A plumber is troubleshooting a persistent odour problem in a residential bathroom. The toilet and sink both have traps, but a faint sewer gas smell is noticeable, especially after heavy rain. All visible traps appear to be filled with water. What is a likely cause and an NPC-compliant solution?

Question: A plumber is troubleshooting a persistent odour problem in a residential bathroom. The toilet and sink both have traps, but a faint sewer gas smell is noticeable, especially after heavy rain. All visible traps appear to be filled with water. What is a likely cause and an NPC-compliant solution?

Answer options:

• The house trap is blocked; clear the house trap. ✅ The venting system is inadequate or blocked; investigate and clear the vent.
• The fixture traps are too deep; install shallower traps.
• The main building drain is sloped excessively; re-lay the main drain.

Correct answer: The venting system is inadequate or blocked; investigate and clear the vent.

Explanation: A persistent sewer gas smell, even with filled traps, often indicates a problem with the venting system, such as an inadequate vent, a blocked vent, or improper vent sizing/connection leading to siphonage or backpressure. The venting system's purpose is to prevent these issues. A blocked house trap would likely cause sewage backup, not just odour. Trap depth is standardized. Excessive main drain slope is unlikely to cause localized odour unless drainage is completely compromised, which isn't indicated by the symptoms.