Metal-Fab 5M18 5" Style M B-Vent 18" Length. These light commercial and residential B-Vents feature a positive twist locking system designed for easy assembly and dis-assembly without bending or harming the product. The B-Vent Locking System comes standard on 3"- 6" pipes. The Metal-Fab 5M18 5" Style M B-Vent 18" Length is ideal for quick installation and long lasting performance. This medium sized length is ideal for connecting runs and improving the ventilation throughout the house.