When is it permissible to share a vent between two fixtures?

Sharing a vent between two fixtures is permissible when the fixtures are located on the same floor and the system is properly designed. This is due to the need to maintain proper air circulation and ventilation within the drainage system, which prevents clogs and siphoning of traps that could result from inadequate venting.

When fixtures are on the same floor, it is typically easier to design a venting system that accommodates both pieces of plumbing effectively. The plumbing codes allow for this shared venting because it simplifies the plumbing layout while ensuring that both fixtures can adequately vent without causing pressure imbalances in the system. Proper design considerations must include aspects like the size of the vent pipe, the distance between fixtures, and the overall plumbing layout to ensure that venting functions effectively.

In contrast, if fixtures are in different rooms, this may complicate the venting requirements, leading to potential issues with drainage and venting compliance. Additionally, sharing a vent based solely on the simultaneous use of fixtures or their designation as water-saving devices does not inherently address the fundamental requirements of proper venting design as dictated by plumbing codes. Thus, the proper conditions and design are critical for effectively sharing a vent between fixtures.