Home/Field notes/FN-002
Finding pressure-loss points in a supply line
Low pressure at a fixture is a symptom. Walking the line with a gauge under flow finds the place where the pressure is lost.
Example note This shows the format. The figures are illustrative and do not come from a real job.
The problem
"Low water pressure" is reported at the fixture, but the cause is rarely there. Static pressure can look fine at every point in the building. The loss only shows when water is moving.
Method
- Read static pressure at the first hose bibb after the meter.
- Open a known flow downstream. A tub spout or a hose bibb with a flow gauge works.
- Read pressure again at each accessible point while that flow runs.
- The loss sits between the last good reading and the first poor one.
Example readings
| Point | Static | Under flow | Drop |
|---|---|---|---|
| After meter | 68 psi | 64 psi | 4 psi |
| After pressure reducing valve | 60 psi | 41 psi | 19 psi |
| Water heater inlet | 60 psi | 39 psi | 21 psi |
| Far bathroom branch | 60 psi | 37 psi | 23 psi |
In this example the reducing valve is the loss point. Everything downstream of it only inherits the problem.
Where the loss usually is
- A pressure reducing valve that has failed or was set low.
- A softener or filter with a restricted bed or bypass.
- A stop or main valve left partly closed.
- Old galvanized pipe narrowed by scale.
- A branch sized for one fixture that now feeds three.
What gets recorded
Static and flowing pressure at each point, the flow used for the test, and the same table repeated after the repair. The second table is the proof.