The Underwriters Laboratories (UL) Master labels designate gas vents as Type B, Type BW or Type BH, according to its website. Individually each vent consists of a support assembly, one or more sections and firestop spacers. Additional fittings include elbows and draft hood adapters.
Type B Gas Vents
Used to vent equipment that is gas fired and gas appliances with draft hoods, Type B gas vents may be installed in single or multiple story buildings. UL signifies approval of gas appliances that are appropriate for Type B vents. Type B gas vents are not intended for use with vented wall furnaces certified for Type BW gas vents, gas-oil combination appliances, appliances required vented with a chimney or appliances easily converted to the use of liquid of solid fuels and incinerators.
Type BW Gas Vents
Recessed heaters with input no greater than the specified capacity of the Type BW gas vents are approved for use by the UL. Install Type BW gas vents with a sole header plate to serve as a firestop, according to the UL website. As with Type B gas vents, Type BW gas vents may be installed in multiple or single story buildings. Attach a Type BW gas vent to a chimney or a Type B gas vent to vent through the roof.
Type BH Gas Vents
Use Type BH gas vents only for positive or negative pressure venting gas appliances producing flue gasses at Class I or Class II temperatures. Class I venting systems temperatures are defined as more than 135 degrees Celsius but less than 245 degrees Celsius. Class II venting systems temperatures are defined as 135 degrees Celsius or less and classified into four different ratings as follows: A equals temperatures up to and including 65 degrees Celsius; B equals temperatures up to and including 90 degrees Celsius; C equals temperatures up to and including 110 degrees Celsius and D equals temperatures up to and including 135 degrees Celsius.