Category - Boilers
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Will Scholfield, Engineer

4 Mar : Updated 15 Mar ● 5 min read

Boiler flue position regulations

Whether you’re a Gas Safe registered engineer or a homeowner researching boiler installations in preparation for buying a new central heating system, it’s important to understand the rules in place to ensure your safety. 

One of the most important parts of any gas boiler is the boiler flue. This is responsible for expelling water vapour, as well as any harmful gases, making it critical for ensuring the health and well-being of everyone living in the home.

To guarantee the proper installation of boilers, boiler flue regulations were introduced to help ensure waste gases are expelled safely. 

Here, we delve into the boiler flue position regulations, explaining the key points of the regulations, including the rules around flue positioning and where you can move your flue pipe to. 

What is a boiler flue?

A boiler flue essentially serves as an exhaust or chimney for gas boilers - safely removing all of the waste gases created during the combustion process. Harmful gases like carbon dioxide, carbon monoxide, and nitrogen oxides are all expelled from the boiler through the flue, keeping the air inside our homes clean and safe. 

Boiler flues are an essential part of the following types of boilers:

  • Gas boilers, including combi, standard and system boilers
  • Back boiler units
  • Oil boilers
  • Biomass boilers
  • Wood-burning boilers and stoves

Where is a boiler flue usually located?

Typically made from metal or plastic, the flue is directly connected to the combustion chamber of the boiler - running vertically or horizontally out of your home. 

The precise location of your boiler flue will depend on where your boiler is positioned, but it can typically be found coming out of one of three locations, which we explore below.

External wall

A horizontal flue, coming out of an external wall, is most commonly used in the UK as it is usually the most direct route from the boiler to the outside.

diagram of a house showing the boiler flue placement


A vertical flue is typically used when a horizontal flue can’t be due to Gas Safe regulations.


Brick chimney

If your boiler is a fire and back boiler unit, the flue normally runs inside the brick chimney that the boiler is fitted within directly to the outside.

Why is a boiler flue important? 

The boiler flue is an important component of boilers for these reasons: 

1. Keeping you safe

The main role of the boiler flue is to expel potentially harmful gases and prevent their build-up - essentially, keeping you and your family safe from things like carbon monoxide poisoning. 

2. Ensuring compliance

To help keep you safe, there are several boiler regulations in place relating to new and existing boiler flues, flue length, location, flue support and materials.  

3. Benefitting performance 

A properly installed and maintained boiler flue will benefit the overall efficiency and performance of your boiler system. By safely venting waste gases and condensation, the flue reduces the risk of damage to your boiler. 

4. Improving air quality

A boiler flue’s main purpose is to expel waste gas and maintain air quality in your home. By removing water vapour, carbon dioxide, and more, the boiler flue reduces the risk of health complications like respiratory issues. 

Ensuring your boiler flue is in good working condition is of the utmost importance to your safety and your boiler’s efficiency - making it vital you book an annual boiler service with a Gas Safe registered engineer. 

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Boiler flue regulations - key points 

In terms of fitting a new boiler flue, this work must always be completed by a Gas Safe registered engineer who will be fully trained on how to ensure a safe installation. 

As no two properties are identical, the installation requirements will be unique to your home, with factors such as the location of your boiler and the proximity of nearby houses and public spaces all having a part to play in deciding how your flue needs to be installed.

Similarly, the specific rules and regulations relating to flue sizes and clearances depend on the manufacturer and model of boiler you have installed. Provided they are Gas Safe registered, the engineer undertaking the work will be aware of these requirements before they start. 

Flue support regulations

As well as the rules regarding placement, there are other boiler flue regulations too, including the support regulations. 

This stipulates that boiler flues must be adequately supported in the correct manner at certain intervals depending on the orientation, as well as before and after every change of direction to ensure they do not dip. Dipping within the flue could create condensation lying within it, which can impact your boiler’s performance or even corrode the flue. 

With some manufacturers, screws should be used to secure the flue and make sure it’s fully supported. The flue support will be reviewed by a Gas Safe registered engineer as part of your boiler’s service, so don’t worry about fittings becoming loose over time. 

Where can you move your boiler flue?

As long as your boiler flue complies with the positioning regulations above, a Gas Safe registered engineer can move it to a suitable location for you. If you currently have a horizontal flue and want to change it to a vertical flue, you should cover the flue to ensure nothing can fall into the pipe, like rain, snow or leaves. 

Any work you want to do to your flue, including moving it, must comply with the relevant boiler flue regulations. To ensure that the work is carried out properly and safely, it should always be carried out by a Gas Safe registered engineer. 

More advice from BOXT  

Buying a new boiler or understanding how to maintain your current boiler can be confusing, but the importance of getting it right cannot be overstated. 

To help you navigate everything from choosing the right type of boiler for your home to making sure it’s working safely and efficiently once installed, our central heating experts are on hand to provide tailored advice. Head over to our boiler guides, where you’ll find tips and guidance on topics like how much it costs to install central heating, how to clear an airlock from your central heating system, why your radiator is whistling and much more

Discover BOXT’s boiler guides.


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