When fixture drains connect at different levels, how must the common vent connect?

The requirement for the common vent to connect with a vertical extension when fixture drains connect at different levels is based on the need to maintain proper venting practices. This vertical connection allows for efficient airflow to and from the drainage system, preventing potential siphoning of trap seals and ensuring that the system effectively accommodates changes in pressure that occur during drainage events.

In scenarios where the fixture drains are at different elevations, using a vertical extension for the common vent ensures that it can effectively ventilate both fixtures. This design helps to equalize pressure throughout the system, which reduces the risk of negative pressure that could lead to trap siphonage. Moreover, the vertical connection also promotes the free flow of air, which is essential for the proper function and performance of the plumbing system.

Inadequate venting can lead to issues such as slow drainage or gurgling sounds in the system, which are signs of trapped air or insufficient ventilation. Therefore, the incorporation of a vertical extension for the common vent is a crucial aspect of maintaining a functional and code-compliant drainage system when dealing with fixtures at varying heights.