Inner and outer pipe The outer pipe, made of galvanized steel, provides cooling air necessary to maintain safe chimney temperatures, while the inner pipe, made of stainless steel, emits gases out of the fireplace Snap- lock design 12 standard pipe (12inner diameter, 15 3/8 outer diameter) UL127 Listed 1700 Degree Pipe 12 Support Pipe (For Systems Over 30") Lgth. In=12