Just emailed Breckwell (P23 insert) directly for their take, but figured I'd ask the experts here and probably get a better answer, faster! See photo and tell me if this should be fine, or no way Jose. 3" elbow with 3" to 4" increaser. Steel, but not stainless. I'll be venting 4" diameter straight up a masonry chimney with approximately 25ft of 316 flex. Yes, new clamps will be used at all connections.