chimney elbow effect

[snowtime]

I understand a 90 degree elbow effects the draft considerably. What I want to know is how much 15 and 30 degree elbows effect draft. This will be a class A elbow situation in the attic. I will have a 18' chimney for a T6. I think I can just get away with a 15 degree but if a 30 is not much worse than I will go with it.

 

[webbie]

In my opinion a straight up chimney through the attic which is round and insulated will not be affected too much by either 15 or 30. I would not hesitate to use them if I was doing the job for myself.