In a bathroom drainage system using circuit vent branching, where should the vent connection be located?

• A. After the first fixture in the sequence
• B. Between the last fixture in the sequence and the next downstream fixture
• C. At the midpoint of the drainage run
• D. Directly to the main stack

Answer: (B)

In circuit venting, the vent must tie into the drain after the last fixture in the bathroom group and before the line continues downstream. This location vents the entire group of fixtures with a single vent, allowing air to enter as the fixtures discharge and preventing trap siphonage. If the vent were placed upstream of the last fixture, some traps in the group would not be vented properly. If it connected directly to the main stack, the vent would not effectively serve the entire bathroom run. Placing the vent at an arbitrary midpoint doesn’t reliably vent all the traps in the group. The midway-to-downstream location ensures the entire bathroom group is vented through the shared vent.