How many fixture drains may be connected to a common vent?

In plumbing, the concept of a common vent is important for maintaining proper air circulation and drainage within a system. A common vent is utilized to ventilate multiple fixture drains, preventing issues like siphoning and backpressure. The code typically allows for a specific number of fixtures to be connected to a single common vent to ensure effective operation.

The correct answer indicates that a common vent may accommodate two fixture drains. This limitation exists to ensure that the vent remains effective in equalizing pressure in the drain system and providing adequate airflow to prevent drainage problems. While more than two fixtures might theoretically be connected to a vent, doing so could lead to malfunction, such as poor drainage or foul odors from the sewer system infiltrating into the building.

In comparison, connecting one fixture to a common vent would be underutilizing the vent system. Allowing three or more fixtures could exceed the capacity and efficiency of the vent, creating the risk of drainage issues. Therefore, the regulation permitting two fixtures to connect to a common vent strikes a balance to maintain the plumbing system's intended functionality and efficiency.