Was talking with someone the other day that changed out a prefab to insert. They had double wall pipe already in 6 inch pipe inside of a i'm guessing a maybe a 11 inch pipe with no insulation. Removed 6 inch pipe and put in 6inch double wall stainless inside of the bigger pipe (11inch) now this 6 inch double is ran up close to top of 11 inch pipe which has rain cap on it. Would this get efficient draft and would it be safe?