How long does it take to fit a new boiler?
Fitting a new boiler takes one day for a straightforward swap and two to three days for a bigger job. The deciding factor is how much changes: the same type of boiler in the same place reuses most of the pipework, while a new position or a new type of system means new pipes, a new flue route and often work in the loft or airing cupboard.
Checkatrade's new boiler cost guide puts the boiler work itself on a like-for-like replacement at about 4 to 6 hours. Most installers still book a full day, because draining the system, cleaning it, fitting a filter, commissioning the boiler and explaining the controls all take time on top of hanging the boiler on the wall.
| Type of job | Typical fitting time | Why it takes that long |
|---|---|---|
| Combi to combi, same position | 1 day | Gas, water, heating and condensate connections are mostly in place |
| System or regular boiler, same type and position | 1 day, sometimes 2 | More pipework to drain and refill; cylinder controls checked |
| New boiler in a new position | 1.5 to 2 days | New gas and heating pipes, new flue route and condensate run |
| Regular boiler to combi conversion | 1.5 to 2 days | Cylinder and loft tanks removed, pipes capped, gas pipe often upsized |
| System boiler to combi conversion | Up to around 3 days | Cylinder, valves and controls removed and the system re-piped |
| Back boiler replacement | 2 to 3 days | Boiler moves out of the fireplace; chimney, fire and pipework reworked |
| Oil boiler replacement | 2 to 3 days, more with a new tank | Oil line, fire valve and flue work; a new tank is a separate stage |
The conversion times are from Checkatrade's published guide; the others are typical estimates from installer guides. Your installer's survey is what sets the real figure, so ask for the number of days in writing with the quote. Back boilers have their own guide, back boiler replacement, and oil jobs are covered in oil boiler replacement.
Can you have a new boiler fitted in a day?
Yes, most like-for-like replacements are fitted in a single day. It works best when these things are true:
- The new boiler is the same type as the old one, for example combi to combi.
- It goes in the same place, so the gas, water and heating pipes line up or need only short adjustments.
- The existing gas supply pipe is big enough for the new boiler's gas rate.
- The new flue can go through the existing hole, or a new hole is simple to core.
- The condensate pipe already runs to a suitable drain.
- The system water is clean enough for a chemical clean rather than a full powerflush.
If any of these is not the case, a one-day job can become two. A good installer spots this at the survey rather than on the day.
Ask the installer what time they plan to start, whether the heating will be working again that evening and what you need to clear beforehand, such as the boiler cupboard, the airing cupboard or a path to the loft hatch.
How long does it take to install a new boiler, step by step?
A new boiler installation follows the same stages whatever the job, and the longest parts are usually the draining, cleaning and commissioning rather than fitting the boiler itself.
- Isolate and drain. The engineer turns off the gas, water and electricity to the boiler and drains the heating system.
- Remove the old boiler. The boiler, flue and any redundant parts come out, and the wall is prepared.
- Fit the new boiler. The bracket, flue, gas, water, heating and condensate connections go in, plus any new valves.
- Clean the system. British Standard BS 7593 sets out how heating water should be cleaned and treated. Most installers flush the system with a cleaning chemical, then add inhibitor and fit a magnetic filter.
- Fill and test. The system is refilled, bled and pressurised, and the gas pipework is tightness tested.
- Commission. The engineer checks the combustion with a flue gas analyser, sets up the controls and fills in the Benchmark commissioning checklist.
- Hand over. You should be shown the controls, the pressure gauge and filling loop, and given the paperwork.
What adds time to a boiler installation
The most common reasons a job runs longer are extra work agreed at the survey and problems found once the old boiler is off.
- A powerflush. Pumping every radiator through one by one takes most of a day for a typical house. Our powerflush cost guide explains when it is needed.
- A bigger gas pipe. BS 6891 limits the pressure drop between the gas meter and the boiler to 1 mbar. A combi burns gas at a high rate, so an old 15 mm run often has to be replaced with a larger pipe.
- Moving the boiler. New pipe runs, a new flue hole and making good all add time. Our guide to the cost to move a boiler covers this.
- Removing a cylinder and tanks. On a conversion, the cylinder and loft tanks come out and old pipes are capped or removed.
- Controls. In England, the Boiler Plus rules require a new combi to be fitted with at least one extra energy saving measure, such as weather compensation, load compensation or a smart thermostat with automation and optimisation. Radiators without thermostatic valves may need them too.
- Surprises. Seized valves, a leak that shows up on refilling or a radiator that fails after cleaning can all add hours.
How long will you be without heating when a boiler is replaced?
On a one-day swap, you are usually without heating and hot water from the morning until late afternoon or early evening. The system is drained soon after the installer arrives and stays off until the new boiler is commissioned.
On a two or three day job, many installers aim to leave the heating running overnight, but that is not always possible. During a conversion to a combi, once the cylinder comes out you have no stored hot water until the new boiler is working. The cold mains supply is normally only off for short periods while pipes are connected.
If your old boiler still works, it can help to book the replacement outside the coldest months. If it has already broken down, a couple of electric heaters and a plan for showers make the gap easier, especially with young children or anyone elderly in the house.
Paperwork and Building Regulations
Installing a gas boiler is notifiable work under the Building Regulations in England and Wales, but you do not have to contact building control yourself. A Gas Safe registered engineer self-certifies the job and notifies it through Gas Safe Register, and the notification must be made within 30 days of the work being completed. Gas Safe Register then issues a Building Regulations Compliance Certificate to the householder.
You should also get the completed Benchmark checklist and the manufacturer's guarantee registration. Several manufacturers, including Baxi and Ideal, require registration within 30 days of installation. For oil boilers, an OFTEC-registered technician self-certifies instead and gives you an installation completion report.
Only a Gas Safe registered engineer can legally fit a gas boiler. Ask to see their Gas Safe ID card and check the back shows they are qualified for the work, then check the number on the Gas Safe Register. Never agree to a cash job with no notification, as it leaves you without a compliance certificate.
What to do next
Get two or three quotes, each based on a proper survey of the gas pipe, flue route, water flow and radiators. Ask each installer for the number of days, whether a flush, filter and controls are included and whether the heating will be on each night. Prices are compared in our guides to new boiler costs and combi boiler installation costs. Before the start date, clear access to the boiler, the airing cupboard and the loft hatch so the installer can start straight away.