4" X 18" Round Rigid Type B Gas Vent Pipe 4GV18 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 1â„2†from each connection of vent. A 1†clearance (air space) to combustible materials must be maintained. To calculate OD add 5â„8â€. For Category 1 gas appliances 1" clearance to combustibles Easy-fit twist-lock design with no fitting hassles 0.012" aluminum inner 0.018" galvanized outer Outer diameter is 5/8" larger than inner diameter