putting a stove(free stand wood burn) in an open all metal building. dont want to go out roof. was going to come out of back (8 in) of stove into 45 elbow thru wall and into another 45 deg straight vert up above roof peak. it seemd to me that two 45 's would give better flow than straight out of the wall into a 90 deg and up? i would greatly appreciate advice. i thought i read where 45 's are not approved by nfpa but doesnt make sence because they approve 30 deg's and 90's? again, thanks for any help.