Water Supply and Water Distribution
Path and pressures
Trace the system: water main → service line → meter → building distribution → risers → fixture supply. Design is governed by pressure, not pipe size alone. You must have enough residual (flowing) pressure at the most remote fixture; static pressure with everything closed tells you very little.
Core concepts
- Static vs residual pressure: static is at rest; residual is while flowing. Sizing uses residual.
- Elevation loss: 0.433 psi per foot of rise.
- Friction loss: grows with flow and with smaller diameter; keep velocities moderate (roughly 5–8 fps is a common design target).
- Thermal expansion: a closed system with a backflow preventer or check valve needs an expansion tank.
- Water hammer: arrestors at quick-closing valves and at the ends of long branches.
- Valves and access: a full-open main shutoff, accessible isolation valves, drain valves at low points.
- Materials and joints: use approved materials and the joining method the listing requires; no lead-bearing solder or flux on potable lines.
Worked example
A three-story building: the top fixture is 30 ft above the meter. Elevation cost = 30 × 0.433 ≈ 13 psi. If the fixture needs 8 psi flowing and friction losses total 15 psi, the meter must deliver at least 36 psi residual while flowing. A 65 psi static reading does not prove that.
Traps
Sizing from static pressure; forgetting elevation loss on upper floors; omitting an expansion tank after installing a pressure-reducing valve or backflow preventer; and assuming a larger meter fixes a friction problem caused by undersized branch piping.