Water Supply and Distribution
Pressure is the core number
Residential systems must deliver a minimum residual pressure at the fixture - roughly 8 psi for tank-type water closets, much higher for flushometer valves - while static pressure is capped at 80 psi. Above 80 psi, a pressure-reducing valve is required, and any closed system (PRV, backflow preventer, check valve) requires thermal expansion control, usually a expansion tank. Know backflow devices by hazard: air gap or RPZ for health hazard, double check valve assembly for non-health hazard, and pressure vacuum breaker or atmospheric vacuum breaker only where backpressure and continuous pressure are not issues.
Sizing and layout
Supply sizing uses water supply fixture units (WSFU), not drainage fixture units, and developed length includes equivalent length for fittings. Service lines go below the frost line; interior piping needs support, accessible shutoffs, and protection from freezing. Water heaters need a listed T&P valve piped full size to a safe drain, a cold-inlet shutoff, and dielectric protection between copper and steel.
Traps
Mixing WSFU with DFU charts, assuming a larger meter cures low pressure (it improves volume, not pressure), omitting the expansion tank downstream of a PRV, and naming a backflow device that cannot be installed in a vault or where it might freeze.
Worked example
A service reads 95 psi static at the meter. Install a PRV set to about 50–60 psi, a strainer ahead of it, and a thermal expansion tank on the cold side downstream of the PRV. If the same house read 40 psi with long runs, investigate pipe size and elevation before adding a booster.