Anti-siphonage connection for a water closet (W.C.) In sanitary plumbing, to prevent trap seal loss due to siphonage, the anti-siphonage (vent) pipe is connected to which location relative to the P-trap of the W.C.?

Introduction / Context:
Trap seals prevent foul gases from the drainage system entering occupied spaces. During discharge, rapid flow can create negative pressure and siphon the water seal. Anti-siphonage (vent) pipes equalise pressure to protect the seal, a critical aspect of healthy plumbing design.

Given Data / Assumptions:

• A P-trap is installed at the W.C. outlet.
• We need to connect a vent that prevents seal depletion during heavy discharge.
• Standard single-stack or two-pipe sanitary systems are considered.

Concept / Approach:
Anti-siphonage pipes are connected near the crown of the trap (top region) so that any pressure drop is immediately communicated to the atmosphere, preventing the formation of a siphon and retaining the water seal depth.

Step-by-Step Solution:

Verification / Alternative check:
Fixture unit calculations often require vent sizing; code diagrams consistently show anti-siphonage taken off at the top of the trap seal region.

Why Other Options Are Wrong:

• Main soil pipe: Venting at the stack alone may not protect the local trap under surge discharge.
• Bottom of P-trap: Does not relieve suction; also risks deposit accumulation.
• Side of W.C.: Not the functional location for pressure equalisation at the seal.
• None of these: Incorrect because the correct connection is at the top of the P-trap.

Common Pitfalls:
Placing the vent too far downstream; using undersized vents; sharp bends near the trap creating air locks.

Final Answer:
Top of P-trap W.C.