CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceEasy
A homeowner reports a slow drain in their kitchen sink. Upon inspection, you notice water backing up into the second basin when the first is draining. What is the most probable cause of this issue?
- AA faulty air admittance valve (AAV) or vent stack issue.
- BA partial blockage in the common drainpipe connecting the two sink basins.
- CA clogged P-trap directly beneath the affected basin.
- DAn obstruction in the main sewer line serving the house.
Show answer & explanationAnswer & explanation
Correct answer: B. A partial blockage in the common drainpipe connecting the two sink basins.
When water from one basin backs up into another connected basin, it indicates a blockage downstream from where the two basins connect into a common drainpipe. The P-traps are usually clear, but the shared section is partially obstructed, causing the water to seek the path of least resistance—up into the other basin.
Why the other options are wrong
- A. A faulty AAV or vent issue would cause gurgling, slow draining, or siphoning, but not typically water backing up from one basin to another connected basin in this manner.
- C. A clogged P-trap would only affect that specific basin, not cause water to back up into the other basin.
- D. A main sewer line obstruction would typically affect all drains in the house, not just the kitchen sink, or at least multiple fixtures.
Kitchen Sink Cross-Contamination
When water from one kitchen sink basin backs up or overflows into the other basin due to a shared drain blockage.
- Indicates a blockage downstream of the common drain connection.
- Often caused by grease, food particles, or soap scum.
- Requires clearing the shared drain line.
- Different from individual P-trap clogs.
Memory trick: Two paths diverge, but one's choked, so back it goes.