The short answer
A straightforward like-for-like boiler swap, such as combi to combi in the same position, is normally completed in one day. Moving the boiler to a new position usually takes one to two days, and converting a regular or system setup with tanks to a combi typically takes two to three days. Adding a power flush, new radiators or extra pipework adds time.
Typical timescales by type of job
Every home is different, but the scope of the job is what drives the time on site far more than the brand of boiler. These are the usual ranges:
- Like-for-like replacement in the same spot: usually one day.
- Replacement with a change of position, such as kitchen to loft or airing cupboard: usually one to two days.
- Conversion from a tank-fed system to a combi, removing cylinders and loft tanks: usually two to three days.
- Back boiler removal and replacement: often two to three days, because the old fireplace unit must be taken out safely.
- Full new central heating with radiators and pipework: several days depending on the size of the property.
What happens before installation day
A survey comes first. The engineer looks at the existing boiler and flue, the gas pipe size, water pressure and flow rate, the number of bathrooms and how you use hot water. From that they recommend a boiler type and size, and plan the route for the flue and condensate pipe. Getting this right is what keeps installation day on schedule — surprises on the day usually come from skipped surveys.
What happens on the day
The system is drained and the old boiler is disconnected and removed. Pipework is adapted to suit the new boiler, and the new flue and condensate route are fitted. The boiler is mounted, connected to gas, water and electrics, and the system is refilled.
Before handover the engineer carries out gas tightness tests, checks combustion readings, sets the controls and runs the heating and hot water to confirm everything works. A good installer will also clean the system and fit a magnetic filter to protect the new boiler from sludge.
Will I be without heating and hot water?
Yes, but only for part of the time. On a one-day swap you are typically without heating and hot water from the morning until the new boiler is commissioned later that day. On longer conversions the engineer will try to restore at least hot water at the end of each day where possible. If you have a vulnerable person at home, mention it at survey stage so the work can be planned around them.
What can make it take longer
A few things commonly extend an installation beyond the usual range:
- An undersized gas pipe that needs upgrading to feed a modern combi.
- Heavy sludge in older systems, meaning a power flush is needed first.
- Difficult flue routes, for example through a flat roof or a long horizontal run.
- Loft installations that need boarding, lighting or a new access route.
- Replacing radiators or valves at the same time.
How to prepare
Clear the area around the boiler and any cupboards or loft space the engineer needs. Make sure there is parking nearby and that pets are kept away from the work area. Think about where you want the thermostat, and have any questions about controls ready for the handover. After installation the boiler must be registered with the manufacturer to activate the warranty, and building regulations notification is completed through Gas Safe.
Frequently asked questions
Can a boiler be installed in a day?
Yes, most like-for-like replacements in the same position are finished in a single day, including testing and handover.
How long does a combi conversion take?
Converting a system with a hot water cylinder and loft tanks to a combi usually takes two to three days, because the old tanks and pipework have to be removed and the system altered.
Do I need to be at home during the installation?
Someone needs to be there at the start to let the engineer in and at the end for the handover, when the controls are explained. You do not need to stay in the room while the work is carried out.
Which boiler brands do you install?
We fit Worcester Bosch, Vaillant, Ideal and Glow-worm boilers, and recommend a model based on your home and hot water use at survey.