How Long Does It Take to Fit a New Boiler?

How long does it take to fit a new boiler?

The honest answer is "one day for most people, two or three for some", and the difference comes down to how much of your heating system changes along with the boiler. The table below gives realistic timings for the jobs UK installers see most often. They assume a competent Gas Safe registered engineer, sometimes with an apprentice or mate, working a normal 8 am to 5 pm day.

These are working hours on site. The whole process from first enquiry to the engineer leaving is longer: a survey or online quote, a few days to a few weeks to get a slot (longer in winter), and the delivery of the boiler itself. This article is part of our complete guide to replacing a boiler, which covers the cost and the decision-making side; here we stick to the clock.

What decides how long a boiler installation takes?

Five things set the pace of the day. If you know where your home sits on each, you can predict the timing before the engineer does.

1. Like for like or a change of type. Combi for combi reuses the existing gas, water and flue positions. Switching from a regular boiler with tanks to a combi means stripping out the cylinder and loft tanks and running new pipework, which is where second and third days come from.
2. Position. A boiler staying on the same wall is the quick option. Moving it to a loft, garage or utility room means new pipe runs, a new flue and a condensate route, all of which need planning and making good.
3. Flue route. A horizontal flue through the same wall is an hour's work. A vertical flue through a roof, or a flue that has to be rerouted to meet current clearance rules, can add half a day and sometimes needs a roofer.
4. Gas supply. Many older boilers run on 15 mm gas pipe. Modern combis usually need 22 mm to the boiler, so a new run from the meter is a common and time-consuming extra.
5. System condition. If the radiators are full of black sludge, the engineer will chemically clean or powerflush before the new boiler goes on. Skipping this can void the guarantee, so it is not optional on a dirty system.

A sixth factor is simply the installer's team size. A two-person crew will finish a conversion in two days where a sole trader takes three. Neither is better; it just changes the timeline.

How long does a like-for-like combi swap take, hour by hour?

Here is a typical day for the most common job in the UK, a combi replaced with a combi on the same wall.

If the quote included a chemical flush rather than a full powerflush, it is usually done while the old boiler is off and adds an hour or two. If the gas pipe needs upgrading, that typically happens in the morning and can push the handover into the early evening. For a fuller account of the engineer's day and how to prepare for it, see our article on what happens during a combi installation.

How long does a conversion to a combi take?

A regular or system boiler changing to a combi is a different scale of job because three appliances become one. A typical schedule looks like this:

• Day one: old boiler, cylinder, cold water storage tank and feed-and-expansion tank removed; pipework capped; new 22 mm gas pipe run if needed; new pipework to the boiler position; wall prepared and boiler hung.
• Day two: flue fitted, all connections made, system chemically cleaned or powerflushed, refilled, tested and commissioned; controls set up; handover.
• Day three (if needed): making good around removed tanks and pipes, a full powerflush on a heavily sludged system, or finishing a flue that needed roof access.

Expect to be without hot water for the whole period. Many engineers can leave the old cylinder connected to an immersion heater overnight if you ask in advance, which makes a two-day job far more comfortable with children in the house. If you are unsure whether a conversion is right for you, the comparison of system versus combi boilers explains who gains and who loses from giving up a cylinder.

How long will the heating and hot water be off?

For a one-day swap, water is usually off from mid-morning until the system is refilled in the early afternoon, and heating returns when commissioning finishes. In practice:

• fill a kettle, a few bottles and the bath before the engineer isolates the mains, if you are on a combi;
• shower early or at a friend's house; the new boiler will be running hot water by late afternoon;
• in winter, a fan heater in the main living room keeps the house tolerable;
• on a conversion, ask whether the immersion heater can stay connected overnight.

Electricity is only off while the boiler's supply is disconnected and reconnected, normally a few minutes at each end of the day.

What can slow a boiler installation down?

Even a well-surveyed job can run late. The problems engineers most often hit once the system is drained are:

• Seized or passing valves on radiators and the old boiler, which prevent a partial drain and force a full one;
• Undersized or corroded gas pipe discovered only when the gas rate is checked;
• A flue position that no longer meets clearance rules after a neighbour's extension or a new window, so it has to move;
• Asbestos in old back-boiler surrounds or flue components, which stops work until it is dealt with by a licensed contractor;
• Sludge: a system so dirty the filter blocks within minutes, meaning a full powerflush before the new boiler is risked;
• No drain for the condensate near the boiler position, requiring a longer run or a pump;
• Scaffolding or roof access for vertical flues on taller houses.

Most of these are the installer's problem to solve, not yours, but they explain why a "one-day" job occasionally finishes the next morning. A written quote that lists assumptions (gas pipe size, flue route, system clean) protects you if extras are needed; our guide on comparing boiler quotes shows what that looks like.

How can you help the day run smoothly?

A few minutes of preparation saves the engineer an hour:

• clear a metre of space around the boiler and the route from the front door;
• empty the cupboard under the boiler and move anything on the wall nearby;
• know where your stopcock, gas meter and consumer unit are;
• move cars so the van can park close, especially on a conversion with tanks coming out;
• keep pets and children out of the working area;
• ask in advance whether you need to be at home all day (usually yes, at least at the start and the end).

Ask to see the engineer's Gas Safe ID card when they arrive. Our checklist on verifying a Gas Safe registered engineer explains what the card shows and how to check the licence number online.

For what the day's work should cost, including labour and the common extras that add hours to the job, see our breakdown of boiler installation labour and parts costs.

Frequently asked questions

How long does it take to replace a combi boiler like for like?

A combi-for-combi swap in the same position normally takes one working day, typically six to eight hours from arrival to handover. The engineer drains the system, removes the old boiler, hangs the new one, connects gas, water, flue and condensate, refills, tests and commissions it. Add two to four hours if the system needs a chemical flush or the gas supply pipe has to be upgraded to 22 mm.

How long does it take to change a regular boiler to a combi?

Allow two to three days. Day one usually covers removing the old boiler, cylinder and loft tanks and running new pipework; day two is fitting, connecting and flushing; a third day is sometimes needed for commissioning, making good and a full powerflush. Houses with awkward loft access, long pipe runs or a boiler moving to a different room sit at the top end.

How long will I be without heating and hot water during the installation?

For a straightforward swap, most of the working day: the water is usually off from mid-morning until the system is refilled in the afternoon, and the heating comes back on once commissioning is complete. On a two or three day conversion, expect no hot water for the whole period unless the engineer leaves the old cylinder connected overnight. Plan showers around it and keep an electric heater handy in winter.

Can a new boiler be fitted the next day?

Often yes for a standard combi swap. Online fixed-price installers and larger firms frequently offer next-day or two-day slots when the model is in stock and the job is straightforward. Jobs that need a survey visit, a different flue route, a cylinder or scaffolding take longer to schedule. In a cold snap waiting times stretch for everyone, so booking a planned swap in spring or summer avoids the queue.

Why did my boiler installation take longer than quoted?

The usual culprits are a dirty system that needed a full powerflush, an undersized gas pipe that had to be rerun, a flue that could not terminate where the old one did, corroded valves that would not shut off, or asbestos found behind an old back boiler. A good engineer will stop and explain the extra work before carrying on. Most of these are hard to spot without draining the system.

Sources and further reading

• Gas Safe Register - find and check a registered engineer
• HSE - Domestic gas health and safety
• GOV.UK - Approved Document L: Conservation of fuel and power
• Worcester Bosch - installation and commissioning guidance
• Energy Saving Trust - Boilers

Prices and scheme details were checked on 22 October 2026 and are shown as typical ranges. Always get written quotes from Gas Safe registered installers.