Plumbing basics: In building drainage, what is the principal function of a trap installed on a fixture connection?

Introduction / Context:
Fixture traps are mandatory in sanitary plumbing to protect indoor air quality. They form a water seal between the fixture and the drainage system, blocking sewer gases that may be hazardous or malodorous from entering living areas.

Given Data / Assumptions:

• Standard P-, S-, or bottle-traps filled with water after each use.
• Proper venting is provided to prevent siphonage of the trap seal.
• Domestic sanitary installations.

Concept / Approach:

A trap relies on a standing water seal of adequate depth (often around 50–75 mm) to resist pressure fluctuations and seal against gas flow. It is not a mechanical pest barrier nor a gas scrubber; instead, it is purely a hydraulic seal that must be maintained and protected from siphonage, back pressure, or evaporation.

Step-by-Step Solution:

Verification / Alternative check:

Plumbing codes specify minimum trap seal depths and venting requirements to ensure seals are not lost, confirming the functional role of traps.

Why Other Options Are Wrong:

Traps do not capture rodents, do not chemically dissolve gases, and are not intended to create siphonic action; air admittance valves are different components used in venting, not substitutes for trap seals.

Common Pitfalls:

Allowing trap seals to evaporate in seldom-used fixtures; improper venting leading to siphonage and loss of seal.

Final Answer:

It is a water seal that prevents the entry of foul sewer gases into occupied spaces