The minimum diameter of a vent serving multiple fixtures should be based on what criterion?

The minimum diameter of a vent serving multiple fixtures is primarily determined by the number of fixtures being served. Each fixture has a specific requirement for venting to maintain proper air circulation and to prevent the siphoning of water from traps, which could lead to the backflow of sewer gases into the living space.

When multiple fixtures are connected to a single vent, it is crucial to ensure that the vent diameter is adequate to accommodate the cumulative drainage load from all the fixtures. As the number of fixtures increases, so too does the need for a larger vent diameter to allow for efficient airflow. This ensures that each fixture remains properly vented, reducing the risk of potential drainage issues and maintaining the integrity of the plumbing system.

The other criteria, such as vent length, fixture type, or pipe temperature, may influence specific aspects of vent design or function, but they do not take precedence over the number of fixtures when determining the minimum vent diameter. Therefore, assessing the number of fixtures served is essential for ensuring compliance with plumbing codes and safe plumbing practices.