Adjustable lengths telescope onto standard AmeriVent pipe sections. AmeriVent Type B Gas Vent sizes are specified by the inside pipe diameter. Model E is available in sizes 3" through 6" diameters and has a ¼" airspace between pipe walls. Model R is available in sizes 7" through 30" diameters and has a ½" airspace between pipe walls. When calculating the assembled length, allow 1" length reduction for the overlap at each joint.