Water Supply and Distribution: Sizing, Materials, and Protection
Water Supply and Distribution: Sizing, Materials, and Protection
Questions here test whether you can size a system for peak demand, still deliver adequate pressure to the farthest fixture, and use materials and joints suited to potable water.
Sizing logic
Convert each fixture to water-supply fixture units (WSFU), add them, then use the code's table for the available pressure and the developed length. Long runs, many fittings and elevation all reduce what reaches the fixture. Codes require a minimum residual pressure - commonly 8 psi for tank-type fixtures and 15 psi for flushometer valves. Where street pressure exceeds 80 psi, a pressure-reducing valve is required, and a closed system downstream of a PRV, check valve or backflow preventer needs a thermal expansion tank.
Pressure accounting
Static pressure minus meter loss, minus friction, minus elevation equals residual pressure. Elevation costs about 0.433 psi per foot of rise (2.31 ft per psi).
Materials and protection
Copper tube, PEX, CPVC and galvanized steel are common; all must be listed for potable water (NSF/ANSI 61). Protect piping from freezing below the frost line, from physical damage and from backflow. Support at code intervals, test before concealment, and disinfect new service lines per the adopted standard.
Worked example. Static street pressure is 60 psi. The highest fixture is 12 ft up: 12 × 0.433 = 5.2 psi of loss. Meter loss is 5 psi and pipe friction 10 psi. Residual = 60 − 5.2 − 5 − 10 = 39.8 psi, comfortably above the 15 psi a flushometer valve needs.
Traps. Sizing from static pressure alone; forgetting elevation; adding an expansion tank only at the heater instead of the closed side of the system; inverting the conversion to 2.31 psi per foot; ignoring friction from a long run of small pipe.