Brian, every B-vent mfr sells a "draft hood adapter or connector" which is your transition piece from single walled pipe to B-vent.

Do you have a report on this black foam as having passed ASTM E-136, which is a 1382F vertical tube furnace? This test utterly destroys type X sheetrock and even Wonderboard. Not sure where you're getting your info. but I can assure you foam insulation indeed is considered combustible.

You just made the point why you don't screw through B-vent into single walled pipe. That's why you use single walled pipe only below B-vent into a draft hood connector, which is made to be screwed into the bottom part while the top uses a std. Bvent twist or snap lock depending upon mfr. You also just explained why almost every HVAC tech installs B-vent improperly---The supply houses don't sell them the proper connector.

Once you are running B-vent, you cannot revert back to single walled pipe in mid run, esp. at a firestop. Also, single walled adjustable elbows are notorious for blowing apart and must be properly supported. Explain to be how you transition from B-vent to single wall back to B-vent while still maintaining the listing all in one run.

There are two single walled pipes in these pics: the adjustable ell at the firestop and the one at the draft hood.



Bob

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disgusted with some people on this forum. Out of here!