipes must be insulated to prevent freezing.
Flue positioning. Minimum distances from windows, doors, air bricks, and boundaries are set by Part J. The fitter sizes this on the site survey.
FAQs
Common questions
How long does combi boiler installation take?v
A like-for-like swap is one day, around 4 to 6 hours of working time. Relocations or back-boiler conversions take 2 days. Full regular-to-combi conversions stretch to 2 or 3 days because tanks come out and the gas pipe is often upgraded.
Can I install a combi boiler myself?v
No. Any work on the gas supply or flue must be done by a Gas Safe registered engineer. DIY installation is illegal, voids insurance, and is unsafe. Verify your fitter at gasaferegister.co.uk before signing.
Will I have heat the same day as installation?v
On a one-day swap, yes, by the evening. Conversions take longer. Ask your fitter to schedule the install in autumn rather than peak winter so a one-day delay does not leave you cold for a weekend.
Do I need a powerflush with a new boiler?v
Recommended if your system is over 10 years old, has visibly black water, or has cold spots in radiators. Skipping it on an old, sludgy system is the most common reason new heat exchangers fail and warranty claims get rejected.
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