How Long Does It Take to Replace a Boiler?

Updated July 2026 · Emergency Plumber Chelsea

Why the type of job matters more than the boiler. Taking an old boiler off the wall and hanging a new one is a few hours' work on its own. What sets the length of the job is everything around it: whether the pipework lines up with the new boiler, whether the flue can go out through the same hole, whether tanks and a cylinder are being removed, and how much of the heating system needs cleaning or upgrading at the same time. That is why two quotes for the same boiler model can come with very different timescales, and why it is worth asking the installer to explain what they are allowing for. Our guide on comparing boiler quotes covers what else a written quote should spell out.

A like-for-like combi swap is usually a one-day job. On a typical day the engineer protects the floors and the area around the boiler, isolates the gas, water and electrics, drains the heating system and removes the old boiler. The new boiler is mounted, often with a new mounting bracket and a new flue, and the gas, heating flow and return, cold mains and hot water outlet are connected. A magnetic system filter is commonly fitted on the return pipe. The system is then refilled, bled and treated with inhibitor, the boiler is commissioned and its gas rate and combustion are checked, and the controls are set up. Most of these jobs are finished and running by the end of the working day, with the heating back on the same evening.

Replacing a system or conventional boiler with the same type takes a little longer. A system boiler works with a hot water cylinder, and a conventional (heat-only) boiler works with a cylinder plus cold water storage and feed tanks, usually in the loft. Keeping that layout and changing only the boiler is often one day, sometimes a day and a half, because there are more controls and components to reconnect and test. If the cylinder or its valves are also being replaced at the same time, add time for that; an unvented cylinder also needs installing by someone qualified for unvented hot water work.

Converting to a combi usually takes two to three days. Moving from a conventional system to a combi means removing the cylinder and loft tanks, capping or rerouting the pipes that served them, running the cold mains to the new boiler and making sure the incoming mains pressure and flow can support a combi at all. In a Chelsea townhouse where the airing cupboard is on the second floor and the tanks are in the roof, that is a lot of pipework spread across the house, and some of it may sit under floorboards or behind finished walls. Our guide to combi versus system boilers explains whether a combi suits your home before you commit to the extra work.

Moving the boiler or the flue adds time. Relocating a boiler to a different room or wall means new gas and water pipework to the new position and a new flue route, often with more making good afterwards. The gas pipe may also need upsizing, because a modern boiler can need a larger supply than the old pipe was sized for, and an installer will check this before starting. In Kensington and Chelsea, where the flue position on a period frontage can be sensitive and some buildings are listed or in conservation areas, sorting out where the flue can go is sometimes the longest part of the whole process, before any work starts on site. See our guide on boiler flues in conservation areas.

A power flush adds around half a day to a day. If the existing radiators and pipework are full of sludge, many installers recommend cleaning the system before the new boiler goes on, because debris from an old system can damage a new heat exchanger and some manufacturers expect the system to have been cleaned as a condition of the warranty. A power flush circulates cleaning chemicals through the system at high flow, radiator by radiator, and on a larger house it can take most of a day by itself. Other cleaning methods take less time, so ask which one is proposed and why.

Mansion blocks and period homes bring their own delays. Plenty of Chelsea boilers are in kitchen cupboards, lofts, basements or tight utility spaces, and getting an old boiler out of a fourth-floor flat without a lift, or working around a concierge's access times, takes longer than the same job in a house with easy access. Managed blocks may also need notice before work starts, and some freeholders want to see the installer's insurance and method statement. Parking and loading on Chelsea streets can eat into the start of the day as well. None of this is unusual, and a good installer will factor it into the timescale they give you.

How long you are without heating and hot water. On a like-for-like swap you normally lose heating and hot water from the morning, when the old boiler is drained down, until the new one is commissioned later that day. On a two or three day conversion there is usually at least one night without hot water, and sometimes without heating, though the work can often be staged so the new boiler is running by the end of the first or second day and the remaining days are spent on pipework and tidying up. If anyone in the household is elderly, unwell or has small children, or the job is in winter, say so when booking so the work can be planned around it.

What happens after the engineers leave. A Gas Safe registered installer self-certifies the work and notifies Building Control, and you should receive a Building Regulations compliance certificate afterwards. The boiler also needs registering with the manufacturer to activate the warranty, which is usually required within a set period after installation, so check whether your installer does this or leaves it to you. You should be shown how to use the controls, where the filling loop is and how to read the pressure gauge. Keep the commissioning checklist in the back of the manual, because it is needed at the first annual boiler service.

How to keep the job to its planned length. Clear the area around the boiler, the airing cupboard and the loft hatch before the engineers arrive. Tell the installer about parking, lifts and access in advance. Decide on controls, thermostat position and any radiator changes before the day rather than on it. And if you are replacing because the old boiler has failed in winter, ask whether a temporary repair is possible while you plan the replacement properly, because an emergency swap that has to work around a flue or gas pipe problem can take longer than one planned a week or two ahead.

If you want a timescale for your own home, Emergency Plumber Chelsea looks at the existing boiler, pipework, flue route and access, then tells you how many days the job needs and what is included, alongside a written quote. See our guide to what a new boiler costs for the price side, or our boiler installation service for what we fit across SW3, SW10, Knightsbridge, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours. Call 020 3002 6667.

Quick tips

• A like-for-like combi swap in the same position is usually one day; conversions and relocations are usually two to three.
• Ask the installer what their timescale includes: flue, gas pipe upgrade, filter, system clean and controls.
• Clear the boiler cupboard, airing cupboard and loft access before the day.
• Tell your managing agent or concierge in advance if you live in a mansion block.
• Check who registers the manufacturer warranty, and keep the Building Regulations certificate and commissioning record.

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