Flues !

[A A Matthews]

I am confused about use of single & twin wall flues,4 inch diameter specifically. Watched numerous videos that show single wall vitreous from stove through boat ceiling/ roof via cast iron collar with cowl on top . So straightforward , but I have also seen single wall up to ceiling then twin wall through roof topped with cowl . So t/ w flue will prevent heat from affecting ceiling / roof structure/ materials . I also note that all combustibles need to be cleared around flue hole and void filled with rock wool or similar. 
Is it the case for new boats the recommended way is for all t / w flue in the installation?  
Can anyone shed light on this business please?

[BEngo]

There is a BS Code of Practice which advocates twin wall flue.  It is not compulsory or required by the BSS and many are sceptical of some of the claimed benefits of a tw flue.

 

If you want anything other than a dead straight flue 113mm steel chs becomes the material of choice because it can be fabricated to run parallel to the slope on the sides, have bespoke bends etc.  Twin wall flue kits have a limited range of bend pieces and lengths of pipe.

 

All flues need a roof collar.  The flue normally enters the collar from  below and is held in place with fire rope in the gap between flue and  collar and sealed with high temp silcone.  The flue should have some give so that it can expand as it gets hot.  There should be a gap round the collar, filled with non flammable insulation such as rock wool.

 

An  external chimney fits over the collar and is removable for bridges, tunnels etc.  It may be twin wall with the inner tube fitting into the flue pipe inside the collar.

 

N

[A A Matthews]

Thanks for swift response