What is a boiler flue and how do I find it?

A boiler flue is the passage that carries gases produced when a boiler burns fuel safely outside your home. It is separate from the pipe that supplies gas to the boiler. On many modern gas boilers, the flue also forms part of the system that draws in air for combustion.

Where can I find my boiler flue?

Look outside near the room where your boiler is installed. A horizontal flue usually ends at an external wall; a vertical flue ends above the roof. Its location and appearance depend on the boiler and the route chosen during installation. You do not need to open the boiler or touch the flue to look for its outlet.

If you cannot identify it, check your boiler’s installation paperwork or ask a Gas Safe registered engineer during a service. An older back boiler may use a chimney or another flue arrangement rather than a terminal that looks like one on a modern combi boiler.

Is a flue the same as a gas pipe?

No. The gas supply pipe brings fuel to the appliance. The flue carries combustion products away after the fuel is burned. You cannot judge whether the gas supply is correctly sized from the shape of a flue or by assuming every boiler needs the same pipe diameter. An engineer checks the installation against the appliance requirements and the gas pressure available when it is running.

Why does the flue position matter?

Combustion gases must discharge safely outside. The required position and clearances depend on the appliance, flue system and surroundings, including nearby openings and boundaries. An installer should use the instructions for the exact boiler and flue system rather than a general distance quoted in an article.

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.

What if I think the flue is blocked or damaged?

Stop using the boiler and arrange an inspection by a Gas Safe registered engineer. Do not remove, extend or repair the flue yourself.

If a carbon monoxide alarm sounds, you suspect carbon monoxide, or you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999 in Great Britain. Follow the emergency service’s instructions. Gas Safe Register’s safety guidance explains the emergency steps.