Water Supply and Distribution Systems
Layout and pressure
Water service enters at the meter, becomes the building main, and branches to risers and fixture supplies. Piping is sized from fixture units and developed length using the code tables, with a minimum size for any fixture branch. Pressure at fixtures must stay in the code range, roughly 20 to 80 psi; where static pressure exceeds 80 psi a pressure-reducing valve is required, and a thermal expansion tank must then be added because the valve creates a closed system.
Rules that show up on exams
- Size from total demand and developed length, not from whatever pipe is on the truck.
- Provide accessible shutoff valves at the meter, on each riser, and at fixture groups.
- Protect piping from freezing, corrosion, and contact with soil or concrete unless approved for direct burial.
- Install water hammer arrestors at quick-closing valves and at the end of long branches.
- Keep hot and cold lines separated in trenches and insulated through unconditioned space.
- Test before covering.
Worked example
A two-story house has 68 psi static at the meter and a shower head 27 feet above it. Elevation costs 1 psi per 2.31 feet, so 27 divided by 2.31 equals 11.7 psi lost to height, leaving about 56 psi upstairs. Subtracting roughly 8 psi of branch friction loss leaves 48 psi, which is acceptable, and no reducing valve is needed because static pressure never exceeds 80 psi.
Traps that cost marks
Sizing from connected fixture count without applying the demand table, ignoring elevation loss at 2.31 feet per psi, and forgetting the expansion tank whenever a reducing valve, check valve, or backflow preventer isolates the system.