Twin wall flue pipe installation guide

If you are installing a wood burning stove, boiler or gas appliance and you are planning to run your flue system either through a ceiling, a floor or externally you will need to use an insulated Twin Wall flue system.

Twin Wall flue features a 1 inch layer of insulation around the pipe which means the external diameter of Twin Wall pipe is always 2 inches larger than the internal diameter. Please bear in mind, all flue pipes are quoted by their internal diameter as this measurement is the crucial figure when designing a wood burning stove flue system. We just like to make sure our customers are aware of this as if you purchase a 5 inch diameter twin wall pipe the external diameter will be 7 inches - a big difference if you were not expecting it!

We highly advise starting the flue system with at least one length of Single Wall enamelled stove pipe (maximum 1.5m) before changing to Twin Wall but you certainly need to convert at least 6 inches (150mm) before passing through a wall or ceiling. Connecting Single Wall pipe to Twin Wall pipe is easy with the correct Adapter, selected from the same range/brand and diameter as the Twin Wall being used. Adapters can either maintain the internal diameter of the system (eg. 5 inch to 5 inch) or can increase the internal diameter (eg. 5 inch Single Wall to 6 inch Twin Wall). Once you have converted you must continue the flue system in Twin Wall.

When creating a turn in your system to pass through a wall or offset around an obstacle, you must never exceed a maximum angle of 45 degrees. 90 degree bends (or right angles) are not permitted in solid fuel flue systems as they prevent the hot gases from rising. Use a Wall Sleeve to protect your wall and finish the job off with Finishing Plates inside and out.

If you are passing through an internal floor you will need to use Ventilated Fire Stop Plates and remain at least 50mm away from combustible materials like joists. These plates are an important safety feature of an internal flue system, as they allow hot air to escape from the joists and in the event of a fire downstairs, the plate delays the spread of fire up through the property. Please note though, different brands of Twin Wall have different safe distances to combustible materials but these are typically either 50 or 60mm.

When passing through a roof, you should use a weather-proofing Flashing and a Storm Collar for added protection against the elements. Flashings are available for both tiled and non-tiled roofs and can be low (black) or high (silver/orange) temperature resistant. If you are less than 5m away from your stove, you need to use the high temperature Flashing.

Finally, terminate the system with a choice of Rain Caps; open-sided, with birdguard or Anti-Wind.

At Glowing Embers we can provide you with a wealth of experience and advice when planning your twin wall system or installing a wood burning stove, you can call us on 01376 804649, email us at [email protected] or comment on this post and we would be pleased to assist you further.

1st June 2013

 

Comments

Neil Crowe says:
26th December 2021 at 1:20 am
Hi, are joints in twin wall flues allowed within the cavity of the building walls ? Does the wall sleeve outside edge need to be sealed at each end to the wall it is passing through ?

Andrew Shuttleworth says:
31st December 2021 at 12:23 pm
Joints between all flue pipes must be accessible so they cannot be hidden within the cavity of a wall or joist I’m afraid. The outisde edge of the wall sleeve doesn’t necessarily need to be sealed but each installer will have their preferred method of installing these to ensure they are water tight (within reason).

Perry Masters says:
18th October 2022 at 7:02 pm
Why can’t you connect twin wall flue directly into your log burner?

Andrew Shuttleworth says:
21st October 2022 at 8:05 am
You can, however the lower sections of the Twinwall can discolour if stainless or the powder coating crack if black due to the high temperatures directly above the stove. You would still need to use a single to Twinwall adaptor from the stoves outlet to the Twinwall.

Andrew Moseley says:
23rd October 2022 at 12:51 pm
Hi When passing twin wall through a warm pitched roof make up. Can I but my PIR insulation directly up to the twin wall or is PIR classed as combustible? If so do people fill the 50 mil gap with rockwool. Thanks

Andrew Shuttleworth says:
25th October 2022 at 9:56 am
Thank you for your enquiry with us today, PIR is classed as a combustible and therefore we would advise that you allow for a 50-60mm (50mm for SFL and 60mm for Shieldmaster) air gap around the Twinwall. You do not need to surround this with anything as the hole surrounding would be hidden by the finishing plate and roof support on the inside. Externally the flashing base would cover the hole required around the pipe.