How to create air gap by joist for flue?

[^woody^]

I don't think that's right. Flue manufactureres give the minimum distance between the flue and the combustible material. They don't expect the user to extrapolate to the inner wall of a twin wall flue.

It is right. It's from the flue, which is the void in the centre of the pipe, not the casing which is not "the flue".

With twin wall or insualted pipes, the manufacturer will allow for the thickness of whatever is around the flue.

 

[geraldthehamster]

It is right. It's from the flue, which is the void in the centre of the pipe, not the casing which is not "the flue".

With twin wall or insualted pipes, the manufacturer will allow for the thickness of whatever is around the flue.

Sorry, but this is an important point. The clearance is to the outside of the twin wall. Here is an example, with accompanying diagrams:

"Building Regulations as well as the approval procedure to which all prefabricated system chimneys are required to be certificated, dictate that an adequate air gap clearance MUST be maintained between the outside skin of the chimney and any combustible material. Clearances becomes more crucial if the chimney is enclosed. The increase in temperature within any enclosure must be dissipated for safety reasons. The approval process for the Selkirk STC Chimney system requires the air gap distance to be not less than 50mm."

http://selkirkchimney.co.uk/-/media...himney/twin-wall-chimney/generalguideline.pdf

 

[^woody^]

Sorry, but this is an important point. The clearance is to the outside of the twin wall. Here is an example, with accompanying diagrams:

"Building Regulations as well as the approval procedure to which all prefabricated system chimneys are required to be certificated, dictate that an adequate air gap clearance MUST be maintained between the outside skin of the chimney and any combustible material. Clearances becomes more crucial if the chimney is enclosed. The increase in temperature within any enclosure must be dissipated for safety reasons. The approval process for the Selkirk STC Chimney system requires the air gap distance to be not less than 50mm."

http://selkirkchimney.co.uk/-/media...himney/twin-wall-chimney/generalguideline.pdf

The manufacturer's details always take precedence over any Buiding Regulations. Each individual manufacturer will specify their own guidelines, and these are just that, guidelines. There are almost always alternative ways to acheive the same result.

So in the case you have quoted, unless you are fitting a Selkirk product, their guides are irelevant.

And I could link you to many more manufacturers or BS standards that measure from the inner flue void, or base a calculation on the actual flue diameter or area and not the lining, but you can find those yorself if you are interested.

 

[geraldthehamster]

I could link you to many more manufacturers or BS standards that measure from the inner flue void

Please do. I'm happy to be educated but I think you're mistaken on this one. It's not the case with the two installations I've been involved with. Here's another, from a quick Google:

"Distance to combustibles 50mm ... A COMMON CAUSE OF CHIMNEY RELATED FIRES IS THE FAILURE TO MAINTAIN THE REQUIRED AIR GAP BETWEEN THE OUTER SURFACE OF THE FLUE AND COMBUSTIBLE MATERIALS. IT IS VITAL THAT THIS FLUE PIPE SHOULD BE INSTALLED WITH THE CORRECT MINIMUM CLEARANCES AS SPECIFIED IN THESE INSTRUCTIONS."

(Clearline AKW Twin Wall Installation Instructions)

https://www.firepowerheating.co.uk/...lation-instructions-and-maintenance-guide.pdf