Water Supply and Drainage Systems
This domain tests how water supply and drainage systems are sized, vented, and protected. Expect questions on pressure, fixture units, drain slopes, venting, cleanouts, backflow, and sump or ejector systems.
Core rules and concepts
- Water supply sizing considers fixture units, developed length, elevation, and available pressure. The system must deliver the minimum residual pressure required by the code and the fixture manufacturer.
- Drainage sizing uses drainage fixture units and pipe size. Horizontal drains need the correct slope, and trap arms must stay within the length allowed for their size.
- Every trap needs a vent. Vent terminals must be located away from openings and air intakes as required by the code; a vent cannot be used as a waste pipe.
- Cleanouts must be accessible and placed at required locations, including changes in direction and long horizontal runs.
- Backflow protection and proper air gaps protect the potable supply. Sewage ejectors and sump pumps must discharge to an approved point, not create a cross-connection.
Worked example
A meter supplies 60 psi static pressure. A fixture is 18 feet above the meter. Elevation loss = 18 × 0.433 = about 7.8 psi, leaving about 52.2 psi before friction. If pipe and fitting friction loss is 10 psi, the available pressure is about 42.2 psi. If the fixture needs 15 psi and the calculation still leaves 42.2 psi, pressure is adequate; if friction were much higher, the pipe would need upsizing. The reasoning is to subtract elevation and friction before comparing with the fixture requirement.
Traps that cost marks
Using static pressure as the pressure at the fixture, ignoring developed length, omitting a vent, or assuming a cleanout can be hidden above a ceiling are common mistakes. Another is connecting a sump pump or ejector in a way that could backflow into the potable system.
Key ideas: calculate pressure at the point of use, keep trap arms vented and within length, and protect every potential cross-connection.