6" x 18" Duravent DuraTech Factory-Built Double-Wall Galvalume Chimney Pipe 6DT-18 This chimney pipe is designed for appliances that are tested and listed for use with a 2100 °F HT chimney system such as wood stoves fireplaces furnaces boilers ranges water heaters or other appliances fueled by wood oil coal or gas. Approved for most fuel types 430-alloy stainless steel inner wall 2" clearance to combustibles ThermalTech blanket insulation eliminates hot spots Twist-lock connections for strong secure fits Able to vent large fireplaces and building heating appliances Clean smooth exterior finish 10â€-24†diameter pipes feature an .027†430 stainless steel inner wall and an outer wall of .029†galvalume or .027†430 stainless steel. A Refractory insulation is encased between walls. Only stainless pipes use the stainless end rings. Regular galvalume pipe uses galvalume end rings.