Which trap type is explicitly prohibited in residential plumbing?

• A. P trap
• B. S trap
• C. Bell trap
• D. Drum trap

Answer: (B)

In plumbing, a trap must hold a water seal to block sewer gases and be vented so that water flowing through the drain doesn’t pull that seal away. An S-shaped trap is problematic because the way it routes the drain path without a proper vent between the trap and the rest of the drainage line allows siphonage. When wastewater moves through the bend, negative pressure can suck the water out of the trap, breaking the seal and letting gases escape. That failure to maintain a reliable seal is why this configuration is prohibited in residential plumbing.

A properly vented P-trap, by contrast, has a trap arm that leads horizontally to a vent, which prevents siphoning and keeps the water seal intact. Bell traps and drum traps are older designs with their own practical drawbacks, such as difficult venting and maintenance, so they’re not used in modern residential work.