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Understanding Boiler Flues: Everything You Need to Know

Whether you’re installing a new boiler or simply maintaining an existing system, it’s important to understand the role of a flue pipe and how it keeps your home safe and comfortable.
ecoTEC plus boiler on the kitchen wall

Definition of a boiler flue What is a Boiler Flue?

A boiler flue is an essential part of your home heating system. It is a pipe that carries exhaust gases produced by a boiler outside the home and releases them safely into the atmosphere.

Through a pipe, the combustion gases produced by your boiler get carried away. These gases, including carbon dioxide and water vapour, are a byproduct of the heating process. The flue directs these gases outside, preventing them from entering your home and ensuring your heating system works efficiently.

The boiler flue is also responsible for bringing fresh air into the system, which is crucial for the combustion process. If this process is interrupted or faulty, the boiler can’t run effectively and can pose serious risks.

TYPES OF BOILER FLUES

What Does A Boiler Flue Look Like?

Nearly all boiler systems have a boiler flue, including combi, system and regular boiler systems. Numerous regulations affect boiler flues, including their size and where they can be placed. Here’s a quick look at the most common flue pipe designs:

  • Vertical Flue

    A vertical flue expels gases directly upward through the roof. It’s typically used in homes with flat roofs or when the boiler is on an upper floor.

  • Horizontal Flue

    A horizontal flue runs through an exterior wall to vent gases directly outside. This is the most common type of flue for homes with external walls.

  • Room-Sealed System

    This sealed system brings in fresh air and vents gases through the same pipe. It’s commonly used in modern gas boilers and is especially effective in smaller spaces where air quality is important.

  • Concentric Flue

    A concentric flue combines the exhaust vent and the air intake in one pipe, making it compact and efficient. This is typically used for condensing boilers.

Where Is My Boiler Flue?

The exit point of a boiler flue depends on the building type and shape. The majority of boiler flues pass horizontally through the external wall next to the boiler. This is because it is often easier and cheaper than passing the flue vertically, which requires extra parts and work to install.

However, in some cases a boiler flue must be placed vertically and exit through the roof of the home. The flue terminal is different for horizontal installations and vertical installations and the correct one should be used.

Housing regulations in the UK require boiler flues to be placed at certain distances away from openable doors, windows and vents to prevent fumes re-entering the home. Boiler flues should also be positioned away from drains, gutters and balconies because the fumes can be hot enough to melt plastic. Flues should also be placed at least 2.1m above the ground if placed on an external wall next to a frequently used path.

Other boiler regulations include the boiler being placed 200mm below roof overhangs, 2000mm below a Velux window and 300mm above a flat roof (for a vertical flue). Refer to the specific guidelines for your boiler model in your boiler manual.

What factors affect the cost of a boiler flue?

The cost of a boiler flue depends on its shape and length. Typically a new appliance will require a new flue so you are likely to incur costs for this to be replaced. Whilst your existing flue may look and operate correctly, it may not be suitable for use with the new appliance.

Likewise, if you are forced to move your flue because of a house extension you will also incur a cost, especially if the flue has to become vertical and leave through the roof. This is because the vertical flues are more complicated to install. For example, the roof of a home is usually not easily accessible, which means a boiler engineer will need to acquire scaffolding to reach the roof. Roof tiles will often also have to be moved.

Problem solving Common Problems with Boiler Flues

Over time, you might encounter issues with your flue pipe. Here are some common problems to watch out for:

  • Blockages: Leaves, bird nests, and debris can block the flue, preventing gases from venting properly. This can cause a buildup of harmful fumes inside the home.
  • Leaks: A damaged flue can leak combustion gases back into the house. Regular checks for cracks or corrosion can help prevent this.
  • Corrosion: Exposure to the elements can cause metal flues to rust or corrode. Over time, this could weaken the structure of the flue and create leaks.

ecoTEC pro boiler in a kitchen cupboard

What happens if a boiler flue is blocked?

If your boiler flue is blocked, you should immediately turn off your boiler system and call a Gas Safe Registered (GSR) engineer to inspect your boiler.

Do not ever attempt to unblock the flue yourself. If possible, open windows and doors to ventilate the home before leaving the premises. If you suspect that the flue has been blocked for some time seek medical assistance in case you have suffered carbon monoxide poisoning.

How to Maintain Your Boiler Flue

1
Inspect Regularly

Check your flue pipe for signs of damage, such as rust, cracks, or corrosion. If you spot any issues, contact an expert.

2
Keep it Clear

It’s always a good idea to have a Gas Safe-registered engineer inspect your boiler flue during your annual boiler service. They’ll check for any blockages or leaks and ensure everything is working properly.

3
Install Carbon Monoxide Detectors

A carbon monoxide detector can alert you to any dangerous gas leaks from your flue. It’s an important safety measure for every home with a gas heating system.

4
Get a Professional Service

It’s always a good idea to have a Gas Safe-registered engineer inspect your boiler flue during your annual boiler service. They’ll check for any blockages or leaks and ensure everything is working properly.

How to Maintain Your Boiler Flue

1 / 4
1. Inspect Regularly 2. Keep it Clear 3. Install Carbon Monoxide Detectors 4. Get a Professional Service

Check your flue pipe for signs of damage, such as rust, cracks, or corrosion. If you spot any issues, contact an expert.

It’s always a good idea to have a Gas Safe-registered engineer inspect your boiler flue during your annual boiler service. They’ll check for any blockages or leaks and ensure everything is working properly.

A carbon monoxide detector can alert you to any dangerous gas leaks from your flue. It’s an important safety measure for every home with a gas heating system.

It’s always a good idea to have a Gas Safe-registered engineer inspect your boiler flue during your annual boiler service. They’ll check for any blockages or leaks and ensure everything is working properly.

Is your boiler flue blocked?

Contact your local installer

Summary

A boiler flue is a pipe that carries by-product gases generated in a boiler system outside the home.

Boiler flues are either round or square, and can be positioned horizontally or vertically.

The regulations on boiler flues are strict, and the specific dimensions for boiler flues depend on the boiler model as well. You should always refer to your boiler manual to find the right flue.

Boiler flues can become blocked by debris, causing a potential carbon monoxide leak in the home. In this situation you should turn off the boiler and call a GSR engineer to inspect the boiler as soon as possible.

FAQs

  • Can I extend my boiler flue?

    If you are carrying out an extension on your home, or suspect that your boiler flue is not placed sufficiently away from a window, vent or door, you may need to extend the flue. You should look at your boiler manual to find the recommended specifications for your boiler flue, at which point you can speak to a qualified engineer to install the correct extension.

  • Does a boiler flue have to be straight?

    Best practice is to install boiler flues that are as short and straight as possible to maximise efficiency. Nevertheless, some boiler models allow for longer boiler flues which sometimes require bends. The number of bends allowed depends on the length of flue allowed.

  • What factors affect the cost of a boiler flue?

    The cost of a boiler flue depends on its shape and length. Typically a new appliance will require a new flue so you are likely to incur costs for this to be replaced. Whilst your existing flue may look and operate correctly, it may not be suitable for use with the new appliance.

    Likewise, if you are forced to move your flue because of a house extension you will also incur a cost, especially if the flue has to become vertical and leave through the roof. This is because the vertical flues are more complicated to install. For example, the roof of a home is usually not easily accessible, which means a boiler engineer will need to acquire scaffolding to reach the roof. Roof tiles will often also have to be moved.