6" DuraVent DuraPlus 24" Square Ceiling Support Box 6DP-CS24 This square ceiling support box is used with flat vaulted or cathedral ceilings. Supports up to 35' of DuraPlus chimney. Starter section built-in. Trim included. Use when air-cooled chimney is required Free-floating inner 430-alloy liner maintains structural integrity in chimney fires 2" clearance to combustibles Outer diameter is 4" larger than inner diameter Triple-wall chimney For uses with wood stoves fireplaces furnaces boilers stoves ranges water heaters or other appliances fueled by wood oil coal or gas 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.