John Arnold knows me too well (much to his chagrin).

OK John Stephenson, to answer your code questions, look in the vent connector sizing tables in the gas code. Do you see where it lists various sizes based upon the amt. of vent rise off the draft hood? Those tables don't start at zero----they start at 12". It is also just good vent design. Check out the design handbooks from Simpson Dura-Vent and Metalbestos for the physics. Think of it as getting up a head of steam before any offsets.

B-vent is a listed product meaning it was tested in accordance with UL 441. From that test, most B-vent I've ever heard of can pass it with a 1" clearance to combustibles. How? Because of a "Dewars Flask". When you create a dead air space or Thermos bottle, the technical name is a Dewar's Flask. It is this insulated section that allows the 1" clearance. Single walled pipe is unlisted so therefore untested. In lieu of testing, the code has provided us with a default value based on generic research conducted by the Gas Research Institute under the auspices of the American Gas Assn. back when they were into design approval ( they sold that right to the Canadian Stds. Assn. or CSA, which I was dissapointed to hear did Not stand for the Confederate States of America ;-) Ol' Southern boy.....

Anyway, in a typical installation such as pictured here, the single walled pipe and draft hood are not part of a tested, listed B-vent system so by default, the code requires a 6" clearance. Now, should a mfr. test their hood and state a reduced clearance in their listed instructions then the listing of the appliance mfr. would apply over the code. Where problems come in is when the mfr. failed to use a draft hood connector and installed B-vent directly to the draft hood, which it was not intended for.

To identify B-vent, first simply look for the listing sticker. Single walled pipe does not carry a listing sticker. Next, look for the 3/8" bead rolled into the pipe at both ends of each section. Single walled pipe does not have such beads (in general. Some pipe does have it as a depth gauge so be careful). As John Arnold noted, the male end crimp is indicative of single walled pipe. B-vent uses some sort or twist or snap lock mechanism and is not crimped. Single walled pipe is smaller in diameter than B-vent. Not always readily apparent until it abuts B-vent. B-vent is made with an aluminum inner liner and galvanized steel outer casing. B-vent is made to only go male end up if you use the listed components properly. There is nothing stopping you from rigging single walled pipe male end down, which is often done to contain condensation. All it takes is a 5 blade crimper and voila'. The female end of B-vent has the two pipes evident if single walled pipe stuffed into it. The only time you should see a female end is actually the draft hood connector only. You should never see the male end. If the B-vent is properly joined, the joint will overlap sufficiently for it to seat at a ring or indentation in the pipe.

I'll get some out of the garage tomorrow and post some pics of various pipes: factory chimney, L vent, B-vent, single walled, various liners, etc.

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