“Class A” Thimble

A “Class A” thimble is also a 6 or 8 inch diameter hole in the wall, but projects several inches into the room, and has black metal trim and a smooth metal interior. It usually has a crimped metal flange for fitting the metal stovepipe. A Class A thimble is used in situations where the wall doesn’t have the required 18” of non-combustible material between the thimble and the wood framing of the house. A Class A thimble is insulated like other Class A chimney components, and therefore requires only 2” clearance to combustibles.