PelletVent Pro pipe features fully unitized twist-lock construction in 3" and 4" diameters. Listed at 1" clearance to combustibles, in the USA, it can be used in both vertical and horizontal installations. Made from 304 stainless steel welded seam inner wall, combined with a galvalume welded seam outer wall for wood pellet, corn, oil fuel, and biofuel burning appliances. Pipe OD is diameter + 5/8"