Water Supply and Distribution
This domain tests how potable water is sized, pressurized, protected, and delivered. Questions focus on minimum and maximum pressures, service sizing, backflow protection, thermal expansion, and the difference between static and flowing pressure.
Pressure rules
Fixtures need a minimum residual pressure of 15 psi to function, and static pressure must not exceed 80 psi; above that, a pressure-reducing valve is required at the service. Because 1 psi equals 2.31 ft of water, elevation always changes pressure: a fixture 20 ft above the meter loses about 8.7 psi static.
Sizing and protection
The building supply for a one- or two-family dwelling is at least 3/4 in. Size by water supply fixture units, developed length, and allowable friction loss, not by matching an existing pipe. Protect the mains with backflow devices chosen by hazard: air gaps for severe hazards, reduced-pressure zone assemblies for high hazard, double-check assemblies for low hazard, and vacuum breakers on hose bibbs. Use dielectric unions at copper-to-steel connections, water hammer arrestors at quick-closing valves, and a thermal expansion tank on a closed system.
Worked example
Static pressure at the meter is 95 psi. The top-floor fixture is 14 ft above the meter (6.1 psi loss) and branch friction is 11 psi. Static 95 psi exceeds 80, so a PRV is mandatory. Set the PRV at 60 psi: flowing pressure at the fixture is 60 − 6.1 − 11 = 42.9 psi, comfortably above 15 psi. Reasoning: check static pressure for the PRV rule, then flowing pressure for the minimum.
Traps
Sizing pipe by diameter alone, forgetting elevation, assuming a PRV removes the need to recheck residual pressure, and omitting expansion control are frequent errors.