How long does it take to replace a boiler?
Swapping an old combi for a new combi in the same place usually takes one working day. Moving the boiler to a different room, changing the flue route or converting from a system or regular boiler with a hot water cylinder to a combi normally takes two to three days, sometimes longer in a large house. Cleaning the heating system, replacing radiators or upgrading the gas supply adds to that. The survey before the job is what turns a rough estimate into a reliable timetable.
Loyal Plumbers · Updated
Discuss a boiler replacementTypical timings by type of job
The boiler itself is rarely what takes the time. Most of the work is in the pipework, the flue, the gas supply, the condensate drain and cleaning the system so the new boiler starts on clean water. The more of those that change, the longer the job.
These are typical ranges for a house or flat where access is reasonable. They are a guide for planning, not a quote for your home.
| Type of replacement | Typical time | Why |
|---|---|---|
| Combi to combi, same position | About 1 day | Existing gas, water and flue routes can often be reused or adapted |
| System to system, or regular to regular, same position | 1 to 2 days | Cylinder, pump and valves may also need attention |
| Boiler moved to another room | 2 to 3 days | New gas pipe, water pipes, condensate route and flue |
| Regular or system boiler converted to a combi | 2 to 3 days | Cylinder and loft tanks removed, pipework reconfigured, mains checked |
| Replacement with full system cleaning or power flush | Add about half a day to a day | The whole circuit is flushed and dosed before the new boiler runs |
| Replacement with new radiators or pipework | Several days | Depends on how many rooms and floors are involved |
What happens on the day
On a straightforward swap, the first job is to protect floors and the route to the boiler, turn off the gas, water and electricity to the old boiler and drain the heating system. The old boiler and flue come off, and any pipework changes are made before the new boiler is hung.
The engineer then connects the gas, water and heating pipes, fits the flue and condensate pipe, and refills and bleeds the system. The final stage is commissioning: checking gas pressure and combustion, testing the controls and filling in the Benchmark checklist. You should then be shown how to use the boiler and controls and given the paperwork.
- Protect the work area and isolate gas, water and power
- Drain the system and remove the old boiler and flue
- Adapt pipework and fit the new boiler, flue and condensate pipe
- Clean the system, add inhibitor and fit a filter where specified
- Refill, bleed and pressure test
- Commission, complete the Benchmark checklist and hand over
What can make a replacement take longer
Most overruns come from something that was hidden until the old boiler came off, or from a decision that was not made before the start. A good survey catches most of them. It cannot see inside walls or under floors, so a sensible installer builds some margin into the plan.
- The gas pipe is too small for the new boiler and needs upsizing back to the meter
- The flue position no longer meets current rules and has to move
- The condensate pipe has no suitable drain nearby
- Sludge in the system is heavier than expected and needs more cleaning
- Seized valves or corroded pipes break when disturbed
- The boiler is in a loft, a tight cupboard or a flat with difficult access
- Radiators or controls are changed at the same time
If an engineer finds a gas or flue problem that makes the old installation unsafe, it has to be dealt with before the new boiler is commissioned. Ask for any change to scope, time or cost to be explained and agreed in writing before the work goes ahead.
How long you will be without heating and hot water
On a one-day swap, expect heating and hot water to be off from the morning until the new boiler has been commissioned, usually late afternoon. On a two or three-day conversion, you may be without hot water for most of that time, although installers can often restore a temporary supply at the end of each day where the layout allows.
Plan for this. Fill a few containers with water if the mains will be off, have an electric heater ready in cold weather, and let anyone vulnerable in the home know in advance. Our guide on preparing for boiler replacement covers access, belongings and what to have ready.
Paperwork after the installation
Replacing a gas boiler is notifiable building work. A Gas Safe registered installer can self-certify it, and the building control compliance certificate is normally sent to you after the installer has notified it. You should also receive a completed Benchmark checklist in the boiler manual.
Most manufacturers require the new boiler to be registered within a set period for the guarantee to apply, and many require an annual service to keep it valid. Check who is registering it and keep a copy of everything with the boiler manual.
Plan your boiler replacement
Loyal Plumbers replaces boilers across London and inside the M25. Gas work is carried out by Gas Safe registered engineers, and we run 10 vans across the area. If your boiler has failed completely, we also attend heating breakdowns 24/7, which gives you time to plan the replacement properly rather than rushing it.
Call us or send an enquiry with the make and age of your current boiler, where it is, whether you have a hot water cylinder and what you would like to change. We will arrange a survey and give you a realistic timetable before any work starts.
Common questions
- Can a boiler be replaced in one day?
- Yes, a like-for-like combi replacement in the same position is normally finished in a day. Moves and conversions take longer.
- How long does a combi conversion take?
- Usually two to three days. The cylinder and loft tanks are removed, pipework is changed and the mains supply has to be checked for the flow a combi needs.
- Do I need to be at home during a boiler replacement?
- Someone should be there at the start to agree the plan and at the end for the handover and demonstration. You do not need to stay in the room while the work is done.
- Is a power flush needed when replacing a boiler?
- The system should be cleaned before a new boiler runs on it, and manufacturers set out what they expect. Whether that means a power flush or another cleaning method depends on the condition of the system.
- Will I get a certificate after a new boiler is fitted?
- Yes. The installer notifies the work, and you should receive a building regulations compliance certificate and a completed Benchmark checklist.
Sources and further reading
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