DuraVent - This Aluminum 18 Inch Length Round Rigid Pipe with 4 Inch Inner Diameter from the Round Gas Vent series is a Type B gas component and consists of the duralock system for a tight seal. Use for venting listed natural gas or liquid propane Category I, draft hood appliances, or appliances that have been specifically tested and listed to use Type B Gas Vent. The appliances listed typically use Type B Gas Vent systems, but not always: Furnaces, Water Heaters, Boilers, Room Heaters, Decorative Gas Appliances, and Unit Heaters. To calculate installed length, subtract 1 1/2 from each connection of vent. A 1 clearance (air space) to combustible materials must be maintained. To calculate OD add 5/8.