PelletVent Pro 3″ Pellet Chimney Cathedral Ceiling Support Box This 3 Inch Pellet Chimney Cathedral Ceiling Support Box from the PelletVent Pro series is a Type L Biofuel Chimney Pipe component and consists of the duralock system for a tight seal. Use to support pipe. May be used on flat ceilings. Painted black. Includes: Trim and clamp. Features: Type: Cathedral Ceiling Support Box. Material: Stainless Steel. Collection: PelletVent Pro. Specifications: Overall Dimensions: 14.25″ (L) x 14.25″ (W) .