Water Supply and Distribution Systems
Sizing and Design Logic
Water supply questions test your ability to reason about pressure, flow, and pipe sizing. Key concepts: fixture units (FU), demand load, developed length, static vs. residual pressure, friction loss, and the minimum residual pressure required at the most remote fixture. Know that water supply sizing uses tables based on fixture units and developed length, and that velocity limits protect against noise and erosion.
Materials and Components
Understand approved materials (copper, PEX, CPVC, galvanized), joining methods, and that dissimilar metals require dielectric protection. Know the roles of shutoff valves, pressure-reducing valves (required above a threshold supply pressure), thermal expansion control, and backflow protection at the service. Water heaters need a temperature-and-pressure relief valve piped to a safe discharge.
Traps
- Confusing static pressure with residual (flowing) pressure.
- Forgetting that developed length includes fittings and elevation.
- Assuming a larger meter always solves low pressure; friction and elevation may dominate.
- Overlooking that a PRV creates a closed system requiring expansion control.
- Mixing up fixture unit values between supply and drainage.
Worked Example / Rule of Thumb
If supply pressure is high enough to require a PRV, the system becomes closed; a thermal expansion tank (or equivalent) is then needed on the water heater. Rule of thumb: size by fixture units and developed length, then verify residual pressure at the farthest fixture meets the code minimum-if not, upsize the critical path, not the whole system.
Always confirm the water heater's relief valve discharge terminates safely and is not reduced in size downstream.