Quick Answer:
If loud banging in the plumbing started right after work on the pressure-reducing valve and occurs when valves close or appliances stop, the likely causes are trapped air, water-hammer from fast-closing valves, loose pipe supports, or a PRV that’s creating unstable pressure. Start by bleeding air at the highest faucet and reopen supplies slowly. Verify or add arrestors at quick-closing fixtures, secure loose pipe runs, and if the issue began with PRV work, re-check for overpressure or unstable regulation.
Why This Happens
Water hammer (a pressure surge or shock) happens when flowing water stops suddenly and the momentum creates a shock wave through the pipes. Quick-closing valves—like those on washing machines, dishwashers, or some solenoids—are common triggers. Loose pipes let that energy translate into loud banging instead of being absorbed.
If the banging began after work on a pressure-reducing valve, the regulator can be part of the problem: it may be letting pressure spike, cycling unpredictably, or failing to dampen transients. In some cases the issue is a stuck PRV; see PRV stuck high for related information.
Step-by-Step What to Do
1. Confirm when and where it happens
Note which fixtures or appliances are running when the noise occurs and whether it follows PRV work. Is it every time a valve closes, or only after certain appliances stop? This helps narrow the cause.
2. Bleed air at the highest faucet
Open the highest faucet in the house (often an upstairs sink) all the way and run it until the flow is steady without sputtering. This releases trapped air that can make banging worse. After bleeding, close the faucet slowly.
3. Reopen supply slowly
If you shut off any branch or the main to check the system, reopen it slowly. Rapid reopening can create a pressure surge and recreate hammer. Turn valves a bit at a time until fully open and watch for noise.
4. Add or verify arrestors for fast-closing valves
Check washing machines, dishwashers, and other fixtures with quick shutoff valves. If there are no mechanical water-hammer arrestors or if existing ones are old and waterlogged, install or replace them with modern arrestors designed for your fixtures. Arrestors placed near the valve help absorb the shock before it travels through the plumbing.
5. Secure loose pipes
Inspect exposed runs and tighten loose straps or add cushioning supports where pipes bang against framing. A properly supported pipe absorbs and disperses the energy instead of transferring loud noise into the structure.
6. If hammer began with PRV work, re-check pressure and regulation
Use any installed pressure gauge to check static pressure. If pressure is higher than normal for your home (commonly above 60–65 psi) or if readings swing significantly when fixtures cycle, the PRV may be overpressurizing or unstable. Do not force the PRV apart — if the gauge shows overpressure or unstable behavior after your checks, call a pro to diagnose and reset or replace the regulator.
What Not to Do
- Don’t snap valves shut to test; you’ll worsen hammer and may create leaks.
- Don’t try to disassemble an unfamiliar PRV under pressure — that can cause injury and water damage.
- Avoid using temporary wooden wedges or makeshift supports as a long-term fix for pipe movement; use proper clamps or cushioning straps.
When to Call a Professional
- Persistent banging after you’ve bled air and secured pipes.
- Pressure readings that are high or unstable after PRV work.
- Signs of leaks, damaged fittings, or if the PRV appears to be stuck or leaking.
- You’re uncomfortable working on pressurized plumbing or installing arrestors.
Safety Notes
- Turn off water at the main before tightening fittings or installing supports on supply lines.
- Release pressure by opening a faucet before working on fixtures; watch for hot water and steam.
- If you smell gas, see electrical arcing, or find major leaks, leave the area and call the appropriate emergency service.
Common Homeowner Questions
- How long should I run the highest faucet to bleed air? Run it until the sputtering stops and the flow is steady — usually a minute or two.
- Will adding arrestors stop all banging? They greatly reduce shock from fast-closing valves but may not fix issues caused by overpressure or very loose pipes.
- Can I adjust the PRV myself? Simple checks are fine, but adjustment or repair of a PRV is best left to a professional if you’re not experienced with plumbing regulators.