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. Designed to be placed within 2" x 4" stud walls (4" and 5" oval pipe). 6" may only be placed in a 2" x 6" O.C. stud space.