Interior Chimney Chase Design Check

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peakbagger

Minister of Fire
Jul 11, 2008
8,845
Northern NH
Working on the new house design and want to get the chimney chase sized right (unlike my existing house that I had to reframe through two floors a ceiling and a roof 30 years ago). Odds are its going to be two 6" double wall flues. The various tables show the OD of a 6" ID flue is 8". So if I draw a box with 2" between the pipes and 2" from the OD of the pipes to the nearest combustible wall I am set with 22" by 12" interior dimensions. The question is the pass through the various floors and roofs, do they make a two pipe transition piece that maintains the 2" clearance or do I need to space things out to whatever size the transitions plates need?

One will be for basement stove/boiler in a cape cod while the other would be on the first floor and the chase would be centered to come out of the roof next to the ridgepole. Plenty of stack height for both. Only weird part is I would run the first floor pipe down to below the basement ceiling to allow cleaning both from the basement.
 
This sounds like it will need a custom fire block between each floor that has two 8.25 or 8.5" holes in it 2" apart.
 
Bump . Must be someone else out there with code requirements, or a link in the right direction?