This flu pipe was for an attic furnace. I know B vents typically need 1" minimum to combustibles. My question is with the adapter that its going into. Does this lower the minimum clearance needed. It was about 1’2 inch from plywood, also it had shingles and tar paper touching it. There was no label on any of these pipes, so im not entirely sure what to make of it. Thanks for the help
The single wall B vent needs 6" to combustibles and the double wall B vent needs 1" to combustibles. Is there a thimble at the floor/ceiling?
Got a tape measure? 
Lol, Larry I do have a tape measure, I didnt have it in the attic though. So the main pipe is single wall and the larger pipe is considered double? The plywood was about a 1/2 " away and the shingles which were touching it I was under the impression are still considered combustible? No thimble just a hole cut through the sheathing then up into the vent stack on the roof
Drew, recommend a qualified HVAC tech correct the vent installation for safety. and yes, shingles are combustible. And one can not use single wall B vent in the attic. 
Got it. Larry to the rescue again I’m putting you on my Hallmark list the Christmas 