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Last updated: 9th May, 2024


What is a Boiler Flue and How Do I Find it?

What is a Boiler Flue and How Do I Find it?

Discover the different typeof boiler flues and what your home currently has and if it is to regulation.

When your boiler burns gas to heat the water that warms your home, excess heat is produced, in turn creating by-products of combustion fumes that are less than favourable, such as carbon dioxide, water vapour and the incredibly harmful carbon monoxide.

These by-products must be safely carried out of your home via something called a flue to prevent carbon monoxide poisoning.

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How to find a boiler flue

To find out where your boiler flue is, go outside and take a look on the outside wall where your boiler is located, if you see a pipe coming from the wall then you have a horizontal flue.

A vertical boiler flue you will find coming out of your roof or maybe a lower part of the building such as a garage. This tends to be the case when your boiler isn't located on an external wall.

What is a Flue?

The flue is a gas pipe that leads from your boiler to an external wall or the roof of your house and can often be seen puffing out steam (the gases), especially in winter.

Types Of Boiler Flue

Modern flues are usually round in shape, though square flues are still common in many UK homes and they can be fitted either horizontally or vertically in order to comply with regulations.

Due to the harmful properties of the waste gases, it is important to ensure the flue is located sensibly to avoid the gases re-entering your home or the home of a neighbour.

Boiler Flue Regulations 2020 prevent a flue from being positioned within 30-60cm of a window of door and they must be at least 2.1m off the ground if pacing a public space to avoid causing harm to any passers by.

Your flue should also be a certain distance from any drains, gutters, soil pipes or balconies as the waste gases can be incredibly hot and melt plastic.

Do All Boilers Have a Flue?

With the exception of back boilers, yes, all homes with a boiler will have a flue as they are required in order to comply with Gas Safe regulations. However, not all homes with a flue comply with Gas Safe regulations.

If you have a square shaped flue, it is worth getting your pipes checked by a Gas Safe registered engineer to ensure they are 22mm as opposed to the old 15mm ones, which are now illegal in some cases.

Different Flue Sizes

This is because combi boilers need a larger amount of gas in a shorter amount of time than regular and system boilers and therefore require wider pipes.

Flue Variations


As previously mentioned, flues can be either round or square shaped, though any boiler fitted within the last 15 years will have a round flue.


Horizontal flues are most commonly used in UK homes, exiting straight out of the external wall near the boiler.

A vertical flue is usually found where a horizontal flue cannot be used due to Gas Safety regulations and they exit up and out of the roof space.

These are usually more expensive than horizontal flues due to the extra parts and labour required to install them, including scaffolding in some cases.

Not only are there additional costs with a vertical flue, but they also require a guard or cover to prevent rain, leaves, snow and other foreign objects from entering, whilst still allowing gases to escape safely, unlike a horizontal flue. Blocked Flue

However, whilst it is less likely for a horizontal flue to become blocked, it certainly isn’t impossible as they can be more prone to birds’ nests.

If you think your flue is blocked, you should turn off your boiler completely, including cutting off the gas supply to the boiler, as opposed to just turning the heating off.

If your boiler’s flue is blocked, the harmful gases have nowhere to exit and will flow back. Call the National Gas Emergencies number immediately on 0800 111 999. More Help?

For more information on boiler flues, please get in touch with a member of our team at iHeat by calling 0333 305 6880 or take a look at our FAQs.

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Frequently Asked Questions

How to Find Boiler Flue?

To find your boiler flue, first, identify where your boiler is installed. The flue typically exits directly through an external wall close to the boiler or extends upward through the roof. It will be a pipe or duct that leads from the top or side of the boiler to the outside of the property. In apartment buildings or houses without direct external wall access near the boiler, the flue may be routed to go through the roof.

What Does a Boiler Flue Look Like?

A boiler flue usually looks like a white or black pipe made from metal or plastic that extends from the boiler to the outdoors. Its diameter can vary but typically ranges from 4 to 8 inches. The exterior part of the flue that vents outside will often have a terminal which is a protective cover that helps keep rain and debris out while allowing gases to escape safely.

Where is the Flue Located?

The flue is generally located close to the boiler, either leading directly out through an external wall or going up through the ceiling and roof, depending on the layout of your home and the boiler's position. Flues must be placed where the exhaust gases can be expelled safely and away from any windows, vents, or other openings that can allow gases back into the dwelling.

Do All Boilers Have a Flue?

Yes, all boilers have a flue. The flue is an essential part of any boiler installation, as it safely discharges harmful combustion gases such as carbon monoxide away from the home.

Does a Combi Boiler Have a Flue?

Yes, combi boilers have a flue. Like any other type of boiler, combi boilers generate combustion gases that need to be safely vented outside through the flue.

Where is the Flue Located in Building Regulations?

Building regulations specify that a boiler flue must terminate at a safe distance from windows, doors, and other building openings to prevent exhaust gases from re-entering the property. The exact requirements can vary depending on local codes, but generally, flues must be at least 30-60 centimeters away from any opening into the building, depending on the angle of the terminal and the output of the boiler.

Can My Boiler Flue Go Through a Roof?

Yes, a boiler flue can go through a roof if it’s not feasible to exit through a wall. This is often necessary in buildings where the boiler is not located on an external wall or in multi-floor properties. When routing a flue through a roof, it's important to ensure proper sealing and waterproofing to prevent leaks and water damage. The installation must comply with building regulations to ensure it is done safely and effectively.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.