Global Protankless Supply is an Authorized Distributor of all Z-Flex Z-Vent stainless steel venting. We provide stainless steel venting in diameters of 3" all the way up to 24". The Z-Flex Z-Vent stainless steel (AL29-4C) venting comes in either single wall or double wall venting. Dimensions on the double wall venting is different than the single wall venting. All Z-Flex Z-Vent double wall venting has an inside diameter dimension and an outside diameter dimension. As an example a piece of Z-Flex Z-Vent double wall stainless steel venting pipe 4" diameter by 4' long would have an inside diameter of 4" and an outside diameter of 6" due to the double wall.