Sanitary Fixtures, Traps and Venting: Protecting the Water Seal
What this domain tests
This domain focuses on the trap and vent system that keeps sewer gas out of the building. Questions cover trap types, seal depth, vent arrangements and the causes of seal loss.
Core rules and concepts
- A trap holds a water seal that blocks sewer gas. The seal depth is commonly 50 mm; confirm the local requirement.
- Common traps: P-trap, bottle trap, bath trap and floor trap. S-traps are prone to self-siphonage and are restricted in many codes; prefer a P-trap with venting.
- Each fixture should have its own trap; double trapping is avoided because it causes air lock and slow flow.
- Vent types include individual vent, common vent, wet vent, stack vent, circuit vent and air admittance valve (AAV).
- Seal loss happens by siphonage, back pressure, evaporation, capillary action, foaming or a moving obstruction.
- Vent pipes must be kept clear and should rise above the roof as the code requires.
Worked example
A trap has a 50 mm water seal. The pressure difference needed to pull the seal out is ρgh = 1000 × 9.81 × 0.05 = 490 Pa, ignoring surface tension. The reasoning: a deeper seal resists a larger pressure difference, but codes fix a minimum depth because too deep a seal is hard to clear and can become foul. A vent limits the pressure swing so the 50 mm seal is not siphoned.
Traps that cost marks
- Fitting a shallow trap that loses its seal easily.
- Double-trapping a fixture and causing air lock.
- Omitting a vent on a long or heavily used branch.
- Using an S-trap where a vented P-trap is required.
- Blocking a vent with a cap or debris.