Ok it is conceded that Grade 8 steel is much harder than 18-8 SS. That still does not answer my question. Does a vent liner need to made from a steel as hard as grade #8 steel just because it is stronger than 18-8 SS? If it is being implied that vent liners should be made out of grade #8 steel instead of 18-8 SS just because grade #8 steel is harder than 18-8 SS is there a steel harder than grade #8? Shouldn't a vent liner be made out of a steel stronger than grade #8 if there is one? Where does the equation end? When is strong enough, strong enough? I am absolutely not taking sides in this I am just being curious and have really learned a lot from this thread.