Water supply and distribution piping
Domain 5 - Water supply and distribution piping
This domain covers service sizing, pressure, materials, testing, and the devices that keep potable water safe. Expect method items (fixture units, developed length) and rule items (pressure limits, valves, testing).
Sizing and pressure
- Water supply fixture units (WSFU) convert to probable demand, then pipe size comes from the code sizing tables using available pressure and developed length - measured length plus equivalent length for fittings, valves, and the meter.
- Minimum residual pressure is 8 psi at fixtures; flushometer valves need at least 15 psi flowing, and manufacturers often require more. Where static pressure exceeds 80 psi, a pressure-reducing valve is required, and that valve creates a closed system needing expansion control.
- Velocity and noise matter: oversizing wastes material and increases stagnation, undersizing produces howling and pressure complaints.
Materials, valves, and protection
- Copper, PEX, CPVC, and galvanized steel each have listed joining methods and restrictions. PEX requires listed fittings plus protection from sunlight and physical damage; copper-to-steel connections need dielectric protection.
- A full-open shutoff valve is required on the service and at fixture supplies or branches, with drain valves at low points and water hammer arrestors at quick-closing valves.
- Backflow protection follows the hazard: air gap for high hazard, testable RPZ where required, vacuum breakers at hose connections, and no bypass around a reduced-pressure assembly.
- Testing: hydrostatically test with water (or air where permitted) at working pressure or the pressure the building official requires, commonly 100 psi held 15 minutes, with the gauge left in place for the inspector.
Worked example
A two-bath house shows 26 WSFU, a ¾ in. service, and 105 psi static pressure at the outdoor sillcock. Reasoning: 105 psi exceeds 80 psi, so a pressure-reducing valve is required and should be set near 55–60 psi to protect fixtures and reduce noise. The 26 WSFU converts to roughly 20–22 gpm probable demand; a ¾ in. service carries that only at high velocity over a long run, so developed length and meter loss must be checked against the sizing table, and a 1 in. service is usually the answer. Once the PRV is installed, add an expansion tank at the water heater.
Where marks are lost
- Using street pressure at the meter rather than residual pressure at the furthest fixture.
- Ignoring equivalent length for fittings and the meter.
- Installing a PRV or backflow assembly without addressing thermal expansion.