Water Supply and Distribution Systems
Pressure is the whole story
Water supply design starts with available pressure at the meter or source, then subtracts losses: static elevation (0.433 psi per foot of rise), friction in pipe and fittings, and the pressure required at the furthest fixture. A pressure-reducing valve is required where static pressure exceeds 80 psi, and a minimum residual (roughly 8 psi flowing at a tank-type water closet, 15 psi at flushometer valves) must be maintained. Rule of thumb: 1 psi = 2.31 feet of head; so 30 feet of rise costs about 13 psi.
Sizing with fixture units
Demand is computed in water supply fixture units (WSFU) using the code tables, converted to gallons per minute, then matched to pipe size for the developed length of the longest run. Include equivalent length for fittings and valves. Sizing tables generally keep velocity near 8 feet per second to limit noise and erosion.
Components you must know
Meter and main, service valve, backflow preventer, PRV, expansion tank on closed systems, water heater with T&P relief valve, recirculation for hot water, isolation valves at fixtures and risers, hammer arrestors at quick-closing valves, and air gaps or vacuum breakers for protection. Disinfect new potable mains and flush thoroughly before turning over.
Traps
Questions mix up fixture units with gallons per minute, ignore elevation loss, or forget that a backflow preventer creates a closed system requiring an expansion tank. Another favorite: confusing static pressure (no flow) with residual pressure (flowing). Worked example: 60 psi at the meter, a 21-foot rise to the top fixture (9.1 psi), 12 psi friction loss, and a 10 psi fixture requirement leaves 28.9 psi - adequate but check the furthest fixture, not the nearest.