In plumbing terminology, a vent serving a lavatory on the second floor is primarily for what purpose?

The correct answer is focused on the primary role of a vent in plumbing systems. A vent serving a lavatory on the second floor is primarily for allowing air flow. This is essential because plumbing vents are designed to provide a pathway for air to enter the piping system. This airflow helps to maintain atmospheric pressure within the pipes, which is crucial for proper drainage. Without sufficient air flow provided by the vent, negative pressure could occur, causing water to be siphoned out of traps. This could lead to potentially hazardous situations, including sewer gases entering the living space and impairing drainage efficiency.

While discharging waste is an important function of plumbing systems, it is primarily achieved through drains rather than vents. Preserving the seal refers to keeping the water in the traps, which is important but relies on proper venting to prevent siphoning. Preventing backflow is a critical function, but it is more related to system design elements such as check valves and backwater valves rather than the specific role of vents. Therefore, the correct answer underscores the importance of air flow in ensuring that plumbing systems operate effectively and safely.