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Boxing in twin wall flue and chimney

doc j

Boxing in twin wall flue pipe - in Document J, page 24 & 25

Stove building regs pages:
Stove building regulations intro
Notice plates
CO Alarm
Installation and Building Control
Stove hearth sizes
Flue outlet heights
Flue sizes
Ventilation for stoves
Flue pipe - distance to combustibles and heat shielding
Flue pipe - recommended length
Twin wall flue - boxing in
Changing from single skin flue to twin wall at a ceiling
Lining existing chimneys
New chimneys

When do you have to box in twin wall flue pipe

When twin wall flue passes through a cupboard, storage space or roof space a guard should be placed that is not closer than the distance to combustibles for that type of flue. Often this is 50mm or 60mm but it does vary. Your twin wall flue should have a label with it which will show the distance to combustibles on it. For example, With Clearline AKC you should see a part of the label which says "G60" - this is the distance to combustibles in mm, 60mm in this instance.

The point of boxing in twin wall flue is that it keeps flammable items safely away from the flue in places where this is likely to happen. A bedroom is a prime example where toys or the contents of "floordrobe" may easily come to rest against the flue/chimney. Twin wall flue can get hot enough on the outside so that you would not want to keep your hand on it for very long but will not normally get much hotter than that so you might not think this is too much of a problem. But when you have a chimney fire then the outside can get very hot as the internal temperature rises up to 1200C. It is really this situation that you are preparing for.

In a roof space this is often done with mesh around the pipe and in bedrooms and storage spaces it is more commonly done by boxing the flue in, although some type of mesh is still an acceptable way to do it.

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