Plumbing Systems and Installation - Traps, Vents, Supports, and Testing
Domain 2 - Plumbing Systems and Installation
This domain tests installation rules you apply in the field and on details: traps, vents, supports, joints, fixture setting, and testing before concealment.
Traps and fixture connections
Every fixture needs a trap except fixtures with integral traps, such as water closets and urinals. Trap seal depth is 2 to 4 inches. The trap arm connects the trap to its vent and must be protected from siphonage. S-traps are prohibited, and double trapping is not allowed. An air-admittance valve is a one-way vent only where the code permits it; it is not a universal substitute for a vent through the roof.
Venting and drainage
Each trap requires a vent. Vent size depends on fixture-unit load and developed length; enlarging pipe without checking the table is an error. Horizontal drainage must slope uniformly toward discharge. Support spacing depends on pipe material, so use the code table for the material installed. Joints must be approved for the material and location, and buried lines must not rely on prohibited threaded joints.
Fixtures and testing
Set fixtures level and secure, and confirm clearances and accessibility. Test drainage and vent systems before concealment by the code-specified water or air method, using the required head or pressure and duration.
Worked example
A commercial kitchen adds a 3-compartment sink. The sink discharges through an air gap into a floor sink; the floor sink is trapped and vented; the discharge then passes through a grease interceptor sized from the fixture load and code table. Reasoning: the air gap protects potable water from soiled sink water, the floor sink is a receptor, and the interceptor keeps fats, oils, and grease out of the building drain.
Traps that cost marks
Direct-connecting a sink without an air gap or air break; omitting a vent; guessing support spacing; skipping the pre-concealment test; placing an air-admittance valve where the code does not allow it.