Water Supply and Distribution
Pressure and protection
The supply chapter is mostly about pressure, sizing, and backflow. Static pressure above 80 psi at any fixture requires a pressure-reducing valve; residual pressure must still satisfy the most remote fixture, with tank-type fixtures needing less than flushometer valves. Backflow protection is chosen by hazard: an air gap for the severest hazard, a reduced-pressure principle assembly where both backpressure and backsiphonage are possible, and a dual check or vacuum breaker for lower hazards.
Sizing logic
- Assign fixture units to every fixture on the branch, subtotal at each junction, and size from the table using the available pressure and developed length.
- Longer developed length or lower pressure means larger pipe for the same load - this is the single most tested relationship.
- Size the service and meter for total demand, not for the largest single fixture.
- Keep velocity near or below roughly 8 feet per second to limit noise and erosion.
- Insulate hot lines and protect all supply piping from freezing.
Worked example
A hose bibb gauge reads 95 psi static. The rule: above 80 psi, install a PRV downstream of the meter. Because the PRV closes the system, also install a thermal expansion tank at the water heater. If the gauge read 60 psi, neither device is triggered by this rule. Note the gauge is measuring static pressure, not residual pressure at the farthest fixture - do not mix the two.
Traps
- Sizing the main from the largest fixture instead of total fixture units.
- Applying the 80-psi threshold to residual rather than static pressure.
- Forgetting the expansion tank whenever a PRV or check valve is added.
- Choosing a vacuum breaker where the hazard requires an air gap or RP assembly.