Which plumbing component helps in preventing negative pressure in drainage systems?

The vent stack is crucial in preventing negative pressure in drainage systems. Its primary function is to allow air to enter the plumbing system, which helps maintain air pressure and ensures that water can flow smoothly through the pipes. Negative pressure, or siphoning, can occur when drainage systems are operating, potentially causing slow drainage or even creating a vacuum that disrupts the flow of wastewater. The vent stack provides a pathway for air to balance the pressure within the drainage system, helping to prevent these issues.

In contrast, the other components have distinct roles. The trap retains a small amount of water to provide a seal against sewer gases, but it does not directly equalize pressures. A septic system is designed for the treatment and disposal of sewage on-site but is not directly involved with pressure dynamics in the drainage pipes. A sanitary sewer ultimately carries wastewater away from buildings but does not play a role in pressure management within the plumbing system. Thus, the vent stack is essential for maintaining the necessary pressure conditions for efficient drainage.