Which statement about circuit vent branching is most accurate?

Which statement about circuit vent branching is most accurate?

• A. Vents must connect to the drainage line immediately after the first fixture.
• B. Vents should be positioned between the last fixture in the sequence and the next downstream fixture.
• C. Vents are never required in sequence of lavatories.
• D. Vents can be eliminated if drains are short.

Answer: (B)

Circuit venting protects a group of fixtures on a single drainage run by allowing air to enter the line as wastewater flows, preventing traps from siphoning. The vent is placed downstream of the last trap in that run, so the air can relieve negative pressure for all the upstream fixtures before the line continues to the next downstream section. That way, all traps in the circuit stay protected. If the vent were located right after the first fixture, the remaining fixtures on the same run wouldn’t be reliably vented—the negative pressure generated by their discharges could siphon their traps before air can reach them. Lavatories, like other fixtures, require proper venting to prevent siphonage, not an exception. And venting isn’t optional simply because a drain is short; even short runs can create negative pressure that traps must be protected against, so they still need a vent. So placing the vent between the last fixture in the sequence and the next downstream fixture best aligns with how circuit venting is intended to work.

Circuit venting protects a group of fixtures on a single drainage run by allowing air to enter the line as wastewater flows, preventing traps from siphoning. The vent is placed downstream of the last trap in that run, so the air can relieve negative pressure for all the upstream fixtures before the line continues to the next downstream section. That way, all traps in the circuit stay protected.

If the vent were located right after the first fixture, the remaining fixtures on the same run wouldn’t be reliably vented—the negative pressure generated by their discharges could siphon their traps before air can reach them. Lavatories, like other fixtures, require proper venting to prevent siphonage, not an exception. And venting isn’t optional simply because a drain is short; even short runs can create negative pressure that traps must be protected against, so they still need a vent.

So placing the vent between the last fixture in the sequence and the next downstream fixture best aligns with how circuit venting is intended to work.