Quote Originally Posted by cvcman View Post
Nope, its single wall galvinized just like the oil furnaces and the older gas funaces, screwed together every joint, yup just reg single wall galv !
Oh yes, I do see this in the mech room sometimes, you are right. Just before it goes into the ceiling it is B-Vent though, usually looks like it gets larger, 5" pipe typically. Or it might dump right into a masonry flue.