A tall building is zoned so that static pressure at the lowest fixture in each zone stays within the 80 pound per square inch ceiling the code places on the water distribution system. The pressure-reducing valve at the top of a zone is set to deliver 62 pounds per square inch. Using 0.433 pound per square inch for each foot of elevation, given, what is the greatest height that zone may extend below the valve?
- AAbout 18 feet
This is the pressure difference itself, 18 pounds per square inch, read as though it were a height in feet. Pressure and head are related by 0.433 pound per square inch per foot, so the two numbers are never interchangeable.
- BAbout 143 feet
This converts the valve setting itself to a height rather than the difference between the setting and the ceiling. The 62 pounds per square inch is already present at the top of the zone and none of it is available to be spent on falling.
- CAbout 185 feet
This converts the entire 80 pound per square inch ceiling to height and ignores the pressure the valve is already delivering at the top of the zone. It would put the lowest fixture at well over twice the permitted static pressure.
- About 41 feet
Why D is correct
Only the difference between the ceiling and the valve setting is available to be spent on elevation: 80 minus 62 equals 18 pounds per square inch, and 18 divided by 0.433 gives about 41 feet of drop. Below that the static pressure at the lowest fixture passes the limit and a second pressure zone is required.
What this question is testing
This checks whether you can work between pressure and elevation head in the correct direction, and whether you recognize that only the margin between the valve setting and the code ceiling is available to spend on height.
On the job
Zoning a high-rise is this calculation repeated up the building, and the setting chosen for each valve trades zone height against the flow pressure the top floor of that zone receives. Set the valve high and the zone gets short and the valve stations multiply; set it low and the top floor showers disappoint. Field crews meet the same arithmetic in reverse when a homeowner in a hillside subdivision reports leaking fixtures on the lowest floor.
Memory technique
Headroom in pounds, divided by 0.433, becomes headroom in feet.
Exam tip
Subtract the setting from the ceiling first, then convert. Converting first is the trap.
Where to look it up
The 80 pound per square inch limit and the requirement for a pressure-reducing valve are in IPC 604.8. UPC states carry a comparable limit in their own water distribution chapter under different numbering.