Water Supply and Distribution Systems
Water Supply and Distribution Systems
This domain covers pressure, pipe sizing, backflow protection, hot-water delivery, and the behavior of water in a closed system. Expect calculation questions on static head and practical questions on backflow devices and thermal expansion.
Core rules
- Pressure and elevation. Static pressure changes by 1 psi per 2.31 ft of elevation. A fixture above the meter loses pressure; a fixture below gains it.
- Service pressure. A pressure-reducing valve is commonly required where street pressure exceeds 80 psi. Minimum residual pressures are roughly 8 psi for flush-tank fixtures and 15 psi for flushometer valves-check the adopted code.
- Sizing. Use fixture units and developed length, including equivalent length for fittings. A long 1/2 in run that serves several fixtures will fail on velocity and pressure loss even if it passes at the meter.
- Backflow. Protect the potable supply with an air gap, reduced-pressure zone assembly, double-check valve, or vacuum breaker as the hazard requires. Irrigation, boilers, and chemical injection need protection.
- Thermal expansion. A closed system with a backflow preventer or check valve needs an expansion tank or another approved means to absorb heated water expansion.
Worked example
A meter supplies 60 psi. The highest fixture is 21 ft above the meter, and friction loss to that fixture is 8 psi. Convert elevation to pressure: 21 ÷ 2.31 = 9.09 psi. Residual pressure = 60 − 9.09 − 8 = 42.9 psi. That is adequate for a tank-type water closet. If the meter pressure were 30 psi, the residual would be 12.9 psi-still enough for a flush tank but marginal or insufficient for a flushometer valve, so the design would need a booster or a different fixture type.
Traps that cost marks
- Ignoring elevation loss when checking fixture pressure.
- Installing a backflow preventer and omitting the expansion tank.
- Sizing by pipe diameter alone without counting fixture units and developed length.
- Using a vacuum breaker as a substitute for an air gap where a high hazard exists.
- Forgetting that a pressure-reducing valve creates a closed system.