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Pressure Drops as Soon as Two Fixtures Run
Static pressure can read perfectly normal while the house still fails the moment two things run. Pressure and flow are not the same measurement.
A gauge with everything closed measures static pressure. Water is not moving, so nothing is being lost to friction. Open two fixtures and the water has to move — and any restriction in the path now shows up as a drop. A house can read a healthy 55 psi static and still be unusable with a shower and a dishwasher running.
The test that separates them
Leave the gauge on the outside tap. Take the static reading, then have someone open two indoor taps and read it again. Write both numbers down.
- Holds steady — supply is fine. Any complaint is at a specific fixture.
- Drops a little — normal. Some loss under demand is expected.
- Collapses — there is a restriction upstream, and this is the number that tells a contractor where to look.
What causes a collapse
- Undersized or corroded supply. A half-inch line that has closed up passes enough for one fixture and not two. Galvanized pipe does exactly this.
- A part-closed valve. Throttling barely shows at low demand and shows badly at high demand. Check both valves.
- A pressure-reducing valve struggling to keep up. More here.
- Well pressure tank. A waterlogged tank cannot buffer demand, so pressure swings as the pump cycles.
Why the distinction matters
It changes the fix. A pressure problem may be solved by adjusting a valve. A flow restriction usually is not — it means something in the path has to be opened up or replaced. Telling a contractor "55 static, drops to 20 with two taps" is far more useful than saying the pressure is bad.
