Journeyman Plumber ExamFixtures, Faucets, and Other AppliancesEasy
A homeowner reports a foul odor emanating from their seldom-used guest bathroom sink. Upon inspection, you find the trap is dry. What is the most likely cause of the dry trap?
- AImproperly vented drain line
- BEvaporation due to lack of use
- CA blockage in the vent pipe
- DExcessive water pressure causing siphoning
Show answer & explanationAnswer & explanation
Correct answer: B. Evaporation due to lack of use
A dry trap in a seldom-used fixture is most commonly caused by the standing water in the trap evaporating over time. The water seal is lost, allowing sewer gases to enter the living space.
Why the other options are wrong
- A. An improperly vented drain line would typically lead to siphoning during use, not just evaporation over time.
- C. A blockage in the vent pipe would also lead to siphoning or gurgling during use, not primarily a dry trap from lack of use.
- D. Excessive water pressure is unlikely to cause siphoning in a properly installed drain system.
Dry Trap
A plumbing trap that has lost its water seal, usually due to evaporation or siphoning, allowing sewer gases to enter the building.
- Common in seldom-used fixtures.
- Causes foul odors from sewer gas.
- Can be remedied by running water to refill the trap.
Memory trick: A trap's water seal is like a liquid door, blocking gases from the floor.