Master Plumber Practice Exam

In a combination waste and vent system, the vent should be approximately equal in area to the cross-sectional area of the drain pipe being served. This is essential to ensure proper air circulation and prevent the creation of negative pressure in the system, which could lead to siphoning of water from traps. By matching the vent size to the drain pipe's cross-sectional area, the system can effectively allow for the escape of sewer gases while also equalizing pressure during drainage, thus preventing any blockages or malfunction.

This alignment in sizes is critical because a vent that is too small may not allow sufficient air flow, potentially leading to slow drainage or even backup. Conversely, if the vent were to exceed the required size significantly, it might not efficiently serve its purpose of venting. Therefore, maintaining an appropriate size in relation to the drain pipe ensures optimal performance and compliance with plumbing codes.