Water Supply and Distribution Systems
Pressure and flow
- Minimum 8 psi residual at the fixture under flow; maximum 80 psi static. Above 80 psi a pressure-reducing valve is required, with a strainer, access, and a thermal expansion tank downstream, because the PRV creates a closed system.
- Sizing method: assign water supply fixture units (WSFU) to each fixture, convert the total to demand in gpm, then select pipe size from the code table using developed length from the meter to the farthest fixture.
- Developed length includes fittings, so a run that looks short on plan can size larger than expected.
Distribution details
Hot water must reach fixtures at a usable temperature without scalding; a master tempering valve is the standard fix where storage temperature is high. Recirculation loops or a manifold layout reduce wait time. Every fixture needs an accessible stop. Hose bibbs, boiler fill, irrigation, and carbonators all require backflow protection. Water hammer arrestors are required where quick-closing valves are installed.
Worked example
A three-story rowhouse reads 95 psi static at the meter and the top-floor shower is weak. Reasoning: 95 psi exceeds 80 psi, so a PRV is mandatory. Installing the PRV without an expansion tank leaves the water heater nowhere to push thermal expansion and the T&P will weep. Set the PRV to roughly 50 to 60 psi, add a listed expansion tank sized for system volume, then recheck residual at the top floor. If residual falls below 8 psi, the branch is undersized and must be increased rather than the PRV raised.
Traps that cost marks
Solving low pressure by raising the PRV setting; forgetting the expansion tank on any closed system; sizing by copying the existing pipe instead of using fixture units and developed length; burying a PRV without access; and ignoring energy code requirements to insulate hot water piping.