DuraBlack 8″ Diameter Slip Increaser 6″-8″ The DuraBlack 6″ – 8″ Single Wall Stovepipe Slip Increaser is a component that is used in a typical venting installation. Use to adapt from 6″ DuraBlack to 8″ DuraTech, DuraPlus, DuraPlus HTC, or DuraLiner at the ceiling support box or finishing collar. Adds 5″ – 15″ of length. Features: Type: Increaser. Used in a Typical Venting Installation. Use to Adapt from 6″ DuraBlack to 8″ DuraTech, DuraPlus or DuraLiner at the Ceiling Support Box. UL Listed. Specifications: Overall Dimensions: 16″ (H). Inner Diameter: 8″. Adds 5″- 15″ of Length.