Vents and vent connectors shall be installed with minimum clearances from combustible material as set forth by the National Board of Fire Underwriters and by local regulating agencies. (NFPA 54, 12.11.5) See Table 188.8.131.52 Clearances for Connectors, NFPA 2015.
Type “B” gas vent material is a double-wall constructed vent with airspace between two walls. The inner wall is usually made of aluminum to resist corrosion, and the outer wall is made of galvanized steel for strength. Type “B” gas vents are round.
Type “B-W” is similar to Type “B”, except that it is oval in shape and is designed to be installed within a stud space.
Type “L” gas vent is similar to Type “B” gas vent, except that the inner pipe of Type “L” vent is stainless steel. Type “L” vents are intended for venting flue gases at higher temperatures than normally produced by gas appliances, such as oil furnaces. (NFPA 54, 12.7).
Type “C” (unlisted single wall) gas vent material used as a vent connector shall not pass through any floor, ceiling or attic and shall have 6 inch clearances to combustible material. Single wall vents designed to be used on high efficiency appliances shall be installed in accordance with manufacturer’s instructions. Listed single wall vent material shall be installed according to its listing.