Ryan Gill, Engineer

31 Jan : Updated 2 Sep ● 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.

Even though flues may appear to be nothing more than a basic pipe, their superpower is the fact that they save lives every day of the year, making them critically important for the well-being of both your family and your boiler.

This article will explain what boiler flues are and why they’re important, along with an overview of how they function as a crucial part of your home’s central heating system.

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. When you’re looking at your boiler, the flue is the large chimney-like pipe often coming from the top or back of your boiler.

How does a boiler flue work?

A boiler flue is effectively a chimney. The flue brings air into the boiler to aid the boiler's 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 water vapour.

Both are by-products of correct and complete combustion, which creates the heat energy that heats our homes. The water vapour cools and turns into condensation, which runs back through the flue system and collects in the boiler; the condensate is then discharged via the condensate pipe to a drain or soak away.

For energy efficiency reasons, all modern gas and oil boilers are condensing boilers, so they all need flues.

In most cases, the flue is connected to a heat exchanger and fan inside the boiler. The fan propels the products of combustion out of the boiler via the flue to the outside air. In most boilers, this fan is connected to the flue system and many other components. These components work in a series, and if any 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, which is known as a horizontal flue. The flue can travel outside in many different ways depending on the layout of your property. Some travel through ceiling voids, and some travel up off the boiler and through the roof vertically.

Whether you have a combi boiler, a standard boiler or a system boiler, you should have a flue connected to it that’s complete throughout its length, supported and in good condition. If this is not the case, you should turn the appliance off and contact a Gas Safe registered engineer immediately in order to get this resolved.

Flue shapes, types and sizes

Flues differ in shape, type and size depending on the kind of boiler you have and where it’s installed. It’s important to know which type and shape of flue your boiler has to make any repairs or central heating system upgrades easier.

If you have a round flue, that usually means your boiler is under 15 years old. 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 when a new boiler is installed. If this is the case, you should discuss what’s needed with a Gas Safe registered heating engineer before organising anything.

Horizontal flue

Horizontal, fanned flues are the most common type of flue found in the UK, especially with all new boiler installations. 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, it’s not possible to install a boiler immediately next to an external wall, meaning you may not be able to fit a horizontal flue.

Vertical flue

When the boiler needs to be located centrally in the home (or anywhere away from an external wall), a vertical boiler flue can be used to get the flue to the outside. A vertical flue extends upwards from the boiler and through your roof, using something called a flashing kit to prevent rainwater from seeping in around the opening.

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 access more awkward-to-reach flues, which will further impact the cost. An experienced Gas Safe registered engineer might be able to carry out this type of work for you, but be prepared to contact a roofing specialist if they’re not.

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 to be replaced.

How much does a boiler flue cost?

This can vary greatly depending on 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 £1,699.

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.

A vertical flue through the roof is subject to an additional charge based on flue parts and materials needed, as well as extra labour costs. Vertical flue installations may also require scaffolding to access them in many cases, and this would be an additional cost.

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Boiler flue maintenance

The boiler flue should be regularly maintained as part of the entire heating system, so you should have the whole system serviced and cleaned at least once a year.

Only Gas Safe registered engineers with flue gas analysers (machines that are 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.

You can maintain the outside space around your flue to ensure it is kept clean and that there is nothing physically blocking the flue, like shrubs or trees, but do not attempt any maintenance work on the flue yourself.

Our top tip: It’s a good idea to book your boiler’s routine service in the spring or summer. In the autumn and winter, boiler engineers tend to be far busier, as boilers are more likely to experience problems in colder weather, so you could find it trickier to get an appointment at short notice.

FAQs

A blocked flue can create a dangerous situation for your home. The primary purpose of the flue is to vent products of combustion out and away from your property. If a flue is blocked up, it could affect combustion, which in turn could cause your boiler to produce carbon monoxide. If this enters your property, it could cause a potentially fatal situation.

You must follow the emergency and safety advice given by the National Grid should you have any problems with your boiler or its flue. If you suspect something is wrong to the extent that a carbon monoxide (CO) alarm has been triggered or there is staining from products of combustion leaking out of the boiler, then you must contact the gas emergency services on 0800 111 999.

The main waste that comes out of a boiler flue is products of combustion, primarily carbon dioxide and water vapour. The gases are expelled to the outside by the flue under normal operation, and the condensation is collected by the boiler and terminated via a pipe out of the bottom of the boiler. If you have an issue with your boiler or its flue, combustion within it could be affected. In this instance, carbon monoxide can be produced, which we want to avoid at all costs.

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 products of combustion your boiler produces.

Generally speaking planning permission is not required when fitting a new flue or replacing one during a domestic boiler installation. If, for example, you live in a listed building, you should contact your local planning authority before any boiler installation work is undertaken. Likewise, if you live in a flat, you may well need permission to move the flue or fit a new one from the property's managing agents.