Metal-Fab - 4MO45F - 4" Oval B-Vent 45 Flat Fixed Angle. Metal-Fab's Round and Oval Type-B Vent Is Listed By Underwriters Laboratories For Use with Approved Category 1 Gas Furnaces, Heaters, and Boilers. A Category 1 Appliance Operates with A Non-Positive Vent Static Pressure. The Double Wall Design Of B-Vent Enables The Product To Be Used In Homes and Commercial Buildings At Close Clearance To Combustible.