Sadlier. After some long consideration and a bit of research, I can see your point. The two re-connecting Y's would have been best I suppose. I can only imagine that the guy was limited on space, and materials; as well as time. Who knows what time it was when it was installed, or what the circumstances were involving the time spent. Perhaps the installers back was to the wall. (We all know that never happens.) Had the collars been properly installed, and all tolerances complied with, and the vent through in one single length section; Sure it is possible, but is it really ethical. It has been many years since I've been in Residential work. I know it is a common practice now to place economizers in central plant flue stacks for pre heating make up water for boiler feedwater systems, as well as other means of hot flue gas heat recovery for commercial or industrial purposes. I suppose I would have had to ask myself, "Would I install it this way in my mothers house?" (I said "Mother", not "mother in law"). Thank you for giving me pause for thought.