Boiler flues protect the occupants of a building from hazardous fumes including deadly carbon monoxide. Gas flues are a standard part of a professional boiler installation and can take gases out through the wall (horizontal flues) or roof (vertical flues). 

In this guide, we’ll look at some common questions, so if you’re asking yourself “What is a flue?” just read on for everything you need to know.

What is a boiler flue?

A boiler flue is a length of pipe connecting the exhaust of your regular gas boiler, combi boiler or system boiler with a safe outdoor position where potentially harmful gases can be released – including carbon monoxide, nitrogen oxides, CO2 and water vapour.

What’s the difference between horizontal flues and vertical flues?

There are two main boiler flue types depending on the position of the boiler inside your property. These two types of flue are horizontal flues and vertical flues, and relate to whether the flue pipe leaves your property through a wall or roof.

Horizontal flue

Horizontal flues are the most common flue type and usually need the boiler to be mounted on the inside of an exterior wall. Horizontal flue regulations include considerations such as the position of the exterior flue terminal, which should be out of reach of passers-by if possible.

Vertical flue

You can also run a vertical boiler flue through roof materials to the open air. This can allow gas boilers to be mounted on interior walls and may be convenient if your boiler is upstairs, or if the boiler itself is in the loft space.

How do I know if I have a flue?

If you know what a flue looks like, you should be able to spot it quite easily as the large exhaust pipe coming out of your boiler. Another option is to look on your outside wall near to where your boiler is, and see if you can find a fairly large pipe coming out of the building – this is the flue terminal. On a cold day, you’re likely to see steam coming from this pipe when your boiler is running.

Do all boilers need a flue pipe? 

Most common boiler types need a flue to get rid of exhaust gases. This includes regular gas boilers, combi boilers and system boilers, but not electric boilers (see below). When it comes to choosing a flue pipe boiler manufacturers usually have a complete kit available, so you get everything you need at a single price. You can find these on our Boilers & Accessories page.

Where should a boiler flue go?

Your boiler flue should take exhaust gases out of your property through a nearby external wall – usually the wall the boiler is mounted on. Where possible, horizontal flue pipes should exit at more than 2m above ground level to keep hot surfaces out of reach. In some circumstances, a vertical flue pipe can be used to create a roof flue and carry gases up and out instead. 

What is a flue terminal guard?

A boiler flue terminal guard can help to reduce the risk of a blocked flue, but is also used to improve safety when the outdoor flue terminal is less than 2m above ground level. Flue terminals can get very hot due to the exhaust gases from the boiler, and a flue terminal guard provides a physical barrier so nobody can accidentally or deliberately touch the hot surface.

Why isn’t a flue needed for electric boilers?

Electric boilers use an element to heat the water, similar to an electric kettle. No gas or other fuel is burned, so there are no harmful gases to remove from your property. Because of this, electric boilers don’t need a flue, and don’t need to be placed against an external wall either.

How much do boiler flues cost?

Boiler flue kits can cost around £80-100, depending on the brand and the size of the kit. Some may cost more – you can see all the prices on our Boiler Flues & Accessories page.

If you don’t need a full kit, you can get individual parts at lower prices. For example, a Worcester Bosch flue terminal outlet bought separately can cost less than £15, whereas a complete Worcester Bosch telescopic flue with terminal, turret elbow and wall plate is around £72. 

How do you know if a flue is blocked? 

Flue guards can help to prevent debris from getting into the flue pipe and causing a blockage, so check for signs of anything stuck in your boiler flue terminal guard for an immediate first sign of trouble.

 A blocked flue can lead to harmful gases accumulating inside your home, ranging from water vapour and carbon dioxide, to nitrogen oxides and deadly carbon monoxide. A carbon monoxide alarm (also known as a CO alarm) can alert you if levels inside your home increase.

Boiler flue regulations explained

A correctly installed boiler should not be dangerous, but they must be treated with respect. Your boiler may have a mains gas supply in addition to a mains electricity connection, very hot water output and hot, potentially harmful exhaust gases. For all these reasons, boiler flue regulations are quite strict in some respects.

Ideally, your outdoor flue terminal should be out of easy reach, and away from neighbours’ windows and public spaces like car parks. If you’re not sure about where to position a boiler flue or whether you need a horizontal or vertical flue, always ask an expert.