What do you think about this vent connector for nat gas hot water tank. It doesnt look like a B vent, it looks like a single wall. Plumber has sticker on tank installed last year. Thanks 
Well, it looks jerry-rigged with many parts…some single wall and some double wall.
Looks like a mixed bag to me also. Ask him if he would do it that way in his own home. That’s my new favorite question to realtors and contractors alike. Love the reaction I get with it.
The 2nd section from the bottom looks like dryer duct. The rest…Lowes must have had a sale on 6" sections.
Actually the draft hood connector “3RDH” is properly installed. Hart & Cooley and other manufacture’s have simplified the transition between single wall and double wall connections. See link http://www.hartandcooley.com/vent/vent%20catalog/010051%20Gas%20Vent%20&%20Chimney%20Systems%20catalog%20-%20Edition%204.pdf page 5 bottom right. Generally this is connected directly to the draft hood. It appears that there maybe a single wall pipe connection further up in the picture, and that is not allowed. Once you start with the double wall pipe it would need to continue to termination above the roof.
Gotta agree with you Gerald. I don’t see any single wall vent after the single to double wall adapter. the screws in the b-vent are not normally required but sometimes we need them and are acceptable if they are less than 1/4 inch so as not to penetrate the interior aluminum liner of the b-vent. I see nothing to report.