Water Supply and Drainage - Fixture Units, Slope, and Residual Pressure
Domain 3 - Water Supply and Drainage
This domain tests sizing and pressure logic. Drainage questions use drainage fixture units (DFU), slope, and pipe capacity. Water-supply questions use water-supply fixture units (WSFU), developed length, elevation, and residual pressure.
Drainage sizing
One DFU approximates 1 cubic foot per minute of probable discharge. Add fixture units by fixture type, then use the drainage tables for the pipe, slope, and occupancy. Do not size a horizontal branch from the building-drain table. Slope must be uniform; flatter slope reduces capacity. Vents are sized separately from the fixture-unit load and developed length.
Water supply sizing
WSFU is not gallons per minute. Convert fixture units to probable demand using the code table, then account for developed length, fittings, meter loss, and elevation. Static pressure drops about 0.433 psi per foot of rise. Size for the minimum residual pressure at the most remote fixture, not just street pressure.
Worked example
Street static pressure is 70 psi; the highest fixture is 30 ft above the main. Elevation loss = 30 × 0.433 = 13 psi, leaving 57 psi. Allow 10 psi for meter and piping friction, leaving 47 psi. Compare 47 psi with the code minimum for the fixtures served. If it were low, increase service size, reduce developed length, or add a booster pump; do not assume street pressure reaches the top floor.
Traps that cost marks
Mixing DFU and WSFU; treating WSFU as gpm; ignoring elevation; sizing from static pressure alone; using the wrong slope; oversizing a vent instead of checking the load table.