Quick Answer:

If the pressure gauge by the pressure reducing valve (PRV) needle chatters even during steady flow, run one steady fixture and watch closely. If the needle continues to jump, common causes are cavitation, a torn diaphragm inside the valve, or sediment in the inlet. Clean any strainer you find and plan to replace the PRV if instability persists.

Why This Happens

Needle chatter or a fluttering gauge usually means the pressure at the PRV is changing rapidly at a small scale. Typical causes are:

• Cavitation — vapor bubbles form and collapse inside the valve when inlet pressure and flow conditions create unstable forces.
• Torn or worn diaphragm — the PRV’s internal diaphragm can fail and react poorly to steady flow.
• Debris or sediment — bits of scale or grit can lodge near the seat or screen and make the valve cycle quickly.
• System vibration or water hammer — less common with a steady single-fixture flow, but possible with upstream issues.

Watch for signs such as noise, vibration, or short, repeating swings of the needle. If you see the gauge jump during a steady draw, that’s more telling than a quick twitch.

Step-by-Step What to Do

Step 1 — Prepare and choose a steady test flow

Pick one fixture that gives a steady, continuous flow—run the tub spout at a steady stream for the test. This is the “one steady flow (tub spout)” check: do not look at the gauge while turning multiple fixtures on and off.

Step 2 — Watch the gauge during that steady flow

Observe the gauge for at least 30–60 seconds. A single brief twitch can be normal; sustained chatter or repeated jumps means instability.

Step 3 — If the needle chatters, suspect cavitation or a torn diaphragm

Needle chatter under steady flow often indicates cavitation or a failing diaphragm inside the PRV. Note any accompanying noise—rattling, humming, or a high-pitched ring—which supports internal valve problems.

Step 4 — Check for sediment and inlet screen issues

Shut off water to the PRV if you are comfortable doing so and check any inlet screen or strainer. Flush sediment from the line or clean the screen per the valve manufacturer’s basic instructions. Small debris can cause rapid cycling.

Step 5 — Re-test and consider replacement

• Restore flow and run the same steady tub spout test again.
• If instability persists after cleaning, replacement of the PRV is often the practical solution—especially if the diaphragm is suspected to be torn.

What Not to Do

• Don’t diagnose from a twitchy gauge alone—test under steady flow conditions.
• Don’t try complex internal repairs unless you have the right valve manual and experience; some PRVs are not serviceable and are safer to replace.
• Don’t ignore repeated noise or hammering—those can damage plumbing and fixtures over time.

When to Call a Professional

• Persistent instability after cleaning the strainer or flushing the line.
• Strong noises (rattling, whine, or continuous vibration) or suspected torn diaphragm.
• Multiple fixtures affected, wide pressure swings, or if you’re unsure how to safely shut down or isolate the valve.

For ongoing issues, a licensed plumber can confirm whether the problem is valve failure, cavitation from upstream pressures, or a larger system issue like incorrect supply pressure. If you’re hearing unusual sounds from the valve, also consider checking related articles on gauge readings unstable and PRV buzzing or humming.

Safety Notes

• Shut off the main or the appropriate isolation valve before opening or removing a PRV. Relieve downstream pressure first.
• Wear eye protection and gloves when working around pressurized water.
• If you are not confident isolating plumbing or replacing a valve, call a plumber. Incorrect work can cause leaks or flooding.

Common Homeowner Questions

• Why does the gauge move a little sometimes? Short, single twitches can happen with transient pressure changes and are not usually a sign of failure.
• Can I repair a PRV diaphragm myself? Only if the valve is designed for field service and you have the correct replacement parts and instructions; otherwise replacement is safer.
• Will flushing the system always fix it? No — flushing or cleaning inlet screens helps if debris is the cause, but a torn diaphragm or cavitation usually requires valve replacement or a system pressure change.