Last year we installed a 5.5" flexible stainless steel liner in our exterior clay tile lined chimney. While the hearth opening is large, surprisingly, the 5.5" liner just fit. A 6" diameter pipe would have required extensive and permanent modifications that we were unwilling to make. Both appliances would be vertically vented with only one forty-five degree dogleg through the damper area. The chimney is twenty feet high. This year we have an opportunity to purchase either a used Hearthstone Phoenix (60,000 btus) or a Regency Classic large insert (75,000 btus). Both of these are factory equipped with 6" flues. Many contemporary stoves I've seen utilizes a 6" outlet regardless of output rating or firebox size. Because of legal considerations, I am reasonably sure that no manufacturer is going to endorse using even a slightly smaller pipe. However I am curious just what the practical consequences of doing so might be. Just off the top of my head, I suspect that if all else is reasonable, this minor reduction in pipe size would not have a dramatic effect on function or safety. While I know that there are other extraneous considerations such as being in compliance with insurance mandates, etc., I am really only interested in gaining some understanding of the likely practical and functional results of such an installation. Thanks to anyone who might respond.