Water Supply and Distribution
Sizing and pressure
Water piping is sized from water-supply fixture units using the available pressure at the source, the developed length (including equivalent lengths for fittings and valves), elevation loss, and device losses. Confirm residual pressure at the most remote fixture: about 8 psi for gravity flush tanks, 15 psi for flushometer valves. Minimum sizes per the table are 3/4 in. for the building supply and 1/2 in. for a fixture branch. Keep velocity near 8 fps to limit noise and erosion.
Backflow and controls
Know the device hierarchy: air gap (highest protection), reduced-pressure zone assembly for high hazard, pressure vacuum breaker for irrigation, atmospheric vacuum breaker for hose bibbs, double check for low hazard. A pressure-reducing valve is required where static pressure exceeds 80 psi, and any closed system downstream needs thermal expansion control. Water hammer arrestors belong at quick-closing valves.
Separation, burial, and testing
Keep potable water at least 5 ft horizontally from sewer pipe and 12 in. above or below it where lines cross. Bury water service below frost depth and not less than 12 in. below grade. Potable lines must be disinfected after repair or new installation before being placed in service, and the system is hydrostatically tested before concealment.
Traps in questions
- Sizing from pipe length alone, ignoring equivalent length of fittings.
- Mismatching hazard level and backflow device.
- Forgetting that an RPZ discharges and needs drainage.
Worked example
Street pressure is 90 psi and the top fixture is 20 ft up (about 8.7 psi loss). Residual pressure is plentiful, but a PRV is still required because static pressure exceeds 80 psi.