24" DuraVent DuraTech Square Ceiling Support Box 7DT-CS24 This ceiling support box supports chimney pipe and provides a positive connection between support box and chimney with a built-in twist lock. Suitable for both flat and cathedral (A-Frame) ceiling applications. Fits between standard 16" on-center joists and most manufactured home applications. Supports up to 60' of DuraTech Chimney. Use adaptor to attach DVL or DuraBlack to the support box. A 2" wide trim collar is included. 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.