Water Supply and Distribution Systems
What this domain tests
Sizing and layout of the water service and distribution system, pressure control, and cross-connection control.
Core rules
- Sizing runs from water-supply fixture units (WSFU) to gallons per minute, then through pipe friction tables using developed length, fittings, and elevation.
- Minimum residual pressure: 15 psi at flushometer valves; 8 psi at other fixtures, or the manufacturer's requirement if greater. Where pressure exceeds 80 psi, a pressure-reducing valve is required, typically set near 50 psi.
- Elevation costs 0.433 psi per foot of rise, so a fixture 20 feet above the meter loses about 8.7 psi before a drop of water moves.
- Water service is buried below local frost depth; in New Hampshire that is commonly 4 feet or deeper, with protection where the pipe enters the building.
- Cross-connection control is matched to hazard: air gap for severe hazards, reduced-pressure zone assembly for high hazard, double check for low hazard, pressure vacuum breaker for irrigation, hose-bibb vacuum breakers for sillcocks.
- Lead-free solder and flux; PEX protected from sunlight and abrasion, with approved fittings and a manifold or trunk-and-branch layout.
Worked example
Street pressure 70 psi, 40 feet of rise to the top-floor showerhead, 90 feet developed length, 12 gpm demand. Elevation loss = 40 × 0.433 = 17.3 psi. Available after elevation = 52.7 psi. Friction at the chosen size must stay under about 44 psi to leave 8 psi at the fixture - a quick check that decides whether the service must be upsized.
Traps that cost marks
Sizing on static pressure alone; ignoring the pressure-reducing valve requirement; adding elevation loss instead of subtracting it; using a double check where the hazard requires an air gap.