I'm finally finishing up my wood furnace installation and have a question on the black pipe. I have about 17" clearance between the joists and the chimney pipe, so I bought an 8" double wall 90 degree elbow to stay within the required safe distance. This elbow will connect to the class A to smoke pipe adapter, but I don't know how to connect the other end. I wanted to join it to a single wall pipe running the rest of the way down to the furnace, but I'm not sure that's possible because of the way the other end of the elbow is designed. It has a smaller crimped end about 7" diameter which is surrounded by the outer pipe and will not join on to either end of a regular 8" single wall pipe. I wish I could describe it better but it's hard to explain without pictures. I'm using Supervent pipe...is there some kind of adapter I need to connect the two or do I have to run double wall all the way to the furnace?