How Long Does a Boiler Installation Take? | CAF Heating

How long does a boiler installation take? A straightforward like-for-like combi swap takes around one day. Converting a system boiler to a combi takes 1.5 to 2 days, moving the boiler to a new location adds roughly half a day, and replacing an old back boiler takes 2 to 3 days.

Those are the honest numbers, and we break each one down below. Most people asking this question are trying to plan time off work, work out whether a tenant can stay in the property, or figure out how long the house will be without hot water. For a standard swap the answer to all three is the same. One booked day is enough, someone only needs to be in to let us start, and the water is back on the same evening. We have been installing boilers across Surrey and South London since 1996, and the vast majority of our boiler installations are finished in a single day.

Boiler installation time by job type

The single biggest factor in how long the job takes is what kind of installation it is. A boiler going back on the same wall with the same flue route is a very different job to one moving across the house. Here are the realistic timescales we quote customers, based on thirty years of doing them.

If you are not sure which category your home falls into - for example whether to keep your cylinder or switch to a combi - our guide to combi vs system boilers walks through the trade-offs. And if you are budgeting for the work as well as scheduling it, our breakdown of new boiler costs in 2026 covers what each of these job types typically costs.

What happens on installation day, hour by hour

Here is how a typical one-day combi swap runs when we do it. Times shift a little from house to house, but the sequence is always the same.

8:00am - arrival and survey recap

We arrive, walk the job with you, and recap what was agreed at the survey: where the boiler is going, the flue route, where the controls will sit, and anything you flagged. Dust sheets go down along the work route and we protect carpets and worktops before a single tool comes out.

8:30am - drain-down and isolation

We isolate the gas and electrical supplies to the boiler and drain the central heating system. This is the point your heating and hot water go off. On a combi swap the drain-down takes 30 to 45 minutes depending on the size of the system and where the drain-off points are.

9:15am - removing the old boiler

The old boiler is disconnected, lifted off the wall and carried out, along with the old flue. We take the old unit away for disposal - you are not left with a dead boiler in the front garden.

10:00am - mounting the new boiler and fitting the flue

The new bracket goes on the wall, the boiler is hung, and the flue is fitted and sealed. If the new flue exits in a slightly different spot we core-drill a fresh hole and make good around the old one. Flue positioning has strict rules on distances from windows, doors and boundaries, so this stage is done carefully rather than quickly.

11:00am - pipework, filter and condensate

We connect the gas, flow, return, hot water and cold mains pipework, fit a magnetic system filter to protect the new boiler, and run the condensate pipe to a suitable drain. This is usually the longest stretch of the day, and it is where older properties can throw up surprises like corroded valves or undersized gas pipe.

1:30pm - cleansing and filling the system

Before the new boiler runs, the system is flushed and treated. On a healthy system that means a chemical cleanse and fresh inhibitor. On a sludged-up system it may mean a full power flush, which we would have identified at the survey and priced separately - our guide to what a power flush is explains when one is needed. The system is then refilled, radiators are bled, and everything is checked for leaks.

3:00pm - commissioning

The boiler is fired up and commissioned to the manufacturer's specification: gas rate and inlet pressure checked, flue gases analysed with a calibrated meter, hot water flow and temperatures verified, and controls set up and paired. Every reading is recorded in the Benchmark commissioning checklist inside the manual - skipping this can invalidate the warranty, which is exactly why manufacturers like Vaillant insist on it.

4:30pm - handover and notification

We walk you through the controls, show you how to top up the pressure, register the warranty, and tidy up. Within a few days of the job we notify the installation to Building Control through the Gas Safe Register, and you receive a Building Regulations compliance certificate in the post. A new boiler is notifiable work under the Building Regulations, and you will want that certificate when you come to sell the house - solicitors ask for it.

What can extend the job beyond a day

Most delays are predictable, which is why the survey matters so much. These are the things that add time, and in almost every case we spot them before installation day and build them into the quote rather than springing them on you:

• ◆Undersized gas pipe - many older homes have 15mm gas runs, and modern boilers often need 22mm, which means running new pipe from the meter
• ◆Moving the boiler - a kitchen-to-loft or kitchen-to-garage move adds new flue work, extended pipework and a longer condensate run
• ◆A flue through the roof instead of the wall - roof access, weatherproofing and sometimes scaffolding all add time
• ◆A badly sludged system that needs a full power flush before the new boiler can safely run
• ◆Old or corroded pipework and valves that fail as soon as they are disturbed and have to be replaced on the spot
• ◆Back boilers - removing the unit from the fireplace is heavy, slow work, and older installations can involve materials that need careful handling
• ◆Awkward access - top-floor flats, boilers boxed into tight cupboards, or listed buildings with restricted flue options

The honest rule of thumb: if your quote came from a proper in-person survey, the timescale you were given should hold. If it came from a photo and a phone call, expect the day to stretch.

How to prepare your home so the install stays on schedule

You do not need to do much, but a little preparation keeps the job on time. Here is what helps:

• ◆Clear a working space around the boiler - empty the cupboard under and around it, and clear a path from the front door
• ◆Clear access to every radiator, because we need to reach the valves to drain, refill and bleed the system
• ◆If the cylinder or loft tanks are coming out, clear the airing cupboard and the area around the loft hatch
• ◆Sort parking if you can - we carry the old boiler out and a lot of kit in, so close access saves real time
• ◆Fill a kettle and a couple of pans before we drain down, because the water to the taps can be off for part of the day
• ◆Keep pets and young children away from the work areas - there will be open pipework, tools and a hot flue
• ◆Decide where you want the thermostat or controls before the day, so we are not waiting on a decision mid-install

How long are you without heating and hot water?

On a standard one-day swap, the heating and hot water go off when we drain down mid-morning and come back on when the boiler is commissioned in the afternoon - typically four to six hours without hot water, and heating restored the same evening. On a two-day conversion, we aim to get hot water running by the end of day one and complete heating and controls on day two. For longer jobs like back boiler replacements, we tell you exactly which services will be off and when before we start, and in cold weather we can often stage the work so you are never without heat overnight. If anyone in the household is elderly or vulnerable, tell us at the survey and we will plan the sequence around them.

Getting an accurate timescale for your home

Every figure above is a typical range, and your house is not a typical house - none of them are. The only way to get a reliable answer is a survey, which is why ours is free and takes 30 to 45 minutes. We confirm the right boiler and size, map the flue route, check the gas supply, and give you a fixed written quote with a firm timescale. The timescale does not drift and nothing surprises you on the day. As a Vaillant Approved Installer we can also register you for Vaillant's 10-year parts and labour warranty on qualifying installations - see our pricing page for how our quotes work, or call us on 01293 774592.

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