Journeyman Plumber ExamFixtures, Faucets, and Other AppliancesHard
A plumber is inspecting a newly installed back-to-back water closet connection. To prevent siphonage and ensure proper venting, which of the following reventing methods is most appropriate for this configuration, according to common plumbing codes (e.g., UPC or IPC)?
- AA single common vent serving both fixtures without a fixture cross.
- BA double-fixture fitting (fixture cross) with a common vent connected above the centerline of the drain.
- CAn individual vent for each water closet connected independently to the stack.
- DA wet vent from a lavatory draining into the fixture cross below the water closets.
Show answer & explanationAnswer & explanation
Correct answer: B. A double-fixture fitting (fixture cross) with a common vent connected above the centerline of the drain.
For back-to-back water closets, a double-fixture fitting (often called a fixture cross or double wye) with a common vent connected above the centerline of the drain is the standard and most appropriate method. This prevents cross-siphonage between the two fixtures and ensures proper air circulation.
Why the other options are wrong
- A. A single common vent without a proper fixture cross or with the vent taken off incorrectly can lead to siphonage or insufficient venting for one or both fixtures.
- C. Individual vents are possible but are often less practical and more costly than a properly installed common vent for back-to-back fixtures.
- D. A wet vent from a lavatory draining *below* the water closets' fixture cross would pose a cross-connection risk and is not an approved method for venting water closets in this configuration.
Back-to-Back Water Closet Venting
The plumbing method for venting two water closets installed back-to-back on a common drain, typically using a double-fixture fitting and a common vent.
- Uses a double-fixture fitting (fixture cross/double wye).
- Common vent connects above the centerline of the drain.
- Prevents both self-siphonage and induced siphonage for both fixtures.
Memory trick: Two toilets back-to-back, a fixture cross and high vent keeps the drain on track.