The Twin Wall Flue 101 Guide

This twin wall guide aims to answer some of the general questions surrounding twin wall flue, when and why it is used, how it connects together and to other parts of your flue system.

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What is twin wall flue?

Twin wall flue is chimney pipe that has a layer of insulation between two walls, this reduces the heat transfer to the outer wall of the pipe, making it a safer to humans and less of a fire risk. It is supplied in sections that are connected together. Remember it is the inner diameter that you refer to when buying twin wall flue.  

How is twin wall flue constructed?

Twin wall flue pipes generally consist of a layer of compressed rockwall insulation sandwiched between two layers of stainless steel.

How do you connect twin wall flue?

Twin wall pipes, elbows, adapters and cowls are generally slotted together and held in place with a twist lock system or locking bands.

What's the difference between twin wall flue and flexible flue liner?

Flexible flue liner is used to line an existing chimney stack, whereas twin wall flue is used where there is no chimney.

Flexible flue liner is double skinned, but differs to twin wall flue in that it doesn't have any insulation between the two layers.

When I see a diameter, is this the internal or external dimension?

When dealing with flue, the diameter generally refers to the internal dimension, as this is the diameter of the actual working part of the chimney. Twin wall will ofer have an outside diameter 50 or 60mm greater than it's internal diameter; so a 150mm (or 6 inch) twin wall pipe will generally have an outside diameter of 200 oe 210mm.

How close can twin wall flue be to combustible materials?

Most twin wall flue has a distance to combustible rating of either 50mm, 60mm or 70mm, in comparison to single wall flue that should be kept three times its diameter away from combustibles.

Can I just connect twin wall flue to my stove?

To connect twin wall flue to a wood burning stove you will either need an adapter or a starter length. Both of these sit inside the spigot of your stove (or stove pipe) and then connect to other twin wall flue pipes. A starter length is an adapter with a section of pipe incorporated into it, this cuts down on the connections close to the stove giving it a cleaner look.