I am new to this forum- just signed on because I can't find an answer anwhere else. I am replacing an old pre-fab fireplace ("Majestic") with a Napoleoan 1100 woodstove. The fireplace flue is 10"; the Napoleon is 6". My major issue is the chimney- it has an interior diameter of 10", and the exterior looks to be about 18"! What kind of a chimney is this? and can I use it safely?