Twin wall flue size guide

1. Pipe and elbow diameters

With twin wall flue pipe, the internal and external diameters are different. This is because they have 1 inch of insulation surrounding the inner pipe that makes the outer pipe 2 inches larger than the inner pipe. The diameters referred to always relate to the INTERNAL diameter of the pipes it is this diameter that is important when connecting your flue pipe to a stove.

2. Brackets and finishing plates

Due to the reasons above all brackets and plates that are cut to fit twin wall pipe will actually measure 2 inches more than their stated diameter.

Stated DiameterI = Internal DiamaterE = External DiameterBrackets & plate Diameter
100mm (4″)100mm150mm150mm
125mm (5″)125mm175mm175mm
150mm (6″)150mm200mm200mm
175mm (7″)175mm225mm225mm
200mm (8″)200mm250mm250mm

Other useful information regarding twin wall flue systems

  • Your flue system (internal diameter) must never be less than the flue collar diameter on your stove and the stated flue requirements of that stove.
  • You can increase but never decrease the diameter of your flue system.
  • For that reason we do not sell reducers or decreasing components.
  • You should have a maximum of 4 elbows in your system.
  • The maximum angle of a bend permitted is 45 degrees other than coming straight out the back of your stove.
  • You should aim for an overall height of at least 4.5 meters from the top of your stove for the flue to work well.
  • The outlet or terminal of your flue system needs to finish at least 1000mm above your roof or 2,300mm horizontally from your roof, or 600mm above the ridge of your roof, whichever comes first.
  • Need help planning your system? Call us on 01376 315 760 to speak to our HETAS approved team of specialists who can plan your building regulation compliant system with you.
Free delivery

On all orders over £125 (ex. VAT)

Easy 30 day returns

30 days money back guarantee

Lifetime warranty

On all Flexible Flue Liners

100% secure checkout

PayPal / MasterCard / Visa