Dura-Vents UL Listed Type B Gas Vent. It has a twist-lock design that is fast and easy to install or disassemble all with no fitting hassles. Dura-Vent Type B Gas Vent can be used to vent Category I gas appliances or whenever R-Vent is specified on the appliance label. It requires 1” clearance to combustible. The inner liner is 012in aluminum and the outer wall is premium grade 018in galvanized steel.