Water Supply and Distribution Systems
What to master
The water supply domain tests three linked skills: sizing, pressure, and protection of the potable supply.
Sizing path. Add the water supply fixture units (WSFU) for every fixture served (a tank-type water closet is commonly 2.5, a lavatory 1, a bathtub or shower 2; confirm against the table in your code book). Convert total WSFU to probable demand in gpm, compare that demand to the developed length from the meter to the farthest fixture, and select the smallest pipe that keeps friction loss acceptable. Then verify velocity stays under roughly 8 to 10 fps and that residual pressure at the fixture meets the code minimum.
Pressure. Static pressure falls 0.433 psi per foot of rise, so a 20-ft rise costs about 8.7 psi. Gravity fixtures need only a few psi flowing; flushometer valves need roughly 15 to 20 psi flowing. Excess street pressure above about 80 psi calls for a pressure-reducing valve.
Protection. Ranking of backflow protection from best: air gap, then reduced-pressure-zone (RPZ) assembly, then double check, then vacuum breaker. Hose bibbs need vacuum breakers; irrigation, boiler feed, and chemical mixing points need RPZ assemblies with annual testing. On a closed system (check valve, PRV, or meter with backflow device), add a thermal expansion tank.
Water heaters. The T&P valve sits within the top 6 in of the tank. Its discharge pipe is full size, unthreaded, gravity-flowing, not trapped, with no valve, ending 6 in above the floor or outside.
Traps. Mixing fixture units up with gallons of demand, sizing by rule of thumb while ignoring developed length, and answering "vacuum breaker" where the code requires an air gap.
Rule of thumb. Elevation gain eats pressure fast; always subtract static loss before checking residual pressure at the top floor.