IMPORTANT NOTICE: All flue installation and modification work must be carried out by a Gas Safe registered engineer. This applies without exception — fitting, extending, or modifying a boiler flue without Gas Safe registration is illegal under the Gas Safety (Installation and Use) Regulations 1998 and can be life-threatening if incorrectly done. Carbon monoxide from an incorrectly flued boiler is odourless and can be fatal.
The flue system safely expels combustion gases while drawing in fresh air for the burner. Modern condensing boilers use balanced flue systems — typically concentric (tube within a tube) or twin-pipe — that draw combustion air from outside the building rather than from the room the boiler is in.
Flue Types
Concentric Flue Kits (Co-axial)
The most common flue system for UK domestic condensing boilers. A concentric flue uses two concentric tubes — the inner tube exhausts combustion gases, the outer tube draws in fresh air. The two gas streams are separated entirely, preventing contamination.
A standard concentric flue kit includes the terminal (the external end), a short horizontal run, and the connection at the boiler.
We stock concentric kits for: Worcester Bosch Greenstar, Vaillant ecoTEC, Ideal Logic, Baxi 600/800, Glow-worm Ultimate, and generic 60/100mm kits compatible with a wide range of boilers.
Twin-Pipe Flue Systems
Some boiler positions require the flue to travel vertically or over longer horizontal distances than a single concentric kit allows. Twin-pipe systems use separate 80mm pipes for the exhaust and air intake, allowing much longer runs (typically up to 10 metres total equivalent length depending on the boiler).
Vertical Flue Systems
Where horizontal flue routes are not possible, a vertical flue exits through the ceiling and roof. Additional parts include 90° bends, vertical extensions, and a pitched or flat roof weather collar.
Individual Flue Components
Flue Extensions
Available in 500mm and 1000mm lengths. Standard concentric extensions are 60/100mm for gas condensing boilers. Always check the maximum approved extension length for your boiler model — exceeding the maximum reduces flue draught and may void the boiler warranty.
Flue Elbows
45° and 90° elbows allow the flue to change direction. Each elbow adds “equivalent length” to the total flue run — typically 1 metre per 90° elbow and 0.5 metres per 45° elbow.
Flue Terminals
The terminal is the external end of the flue. It prevents rain ingress and directs exhaust gases downward and away from the building fabric. Terminals are brand and system specific — always replace with the correct terminal for the flue system.
Weather Collars and Wall Plates
Where the flue passes through an external wall, a weather collar seals the gap between the flue pipe and the masonry to prevent draughts and water ingress.
Condensate Drainage in Flue Systems
Condensing boilers produce water vapour as a byproduct of combustion. Flue components for condensing boilers are specifically designed with condensate drainage profiles to prevent condensate pooling in the flue system. Using non-condensing flue components on a condensing boiler is incorrect.
Flue Regulations
The Gas Safety (Installation and Use) Regulations 1998 require that all gas boiler flue work is carried out by a Gas Safe registered engineer. Approved Document J of the Building Regulations sets minimum distances for flue terminals from windows, doors, and ventilation openings.
A homeowner who permits unregistered gas or flue work may face prosecution and will face difficulty with building insurance claims if an incident occurs.
Need a Recommended Installer?
Flue work is a non-negotiable Gas Safe job. Our Recommended Installers cover flue replacement and extension work alongside full boiler installation.
We supply the parts. They do the fitting — safely and legally.