Water Supply and Distribution Systems
What this domain tests
This domain tests pressure, sizing, backflow protection, hot-water distribution, and the causes of poor flow. You should be able to convert pressure to elevation, recognize the devices that protect potable water, and explain why a closed system needs expansion control.
Core water-supply rules
- Pressure and elevation: 1 psi equals about 2.31 ft of water column. A 20-ft rise costs roughly 8.7 psi of static pressure.
- Size pipe by fixture units, developed length, available pressure, and friction loss. Undersizing shows up as low residual pressure at the farthest fixture.
- Backflow protection: an air gap is the strongest protection; an RPZ is used for high hazard; a PVB or AVB protects against backsiphonage; a double-check assembly is for lower hazard. Test reduced-pressure assemblies annually.
- A closed system with a water heater needs a thermal expansion tank. The T&P valve protects against over-pressure and over-temperature, but it is not a substitute for expansion control.
- Water hammer is controlled with arrestors, proper pipe support, and pressure regulation.
- Hot-water recirculation reduces wait time; insulate hot lines and protect exposed piping from freezing.
Worked example
A street pressure of 60 psi serves a fixture 20 ft above the meter. Static pressure loss is 20 ÷ 2.31 = 8.7 psi, leaving about 51.3 psi before friction. If the pipe and fittings lose another 10 psi, the fixture sees about 41 psi. That is adequate, but a long, undersized branch could drop it below the minimum residual pressure.
Traps that cost marks
Ignoring elevation loss, installing a backflow device backwards, omitting an expansion tank on a closed system, and sizing from pipe diameter alone without fixture-unit load and developed length.
Key points
- 1 psi ≈ 2.31 ft of water column.
- Size by fixture units, length, pressure, and friction loss.
- Air gap is best; RPZ for high hazard; test annually.
- Closed systems with water heaters need expansion tanks.
- Water hammer needs arrestors, support, and pressure control.