in another thred Don said something about chimney height increasing for each use of an elbow. does this include a 90 elbow used on a rear exit? I am installing a jotul 602 and have been trying to decide between a top and rear exit. total height of stove pipe and class a will be around 13 ft, and with the rear exit I would use doublewall. is this sufficent? don, can you explain again the requirements associated with elbows/chimney height? thanks.