Water Supply and Distribution Systems
Supply questions test sizing, pressure and protection. You must convert fixtures to water supply fixture units (WSFU), check residual pressure at the farthest outlet, select materials California accepts, and prove the potable system is protected from cross-connections.
Sizing and pressure
- Typical WSFU: tank-type water closet 2.5 (flushometer 10), lavatory 1, bathtub 2, shower 2, kitchen sink 1.5, dishwasher 1.5, clothes washer 2, hose bibb 2.5.
- Minimum residual at fixtures is 15 psi; above 80 psi static a pressure-reducing valve is required.
- Keep velocity at or below about 8 fps to limit noise and pipe erosion.
- Size on developed length - the longest actual run from the meter, plus the fitting allowance.
Materials and protection
Copper (Type L; Type M is restricted underground), PEX to ASTM F877, CPVC to ASTM D2846, and galvanized steel are the common choices. Solder and flux must be lead-free, and wetted surfaces meet the 0.25 percent weighted-average lead limit. Cross-connection control is the exam favorite: hose bibbs get vacuum breakers, irrigation gets a pressure vacuum breaker or RPZ, boiler makeup and chemical injection need an air gap or RPZ, and a closed system (check valve, PRV or backflow device on the inlet) requires a thermal expansion tank.
Testing
Water piping is hydrostatically tested, commonly at 100 psi for one hour, with the gauge left on and no drop.
Worked example
Street pressure is 62 psi at the meter. The highest fixture is 12 ft above: 12 x 0.433 = 5.2 psi lost to elevation, leaving 56.8 psi. Friction through the 3/4 inch run and the softener totals 14 psi, so residual at the shower is 42.8 psi - well above 15 psi and below 80 psi, so no PRV is needed. Raise the building to 60 ft and elevation alone consumes 26 psi, so the sizing must be reworked.
Traps
- Forgetting elevation loss in multi-story or hillside work.
- Confusing WSFU with DFU and sizing supply pipe from the drainage table.
- Installing a backflow preventer without adding an expansion tank.
- No vacuum breaker on a hose bibb or a boiler fill connection.