How long does it take to replace a boiler?

If your boiler is on its way out, one of the first things you'll want to know is how long you'll be without heating and hot water. For most homes, swapping one combi boiler for another in the same spot is a day's work. Changing to a different type of boiler, moving it to another room or replacing radiators at the same time can stretch the job to two or three days.

This post explains what happens on installation day, what can make the job take longer and how to plan around it as the weather turns colder. There's nothing here for you to do to the boiler yourself: any work on a gas boiler must be done by a Gas Safe registered engineer.

Typical timescales for different boiler jobs

Every home is different, so treat these as a rough guide. The survey before your installation is where you get a timescale for your own job.

• Like-for-like combi swap in the same place: usually one day.

• Replacing a system or regular boiler with the same type: often one to two days, because the hot water cylinder and its controls need reconnecting too.

• Changing from a regular or system boiler to a combi: often two to three days. A regular boiler (also called heat-only) works with water tanks in the loft and a hot water cylinder. A combi heats water straight from the mains, so the tanks and cylinder usually come out and the pipework changes.

• Moving the boiler to a new position: this can add up to a day for the new gas pipe, flue and drain routes.

Some jobs come in under these times. One of our customers described their boiler change as done "within only a few hours", and another had their boiler moved to the other side of the house, plus a new radiator fitted, in a single day.

What happens on installation day

A one-day swap follows the same pattern in most homes. We put dust sheets down first, then turn off the gas, water and electricity to the boiler and drain the heating system.

The old boiler comes off the wall and the new one goes up, with its flue (the pipe that carries waste gases outside) and its condensate pipe. That's the small plastic pipe that carries the slightly acidic water a modern boiler makes away to a drain.

Once everything is connected, the system is refilled and the engineer tests the gas pipework for leaks. Then comes commissioning: the checks that prove the boiler burns gas safely and correctly, recorded on a form called the Benchmark checklist. Before we leave, we show you how to use the new boiler and its controls, and take the old boiler away.

Why the system is cleaned before the new boiler goes in

Part of the day goes on the water inside your radiators and pipes. Building Regulations in England say the heating system should be thoroughly cleaned and flushed before a new boiler is fitted, then treated with inhibitor, a chemical that slows down rust and sludge.

A filter is also fitted on the pipework to catch any dirt before it reaches the boiler. On a fairly clean system, a chemical clean adds a few hours. On a system full of sludge, a power flush may be needed, which can take most of a day. Our guide to power flushing and chemical flushing explains the difference.

What can make the job take longer

The survey is there to spot these before the day, so they're built into your timescale rather than coming as a surprise.

• The gas pipe is too small for the new boiler and a larger one needs running from the meter.

• The flue can't use the same route, for example because it would sit too close to a window.

• There's no easy route for the condensate pipe, ideally kept inside so it can't freeze in winter.

• The gas meter needs moving, which only your gas supplier can arrange. One customer waited several weeks for this before their installation could go ahead.

• Old valves or fittings are seized or corroded and need replacing as the work goes on.

How long you'll be without heating and hot water

On a one-day swap, you'll usually be without heating and hot water from the morning until late afternoon or early evening. The cold water at your taps is normally on for most of the day, with short breaks while pipes are connected.

On a job that runs over two or three days, ask your installer before the work starts whether you'll have heating and hot water overnight in between. A few simple steps make the day easier:

• Have a shower or bath the evening before.

• Clear a path to the boiler, the gas meter and, for a conversion to a combi, the loft.

• If someone in your home is elderly, unwell or very young, plan a warm room with a plug-in electric heater, or a day out.

Planning a replacement before the cold sets in

The first cold weeks of the year are when tired boilers tend to give up, and when installers are at their busiest. If your boiler is old or the repairs keep coming, booking a survey now gives you the most choice over the day it's fitted. Our post on deciding between boiler repair and replacement can help you weigh it up.

When a boiler fails without warning, a replacement can sometimes go in within a day or two, but that depends on getting the boiler and the extra work needed. It's not something to count on in a cold snap.