How long does it take to replace a boiler

How Long Does It Take To Replace A Boiler? From 3 Hours To Several Days


Your boiler installation can take as little as 3-4 hours to several days, depending on the size of the job and the type of boiler being fitted. Our guide will give you an overview of how long it will take to get the old boiler removed and the new one installed.

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How long does a new combi boiler installation take? 3-5 hours

replacing combi boiler with combi boiler

If you already have a combi boiler and want to replace the old one with a new version, this will generally take around 3-5 hours. Also known as a “combi swap”, this is the easiest of combi boiler installations.

This might not seem like a lot of time, but since most of the work is already done, there isn’t a great deal for the gas safe engineer to do to fit a new boiler in this installation scenario.

Once the old boiler is removed, it’s usually just a case of mounting the new boiler, connecting up the pipework, installing the flue, and going through the commissioning process.

This, of course, is based on a “boiler swap” or “combi swap”, with no additional work being required. As you will read further on, sometimes these boiler installations aren’t straightforward.

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How long does it take to change from a conventional boiler with a cylinder to a combi boiler? 1.5 days.

replacing regular boiler with combi boiler

Commonly referred to in the industry as a Combi Conversion, this type of installation takes longer than a like for like swap. The additional time will also add to the price.

Why does a combi conversion take longer? There are several additional tasks the heating installer must complete to install a new combi boiler in this scenario. The main ones are:

gas pipe upgrade

Boiler type requires gas pipe upgrade.

The gas run to your existing boiler is likely in 15mm pipe though it’s not always the case. In this case, the heating installer will need to upgrade the gas supply pipe to 22mm to comply with UK gas safe regulations.

If the gas meter is close to the boiler with no obstructions, this is not a problem. But in cases where the gas meter is a significant distance from the boiler, with several obstacles in the way, it can add complexity and time to the gas run.

A particularly long gas run can add to the price, so you must get a fixed price upfront.

Removal of cylinder and header water tank

heating header tank

Your old conventional boiler likely relied on a feed of water from a header water tank in the loft, and your hot water would be stored in a hot water cylinder, commonly found in an airing cupboard.

Removing the cylinder and the header tank will undoubtedly add time, mainly if the header tank in the loft requires to be broken down to fit it through the hatch in the attic.

Pipe reconfiguration

central heating pipework installation

When changing from a conventional boiler to a combi boiler, the heating and hot water pipework will have to be configured differently. For example, combi boilers supply hot water directly from the boiler rather than a cylinder, and so certain pipes which used to go to the cylinder will now need to be routed to the boiler.

Depending on the proximity of the cylinder to the location of the new boiler, this could range from a straightforward task to one that adds a few hours extra.

General tidying up and housekeeping

tidying up home after boiler installation

As this type of new boiler installation is more involved and has the potential to cause more disturbance, it will take your installer longer to ensure they are taking proper care.

It would be irresponsible to suggest that there would be “no mess” when having a combi conversion carried out. It involves soldering copper pipes, drilling holes, lifting floorboards, and removing items that have been there for many years.

Thankfully most gas safe engineers take pride in ensuring a tidy job, and so extra time should be allowed for the gas safe registered engineer to ensure that they can leave your home as they found it, with the addition of a new combi.

We always recommend getting a fixed price for peace of mind.

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How long does it take to have a new heat only or system boiler fitted? 0.5 – 1.5 days

replacing a regular boiler with another regular boiler

Not everyone wants a new combi installed, and that’s OK. These boilers are excellent in many homes, but they’re not the best choice for all homes. Particularly in cases where there is a high hot water demand, holding onto your cylinder may be the best choice.

A quick note: there are differences between a heat-only and a system boiler, but this won’t matter for the sake of this guide. If you would like to learn about the different boiler types, system boiler included, check out our guide on exactly that.

You may think that it would be easy to get the new boiler installed and that it would take 3-5 hours, but it’s not that simple.

Regulations have changed over the years, and nowadays, to install a heat-only boiler, the system must be configured in a particular way. Here’s how:

Independent control of heating and hot water

homeowner adjusting room thermostat

Most heat only boilers installed over 20 years ago didn’t have much in the way of controls.

If the heating was on, the cylinder was heating the hot water, and vice-versa. This isn’t great because, particularly in the Summer, you might require hot water for showering and baths, but you probably won’t require the heating to be on.

The only way to get independent control of your heating and hot water is to have the system reconfigured slightly and new controls fitted.

This reconfiguring of your central heating system and installing new controls can easily add from a few hours, up to a whole day, onto the job.

The good news for homeowners (and business owners) is that this will have the effect of reducing your utility bills, which should offset any increase in price.

It may be the case that your system already has modern controls installed, and if it does, it usually reverts to being a “straight swap”, and the price won’t be affected. But it’s worth keeping in mind that the extra work described above may be required along with your new boiler.

A fixed price is recommended, particularly if you are interested in getting a smart thermostat as well.

How long does it take to replace a back boiler? 1.5 to 2 days

baxi bermuda back boiler

Replacing a back boiler these days normally involves the installation of a combi boiler. This is because combi boilers tend to work well in the types of properties in which back boilers are installed and because (as things stand) there are no new back boilers on sale in the UK.

Back boilers are not “illegal”, as is sometimes incorrectly claimed. It’s just that there is no equivalent appliance to replace them with, so customers aren’t able to have a new back boiler fitted in place of their existing one – simple as that.

Assuming that you will have a new combi boiler fitted in place of your old boiler, you should allow a day and a half to two days for this.

This is because the work is not just a “Combi conversion” as a back boiler is essentially a heat-only boiler behind a fire; there is also the issue of what happens to the fire in addition to the old boiler.

Some gas-safe installers rip out the back boiler and the fire at the customer’s request, which allows the customer to have a new fireplace installed. And often, customers don’t want the expense of a new fireplace, so the installer will decommission the back boiler but leave the fire in situ.

Sometimes your gas safe registered engineer will even be able to leave your existing gas fire working if it is gas safe to do so.

Either way, there is even more work involved here than a combi conversion, so allow for a day and a half to get this boiler installation done.

We don’t like to see the fire and back boiler being ripped out and then boarded up with no thought given to what would happen next, leaving the customer disappointed. So definitely ask questions about how your potential installers will handle this before committing to the work.

Getting a fixed price from your gas engineer is highly important.

Reasons why getting a boiler installed may take longer (and add a little to the price)

Everyone’s house is different, as are the heating systems within them, and so it can quite often take a bit longer to have your new boiler fitted. Just because your neighbour’s job took half a day doesn’t mean yours will. And conversely, the installation of a boiler at your home may be quicker than expected, too. Here are a few of the reasons why a boiler installation may take longer than average.

The boiler is to be relocated to another area in your home

Customers often desire their boiler to be fitted in a new position within the home, especially when the home usage has changed or will change in the future.

Provided that the location for the new boiler to be installed meets the relevant regulations, there is usually no problem at all.

The one issue with having the new boiler installed in a different location is that the time to install will likely increase, depending on exactly where it is to be installed.

Moving the boiler elsewhere in the home can have a knock-on effect on other parts of the installation. For example, if the boiler is relocated into an airing cupboard, there is a chance that a vertical flue may need to be installed, and this will also increase the time it will take to install.

If you are looking to have the boiler moved a couple of feet within the same room, it might not add any time to the job. But if you are looking to have the boiler moved a significant distance, it will take considerably longer.

Existing boiler not meeting regulations

gas safe regulations

If the boiler in its current position does not meet current gas-safe regulations or regulations relevant to your boilers fuel type, the chances are it may need to be moved. There are several possible reasons for your old boiler not meeting current regulations, and it’s important to note that it doesn’t mean that your boiler wasn’t installed correctly back when it was new.

Either way, the new combi boiler will have to meet the current regulations, adding time to the installation.

The boiler is in a difficult location to access

loft access to install boiler

If your boiler is in a loft, for example, this will likely add some time to fit a new boiler.

Even the most organised gas engineers will need to make trips out to their van, or at the very least to other parts of the central heating system throughout the process.

If they need to go via the ladders to the loft each time, this will add some time to the job.

Other issues, such as the boiler being in a cupboard where space is limited, can mean that time to install a new boiler will be increased.

Vertical flue installation

ladders leading to roof where vertical flue will be installed

In cases where a vertical flue is required, this will undoubtedly add additional time to the installation process.

This is because it is highly likely that the engineer will need to get onto your roof to ensure the new flue is installed correctly.

Depending on whether ladders are involved or a full access tower in certain circumstances, you should allow for at least a couple of extra hours. Vertical flues commonly turn a 3-5 hour combi swap into a whole day job and can add £100-£400 to the price.

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Smart thermostat installation

Nest smart thermostat installation

A smart thermostat has the potential to make your heating system a joy to control, but spare a thought for the engineer fitting it!

Although a smart thermostat is easy to use, installing a new smart thermostat is not always the easiest thing, and it may add on a couple of hours.

Power flushing

Powerflushing machine

Most installers will carry out a mild cleanse of your heating system before fitting the new boiler; however, in cases where the system is ancient, a deeper cleanse may be required.

There isn’t much point in fitting a new combi onto a sludged up system after all!

This process can add 3-4 hours to the boiler replacement, and you can read more about this process here.

Magnetic boiler filter

Adey magnaclean magnetic filter

It’s a great idea to get a magnetic boiler filter installed, as this will increase the lifespan of your boiler without adding much to the price. Many providers now include a magnetic filter as part of the overall price quote with nothing extra to pay.

Fitting radiators

standard white compact style radiators

If you have some radiators replaced on your system simultaneously as your new boiler, this will add some time.

Budget (very, very roughly) for around 30 mins per additional radiator, assuming no pipework alterations need to be made to your heating system.

Thermostatic radiator valves

Danfoss thermostatic radiator valves

If you have thermostatic radiator valves installed onto your system simultaneously as the boiler, this will also add a little time.

Expect your engineer to take an additional couple of hours to add new radiator valves to an average-sized heating system.


A lot rides on the boiler type you have at present and the proposed location of your new boiler system.

Online boiler replacement specialists will factor in the points mentioned above and let you know how long it will take to install, and give you an instant price quote for your new boiler. Many providers also finance options available to make it more affordable too.

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