Water Supply and Distribution
Water Supply and Distribution. This domain tests how potable water is sized, delivered, protected, and maintained at adequate pressure. Expect questions on fixture-unit sizing, developed length, elevation loss, pressure loss, velocity, water heaters, thermal expansion, and protection from contamination.
Core rules
- Size water piping from the water supply fixture units (WSFU), the available pressure, the developed length, and the elevation change. The developed length includes the actual pipe run plus fittings and valves as allowed by the code.
- Static pressure is reduced by elevation: about 0.433 psi per foot of rise for water. Friction loss through pipe, fittings, and devices reduces residual pressure further.
- Fixtures must receive at least the code minimum residual pressure for proper operation; flush valves typically need more than ordinary faucets.
- Provide backflow protection, thermal expansion control, and approved materials. Water heaters need a temperature-and-pressure relief valve piped to a safe discharge.
Worked example
A supply main has 60 psi static pressure. A fixture is 12 ft above the meter, and calculated friction loss is 9 psi. Elevation loss is 12 × 0.433 = 5.2 psi. Residual pressure is 60 − 5.2 − 9 = 45.8 psi. The reasoning: elevation and friction are separate deductions from static pressure. If the result is below the minimum for that fixture, increase pipe size or reduce losses.
Common traps
Do not size by pipe length alone. Do not ignore elevation in tall buildings. Do not confuse static pressure with flowing residual pressure. Do not install a relief valve without a safe, approved discharge.