What is the purpose of an extension on a stack vent?

The purpose of an extension on a stack vent is to provide venting above the highest horizontal drain. This is crucial for proper venting in plumbing systems, as the vent allows air to enter the drainage system, which helps maintain neutral air pressure within the pipes. By extending the stack vent above the highest horizontal drain, it ensures that any gases produced in the drainage system can escape safely and that the flow of wastewater is unhindered.

When a vent is adequately positioned and extends above the highest point of drainage, it helps prevent potential siphoning of water from traps in plumbing fixtures. This venting process plays a significant role in preventing sewer gases from entering the building and maintaining efficient drainage.

The other aspects mentioned in the choices, such as increasing the capacity of waste drainage or connecting multiple drains, relate to different functionalities of a plumbing system but do not specifically describe the primary role of a stack vent extension. The management of overflow from rainwater also does not pertain to the function of a stack vent, as its purpose is strictly for venting and air pressure management within the waste drainage system.