The short version
- Swapping a boiler for the same type in the same place is usually a 1 day job.
- Moving the boiler or converting to a combi usually takes 2 days, and occasionally 3.
- Loft tanks, gas pipe upgrades, a power flush or a tricky flue can all add time.
- You’re only without heating and hot water while the work is happening.
- A good installer tells you the plan and the number of days before they start.
The short answer
Replacing a boiler with the same type, in the same position, is usually done in a day. That covers taking the old one out, cleaning the system, fitting the new boiler, flue and controls, and testing everything. Moving the boiler to a new spot, or converting from a regular or system boiler to a combi, usually takes 2 days, and sometimes 3 when there are tanks to remove from the loft or a lot of pipework to change.
| Job | Typical time |
|---|---|
| Combi to combi, same position | Usually 1 day |
| Regular or system boiler, like for like | Usually 1 day |
| Boiler moved to a new position | Usually 1 to 2 days |
| Regular or system boiler to combi | Usually 2 days, sometimes 3 |
| Back boiler replacement | Usually 2 to 3 days |
These are typical times, not promises. Your installer should give you the number of days for your job, in writing, before they start.
Why a like-for-like swap is quick
When the new boiler goes where the old one was, most of the work is already in place. The gas pipe, water connections, condensate drain and flue route are there, and they only need adapting to the new boiler. The day is spent removing the old boiler, cleaning the system, fitting the new one with a new flue kit and filter, then commissioning and testing.
Even here, a few things can stretch the day. A very sludged system takes longer to clean. A flue that has to change position slightly to meet the rules needs more work outside. Our boiler replacement page covers what a standard swap includes.
Moves and conversions: why they take longer
Moving a boiler means new gas, water and condensate pipe runs to the new position and a new flue through a different wall or the roof. Floorboards may need lifting and pipes may need running through other rooms. That is usually an extra day. See boiler relocation.
Converting to a combi adds more. The cylinder and loft tanks come out, the pipework is re-routed, and the heating system is sealed and filled to pressure. The system is then checked for leaks, because older joints and radiators sometimes weep once the pressure goes up. Removing tanks from a loft with a small hatch takes time on its own. Read more on boiler conversions.
Replacing a back boiler behind a fireplace is usually the longest job. The old unit and its flue in the chimney have to be removed and made safe, and the new boiler goes somewhere else entirely, often the kitchen. See back boiler replacement.
Find out how long your job takesTwo minutes, no obligation, a Gas Safe engineer replies.
Start my quoteWhat makes an installation take longer
- Gas supply upgrades: a larger boiler can need a bigger pipe from the meter. Running a new one takes extra hours, more if it crosses several rooms.
- A power flush: a heavily sludged system may need a power flush instead of a chemical clean, which adds time.
- Roof or long flues: a flue through a pitched or flat roof, or a long horizontal run, needs more parts and more work at height.
- Access: a boiler in a loft, a top-floor flat or a tight cupboard is slower to work on than one on a ground floor kitchen wall.
- Cylinder work: fitting a new cylinder, especially an unvented one, adds its own connections, controls and safety checks.
- Surprises under the floor: old pipework is sometimes in worse shape than it looked. A good installer tells you before carrying on.
Why a faster job isn’t always a better one
It is natural to want the shortest possible time without heating. But some of the hours on installation day are what make a boiler last, and they are the easiest to cut when someone is in a hurry.
- The system clean. Connecting a new boiler to dirty water is the quickest way to shorten its life. The clean to BS 7593 takes time and should not be skipped.
- The flue check. Reusing an old flue route without checking it against the manufacturer’s instructions and the standards can mean a flue in the wrong place.
- Commissioning. Testing gas tightness, checking combustion with an analyser and filling in the Benchmark checklist properly takes a while. It is also what the manufacturer will ask about if you ever claim on the warranty.
- The handover. Ten minutes being shown the controls saves weeks of the heating not doing what you want.
A realistic time from an installer who explains how the day is spent is worth more than a short one with no detail.
How long you’ll be without heating and hot water
On a 1 day job, you are without heating and hot water from the morning until the new boiler is commissioned later in the day. On a 2 or 3 day job, the work can often be planned so you are not left cold overnight, for example by doing the disruptive work early and getting the boiler running by the end of the first day. Ask what the plan is before the work starts.
In winter, ask what happens on the first night if a job runs over. An electric heater or two in the rooms you use most helps. If you have a cylinder with an immersion heater that is staying, it may give you hot water while the boiler is off.
Planning around the work
Someone needs to be home to let the engineers in and, at the end, to be shown the new boiler and controls. You do not have to be there all day. Clearing the space around the boiler and the route to it the night before saves time in the morning. Our guide to installation day has a full checklist.
If you want a firm idea of your own timescale, send us photos of the boiler, the flue outside and the pipes underneath through the quote form. We’ll tell you how many days your job needs along with the fixed price.