Category - Boilers
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Ryan Gill, Engineer

31 Jan : Updated 11 May ● 7 min read

What is a boiler flue & how do they work?

The boiler flue is one of the most important parts of your home's central heating system. It takes away waste products of combustion and excess water from your boiler. Flues come in a variety of shapes, colours, and styles, but all have the same job and function the same way. They are critically important to both your family and your boiler.

Some older flue systems are integral to the boiler's actual casing, and therefore if the flue fails, the boiler could actually need replacing as the flue forms part of the main body of the boiler. 

This article will answer the question, what is a flue? And explain the importance of a boiler flue along with how they function.

What is a boiler flue?

The boiler flue is a large pipe that travels from the boiler to an external wall of the house. It allows the exhaust gases and fumes created by the boiler's operation to escape outside the house instead of inside the home. Looking at your boiler, the flue is the large chimney-like pipe often coming from the top or off the back of your boiler. 

How does a boiler flue work?

A boiler flue is effectively a chimney. The flue brings air into the boiler for combustion and expels the products of that combustion back to the outside. There are typically two waste products that a properly functioning modern boiler creates; carbon dioxide and condensation, both by-products of how boilers heat water. A modern boiler does condense, and this condensation runs back through the flue system and collects in the boiler.

The flue is connected (in most cases) to a fan inside the boiler, which helps propel the products of combustion out of the boiler through the wall to the outside air. In most boilers, this fan is connected to the flue system and many other components. These components work in series, and if any one faulty part breaks this series, the boiler will not operate - the boiler is designed in this way to ensure safety. 

Where is my boiler flue?

Most commonly, the exit point for the boiler flue pipe will be through the closest external wall off the top of the boiler or out the back of the boiler and is known as a horizontal flue. The flue can exit, however, in many different ways; some travel through ceiling voids and some travel up off the boiler and through the roof vertically (also known as a vertical flue).

Whether you have a combi boiler, a regular boiler or a system boiler, you will have a boiler flue with your system. If you don’t, your heating system isn’t meeting Gas Safe regulations, and you should call out an engineer immediately to get this resolved.

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Flue shapes, types and sizes

Boiler flues have different shapes, types and sizes that vary based on the kind of boiler installed. It’s important to know which shape your existing flue is so that when you come to upgrade your heating system, you will have an idea of how much work needs to be undertaken.

If you have a round flue, that usually means your boiler is under 15 years old. On the other hand, if you have a square flue, that usually indicates your heating system and boiler are a lot older and potentially require more work like bricking up and patching the wall internally. If this is the case. This is something you should discuss with a heating engineer before any work is undertaken.

Horizontal flue

Horizontal flues are the most common type of flue found in the UK, especially with all new boiler installations. So chances are this is what you currently have or what your new boiler will use.

As the name suggests, these flues extend out horizontally from the boiler through the wall, are made of round pipes and come in either white or black. 

Though the most common type, the biggest factor is your home's floor plan. In some instances, the ability for the boiler to be near an external wall is not possible, and thus this type of boiler flue may not be suitable and/or able to meet regulations.

Vertical flue

When the boiler needs to be located centrally in the home, or your home has a unique floorplan, then vertical flues can be used to get the flue to the outside. A vertical flue extends upwards from the boiler to the roof but should not be confused with your chimney; this is very different! As there are many different roofs, there are many different flashing kits and types of flue to accommodate every shape of roof.  

Vertical flues are often more expensive than horizontal flues due to the extra components and labour required to install them. Sometimes there may even be a need for scaffolding to get to more awkward to reach flues which will further impact the cost. Experienced Gas Safe engineers should have no problem carrying out this type of work; however, in most cases, only specific types of roof work would require a roofer.

When you install a new boiler, you have to change the flue to ensure you are meeting current gas safety legislation and make sure the new flue is suitable for the new boiler, as you cannot mix different manufacturers' flue systems with other boilers. Even if you currently have a flue pipe that meets regulations the new boiler will still need the flue pipe replacing. 

Get a quote for your new boiler installation today and lock in the price it would cost for one of our engineers to complete the work for you.

How much does a boiler flue cost?

This can vary greatly depending upon the boiler's model, the flue's style, and whether or not you need to pay for installation. Our new boiler installation, with flue included, starts from as little as £1520. Discover your fixed price now to get the best deal.

The flue will rarely need to be replaced independently of the boiler, as the flue system should last the lifetime of the boiler. In most cases, if there are problems with the flue, it could be an indicator that you need a new boiler. 

As part of the installation with BOXT, we provide a horizontal flue through the wall for no charge at all. 

A vertical flue through the roof is subject to a charge due to it being an extensive list of materials and extra labour costs, but with our fixed price promise, there will not be any hidden fees for you to worry about. 

Boiler flue maintenance

The boiler flue should be regularly maintained as part of the entire heating system. Thus, you should have the entire system serviced and cleaned at least once a year. Discover more about a BOXT boiler service and get your service booked today!

Only engineers with flue gas analysers (a machine that is designed to check flues) can check the operation of the flue is correct and sign off on the flue integrity; as the homeowner, you cannot do this yourself. Maintain the outside space around your flue to ensure it is kept clean, and there is nothing physically blocking the flue, like shrubs or trees but do not do any maintenance work on the flue yourself.  

Frequently asked questions

Though among the least expensive parts of a boiler, the flue is also one of the most important. The flue ensures that the boiler operates safely and effectively and that your home and family are also safe from any of the by-products which boilers naturally make. Flues may appear to be nothing more than a basic pipe, but their superpower is the fact that they save lives every day of the year.

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