Journeyman Plumber ExamWater Supply SystemsHard
A plumber is troubleshooting a potable water distribution system in a multi-story building. Occupants on the upper floors are reporting consistently low water pressure, even when fixtures on lower floors are not in use. The water service enters the building at ground level with adequate pressure. What is the most likely cause of this issue?
- AExcessive demand from lower-floor fixtures.
- BInsufficient pipe sizing for the vertical risers.
- CA malfunctioning water heater.
- DA faulty pressure reducing valve (PRV) at the building entrance.
Show answer & explanationAnswer & explanation
Correct answer: B. Insufficient pipe sizing for the vertical risers.
When water flows upwards, it loses pressure due to gravity (static head). If the vertical risers supplying the upper floors are undersized, the friction loss in these pipes, combined with the static head loss, will result in significantly reduced pressure at the higher elevations, even without high demand.
Why the other options are wrong
- A. The question states the issue persists 'even when fixtures on lower floors are not in use,' ruling out excessive demand as the primary cause.
- C. A malfunctioning water heater would primarily affect hot water temperature or flow if severely restricted, but not the overall cold water pressure specifically on upper floors.
- D. A faulty PRV would typically affect pressure throughout the entire building, not just specifically the upper floors, unless it's a zone PRV, which isn't indicated.
Static Head Pressure Loss
The reduction in water pressure due to elevation gain, where water must be lifted against gravity, resulting in approximately 0.433 psi loss per foot of rise.
- Significant factor in multi-story buildings.
- Separate from friction loss.
- Must be accounted for in pipe sizing and pump selection.
Memory trick: Upper floors' low flow, means risers are too slow, or static head makes pressure go low.