Quick Answer:
You’ll see normal pressure at a single faucet, but when multiple fixtures run (showers + laundry, etc.) the pressure drops because the pressure‑reducing valve (PRV) can’t pass the required flow. Confirm by measuring static pressure, then open 2–3 fixtures at once to watch flowing pressure collapse. Check the PRV’s flow capacity and its inlet screen for clogging; if the valve can’t pass the demand, replace it with a properly sized unit.
Why This Happens
A PRV reduces supply pressure to a set level, but it also restricts flow. With low demand (one fixture) the valve holds set pressure. Under higher simultaneous demand, flow through the PRV increases and pressure downstream can fall if the valve is:
- Undersized for the total demand (many older or small PRVs are 3/4″ and may not pass household peak flows).
- Clogged by sediment caught in the small inlet screen or strainer.
- Worn internally and unable to hold pressure under flow.
Other contributors include long runs, small branch piping and excessive fittings that add pressure loss, but the immediate test is whether the pressure drops only when several fixtures run.
Step-by-Step What to Do
1. Measure static pressure
Attach a pressure gauge to an outdoor hose bib or a drain port downstream of the PRV. With all fixtures closed, read the static pressure. A typical target is 50–60 psi in many homes, but note whatever your static reading is — it should be stable with no flow.
2. Reproduce the problem with flowing pressure
With the gauge still connected, open 2–3 fixtures simultaneously (example: one shower, washing machine fill, kitchen sink). Watch the gauge while those fixtures run.
- If the gauge drops sharply when multiple fixtures run, the PRV or piping is restricting flow.
- If the pressure stays near the static reading, the PRV is passing the demand and look for other restrictions or fixture problems.
3. Inspect PRV size and screen
Locate the PRV (usually near the main shutoff). Check its stamped size — common sizes are 3/4″ and 1″. A 3/4″ PRV often cannot pass peak household flows without a pressure drop. Also find the upstream screen or strainer (a small removable basket or mesh) and inspect it.
- Turn off the main supply, relieve pressure downstream by opening a faucet, then remove and clean the screen if present.
- Note visible corrosion, debris or a damaged seat — these are signs the valve may need replacement.
4. Decide whether to replace the PRV
If cleaning the screen does not restore flowing pressure, or the valve is undersized (3/4″ limiting flow) or shows wear, replace it. When replacing, choose a valve sized for the peak flow you need (1″ or a valve with published flow capacity) rather than simply matching pipe labels.
If you change valve size, remember the entire plumbing system’s fittings, meter and main pipe size affect flow. If in doubt, replace the PRV with a higher‑capacity model or call a plumber for a flow calculation.
What Not to Do
- Don’t size a replacement PRV by the pipe’s printed label or by eyeballing the pipe alone — demand flow and pressure losses matter.
- Don’t force or overtighten internal PRV parts when cleaning; that can damage the valve and cause leaks.
- Don’t ignore other possible restrictions (kitchen or laundry supply valves, old washers) before replacing the PRV — but do not assume the PRV is fine just because static pressure looks normal.
When to Call a Professional
Call a licensed plumber if you can’t isolate the valve, if the PRV requires replacement and you’re not comfortable working on the main, or if you need a larger valve installed to match household peak flows. If you see loud banging or suspect the valve change will affect system dynamics, consult a pro — read more about PRV causing hammer in pipes for what to watch for.
Safety Notes
- Always shut off the water at the main before removing a PRV or screen. Open a downstream faucet to relieve pressure.
- Wear eye protection and gloves when working with pressurized plumbing and when cleaning debris.
- If you’re unsure about isolating the system, or the water meter and street valve are involved, stop and call your water utility or a plumber — don’t work on a live high‑pressure main.
Common Homeowner Questions
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Q: Why does the pressure drop only when multiple fixtures run?
A: Because the PRV or piping can’t pass the combined flow; single fixtures don’t demand enough to show the restriction. -
Q: Can I just open the PRV all the way to fix it?
A: No — fully opening the adjusting screw can raise downstream pressure dangerously and won’t fix a valve that is physically undersized or clogged. -
Q: What static pressure should I aim for after replacement?
A: Many homes set PRVs to 50–60 psi, but set the valve to the pressure that balances comfort, appliance requirements and local recommendations.