Purpose of the Trap
Every plumbing fixture drain must be protected by a trap — a liquid-sealed fitting that blocks sewer gases (methane, hydrogen sulfide, carbon dioxide) and vermin from entering occupied spaces through the drain opening. The P-trap is the universal standard: its geometry maintains a standing water seal of 2–4 inches (IPC §1002.2). The seal is vulnerable to four failure modes: (1) self-siphonage, where fixture discharge velocity evacuates the seal; (2) induced siphonage, where flow in an adjacent drain creates negative pressure; (3) back pressure, where positive pressure elsewhere in the drain pushes the water seal up into the fixture instead of out through the trap; and (4) evaporation in infrequently used fixtures. The venting system exists specifically to neutralize siphonage and back pressure by equalizing air pressure across the trap; only trap primers address evaporation.