Water Supply and Distribution
Water Supply and Distribution tests pressure, sizing, materials, valves, backflow control, and thermal expansion in potable-water systems. Questions often require you to separate static pressure from flowing pressure and to account for elevation.
Core rules and concepts
- Size water piping from fixture units, developed length, and available pressure, not from pipe diameter alone.
- Static pressure changes about 0.433 psi per foot of elevation; pressure falls when a fixture is above the supply source.
- Maintain the code-required residual pressure at each fixture type; flushometer fixtures generally need more than conventional tank-type fixtures.
- Provide backflow protection for cross-connections and thermal expansion control where a closed system can raise pressure.
- Install accessible shutoff valves, unions, and drains where required; support piping to prevent stress and water hammer.
Worked example
A building has 60 psi at the street main. A fixture is 30 feet above that point. Elevation loss is 30 × 0.433 = 12.99 psi, so static pressure at the fixture is about 47 psi before friction losses. If the fixture needs 15 psi residual and the piping loses 20 psi, the available pressure is 47 − 20 = 27 psi, which is adequate. If friction loss were 40 psi, the result would be 7 psi and the design would fail.
Traps that cost marks
Do not subtract elevation loss twice or ignore it. Do not use static pressure as if it were residual pressure. Do not size a long run without adding fittings and developed length. Do not omit thermal expansion control on a closed system.
Exam focus
Be ready to calculate pressure loss by elevation and to explain why a pressure booster or larger pipe may be needed.