4" x 12" Pelletvent Pro Pipe, 304-alloy Stainless Inner Liner, Black OuterFeatures fully unitized, twist-lock construction in 3" and 4" diameters. Listed at 1" clearance to combustibles in the USA and Canada. It can be used in both vertical and horizontal installations. Stainless steel welded seam inner wall, combined with a galvalume welded seam outer wall. Pipe OD is diameter plus 518". Galvalume or painted black outer finish. 12" length available with stainless steel outer 55. To calculate installed length, subtract 1 %" from each length of pipe.