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What is a Boiler Flue? Complete Guide + Regulations | 2023

what is a boiler flue

Boiler flues are an important safety feature that help to protect you and your family from the harmful by-products of combustion. As these by-products pose a risk to health and can even damage your home, it is essential to have a correctly installed boiler flue. Let’s explore more in this guide – keep reading!


Short on Time? Here’s a Quick Summary

  • A boiler flue works to remove harmful gas boiler fumes, taking them outside of the home. 
  • UK regulations include using Gas Safe engineers to install and maintain your flue and minimum clearances from certain objects such as windows, as well as specific rules for rental accommodation. 
  • The type of boiler flue required will depend on your boiler type, with replacement costs being up to £700. 
  • Common issues with boiler flues include blockages, corrosion and leaks.

What is a Boiler Flue?

A boiler flue is a pipe that carries exhaust gases from your boiler to outside the home and releases them into the atmosphere. Boilers burn fuels such as gas or oil to heat water, but this process creates harmful by-products such as carbon monoxide. It’s the role of the boiler flue to expel these by-products outside the home, for the safety of those living inside.

A boiler flue is typically made of metal or plastic, and it can be either vertical or horizontal and is connected to the boiler at one end and to the outside of the home at the other end. The flue works by creating a draft.

The draft is created by the difference in temperature between the air inside the flue and the air outside the home. The air inside the flue is hotter than the air outside, so it rises up and out of the home which creates a vacuum that pulls the exhaust gases up the flue and out of the home.

It’s important to have your boiler flue inspected regularly by a qualified professional to ensure that it is in good condition as a blocked or damaged flue can be a fire hazard.

What Comes Out of a Boiler Flue?

steam coming out of boiler flue

The main things that come out of a boiler flue are:

Combustion Gases

These are the gases that are produced when the fuel is burned. They can include carbon monoxide, nitrogen dioxide, and water vapour.

Water Vapour

This is produced when the combustion gases cool down.

Particulate Matter

This is a mixture of solid particles and droplets that are suspended in the air. It can include soot, ash, and other pollutants.

How to Find Your Boiler Flue

If you aren’t quite sure where boiler flue is located, there are a few things you can look for to help locate it. Typically, the boiler flue pipe will come out of the top or side of your boiler and run outside of your home, however, the location of the flue can vary depending on the boiler and the layout of your home.

One way to locate your boiler flue is to look for the pipe outside your home. Usually made of metal, it can be found on an exterior wall, it is typically between 30-60cm long and protrudes slightly from the wall. You may also notice a small, round boiler vent cover near the flue exit, which is designed to allow air to flow into the flue.

If you can’t find your boiler flue outside your home, you may need to head inside, to an area such as the bathroom. The boiler flue pipe will usually connect to your boiler at the top or side, but you may need to remove a cover or panel to see the flue pipe. Once you’ve located the flue, you should check that it’s not obstructed and that there are no signs of corrosion or damage.

If you’re still having trouble locating your boiler flue, consult the manual that came with your boiler or contact a qualified heating engineer who will be able to help.

Does a Boiler Flue Vary Depending on Boiler Type?

Different boiler types produce different types of gases which means that the type of flue you need will vary based on your boiler type. Condensing boilers, which are more efficient than traditional boilers, produce less water vapour than traditional boilers.

This means that they need a flue that is designed to handle the condensation that they produce. Condensing boiler flues are typically made of stainless steel, which is resistant to corrosion from condensation.

Traditional boilers need a flue that is designed to handle the extra water vapour that they produce. Traditional flues are typically made of galvanised steel, which is more resistant to corrosion from water vapour than stainless steel.

If you’re not sure what type of boiler flue you need, it is best to consult with a qualified professional as they will be able to help you choose the right flue for your boiler and ensure that it is installed correctly.

What Are the Most Recent Boiler Flue Regulations?

boiler flue regulations

The most recent boiler flue regulations in the UK are found in the Building Regulations 2010. These regulations set out the standards that must be met for all new boiler flues.

As boiler flues are such an important safety feature, there are a number of boiler regulations that govern their installation, including around distance, and these are designed to protect you from harmful gases. These regulations include:

Minimum Clearances

Flues must be installed with a minimum clearance from certain objects, including:

  • Windows: 300 mm
  • Air bricks: 300 mm
  • Openings: 300 mm
  • Corners: 200 mm
  • Ground: 300 mm

Distance from Public Thoroughfares

If the flue faces a public thoroughfare or frequently used pathway, it must be at least 2.1m above ground level.

Distance from Velux Windows

If the flue is below a Velux window, it must be at least 2000 mm away.

Distance from Roof Pitch or Flat Roof

If the flue is on a vertical roof, it must be at least 300 mm above the roof pitch or flat roof.

More Powerful Boilers

More powerful boilers may require larger clearances between the flue and the relevant object. It is always best to consult with a qualified professional to ensure that the flue is installed correctly.

Gas Safe Installers

Gas Safe installers are qualified to install and maintain boiler flues in accordance with the regulations. If you are having a new boiler installed, it’s important to use a Gas Safe installer to ensure that the flue is installed safely.

Rented Accommodation

If you live in rented accommodation and suspect that the flue is not installed correctly, you should contact your landlord. A poorly-installed flue could be a safety hazard, and it is the landlord’s responsibility to ensure that the flue is safe.

Common Problems Associated with Boiler Flues

dipping boiler flue

Despite the regulations surrounding boiler flues, problems can still occur. Some of the most common problems with boiler flues include:


One of the most common issues is flue blockages, which can be caused by a variety of factors, including bird nests, leaves and debris. Flue blockages can prevent the flue gases from escaping, leading to a buildup of carbon monoxide in the home. Carbon monoxide is a highly poisonous gas that can cause serious health problems or even death, therefore, it is crucial to ensure that your boiler flue is regularly inspected and cleaned to prevent blockages.


Another common problem associated with boiler flues is corrosion. This can occur when the flue gases are not adequately dispersed, causing moisture to build up in the flue which can lead to rusting and eventually, holes in the flue. Corrosion can also occur when the flue is not made from a suitable material or is incorrectly insulated. Again, regular inspection and maintenance of your boiler flue can help prevent corrosion and ensure that your boiler is operating safely and efficiently.


Flue leaks can occur when there are gaps or cracks in the flue, allowing harmful gases to escape into your home. This can be especially dangerous if the leak is undetected for a long period, as it can cause carbon monoxide poisoning or other health hazards. Flue leaks can also lead to a decrease in boiler efficiency, resulting in increased energy bills.

Improper Installation

An improperly installed flue can lead to a range of problems, including inadequate ventilation and improper gas combustion. This can result in carbon monoxide buildup, reduced boiler efficiency, and increased energy costs making it essential to hire a qualified and experienced professional to install your boiler flue.

Older Boilers

Some older boilers may not have a flue at all. These boilers are known as open-flued boilers and can be dangerous, as they draw air from the room in which they are located. This can result in inadequate ventilation, which can cause carbon monoxide poisoning or other health problems. If you have an open-flued boiler, it is highly recommended that you replace it with a modern, condensing boiler that has a flue to avoid problems.

Average Costs of a Boiler Flue

The average cost to replace a boiler flue in the UK is £200-£700 although this can vary depending on certain factors, these include:

  • Type of flue – The type of flue will affect the cost of replacement. There are two main types of flues: horizontal flues and vertical flues. Horizontal flues are less expensive to install than vertical flue. 
  • Size of flue – The size of the flue will also affect the cost of replacement. Larger flues are more expensive to install than smaller flues.
  • Location of flue – The location of the flue will also affect the cost of replacement. Flues that are located in difficult-to-access areas are more expensive to install than flues that are located in easy-to-access areas.
  • Complexity of installation – The complexity of the installation will also affect the cost of replacement. More complex installations are more expensive than less complex installations.


Boiler flues are an important safety feature that will keep you and your loved ones safe in your home. It’s essential to keep on top of boiler flue maintenance, regularly inspecting them to check for any signs of common malfunctions such as corrosion or leaks. Always consult with a qualified Gas Safe engineer who can help check that your flue is in good working order and meets all necessary regulations.

Boiler Choice Director & Founder

This blog was written in collaboration with Eddie Scoffin, the founder of Boiler Choice. Eddie has been installing boilers for over a decade. With a passion for excellence and customer satisfaction, Eddie’s leadership and expertise have been instrumental in the success of Boiler Choice. His commitment to making the boiler installation process hassle-free for customers has earned him a respected reputation in the industry.

Updated on May 12, 2023