Well the subject line tells all. Trying to help a member with his chimney set up, it looks like there is about a 3' run. I dont remember the deduction for that run. Also, in wood venting, the first 90* in the system doenst count against you, the second counts as 5' of deduction. Does the reversed flue collar count as the free one? or is the T going up the chimney count as the first one? The point im getting at, lets say you have a 15 foot chimney. If you take 1 foot off per foot of horizontal run, that would equal a 12 foot chimney, then the second 90* (if it applies in this situation) would deduct 5 more. So now your 15' chimney acts like a 7' chimney. So the unknown in my book is the penalty for the horizontal run assuming that you have a 1/4" of rise per foot.