Our non-positional lock system on the Duravent® BV Gas Vent is the best lock system on the market today. The incorporation of the DuraTab® feature simply improves our lock system by adding flexibility and meeting market needs. The BV line of Duravent round vent pipe and fittings is designed for use on approved gas appliances in residential and light commercial and industrial applications. What's Included (1) Large Diameter Increaser Features The increaser is used to increase the vent size. The maximum increase is 4 sizes. Specifications Item Number 10BVX16-1 Series Round Type B Gas Vent Model BV (3" - 30") Product Type Increaser Inside Wall Aluminum Outside Wall Galvanized Steel Size (Diameter) 10" x 16" Length 7 3/4”