Water Supply and Distribution Systems
Water Supply and Distribution Systems tests pressure, pipe sizing, backflow protection, and the components that keep potable water separated from contaminants. Questions often ask what device is required at a given cross-connection or what happens to pressure at an elevation change.
Core rules
- A pressure-reducing valve is required when static pressure exceeds 80 psi; thermal expansion control is then needed on a closed system.
- An air gap is the strongest backflow protection; an RPZ is used for high-hazard connections; a DCVA for lower-hazard cross-connections; vacuum breakers protect hose bibbs and similar outlets.
- Water-supply sizing uses fixture units, developed length, elevation, and available pressure.
- Shutoff valves are required at fixtures and building supply points; drain valves are needed where water could be trapped.
- Piping must be protected from freezing and supported per code.
Worked example
A supply main reads 60 psi. A fixture outlet is 23 feet above the main. Elevation loss is 23 ÷ 2.31 = 9.96 psi, so static pressure at the fixture is about 60 − 9.96 = 50.0 psi. Reasoning: water loses pressure as it rises; divide feet by 2.31 to convert head to psi, then subtract. Friction loss would reduce it further under flow.
Traps that cost marks
- Installing a PRV but no expansion tank on a closed system.
- Using a vacuum breaker where an air gap is required.
- Ignoring developed length when sizing.
- Forgetting that pressure at an upper floor is lower than at the main.