The Type B designation is assigned to gas venting systems that meet the safety requirements of Underwriters Laboratories, Inc. The AmeriVent all-metal, double-wall gas vent is listed as Type B by UL and is approved for residential and commercial applications such as Category 1 water heaters, boilers, furnaces, space heaters, or wall heaters. UL-approved Type B gas vent products form a continuous passageway from an approved gas appliance to the termination of the vent above the roof of the structure.