Quick Answer:
If water hammer—loud banging or a sharp knock—happens only after you move the main valve, it’s usually a pressure surge or trapped air reacting to sudden flow changes. Check the valve operation, reopen it slowly, purge any trapped air, and consider installing arrestors to absorb shocks.
Why This Happens
Water hammer is a pressure wave that travels through the pipes when flow is started or stopped quickly. When the main valve is moved, even a partial closure or quick reopening can create a sudden change in flow velocity. That sudden change causes a pressure spike that makes pipes bang or vibrate. Trapped air pockets amplify the noise because they compress and release unpredictably. A worn or partly closed valve can also cause vibrations long after you touch the valve—if you see continuous noise rather than a single knock, read about main shutoff buzzing or vibrating for related causes.
Step-by-Step What to Do
1. Confirm the pattern
- Note exactly when the hammer occurs: only immediately after you open or close the valve, or continuously afterward.
- Try a quick test: close and then reopen the valve slowly and listen. This helps confirm the valve manipulation link.
2. Reopen slowly
- Open the main valve slowly over 10–20 seconds rather than snapping it. Smooth movement reduces the pressure wave size.
- If the valve is stiff, stop and address the stiffness—forcing it can damage the valve or piping.
3. Purge trapped air
- Bleed high fixtures first: open faucets at the highest point in the house, then work toward lower fixtures until water runs steady and free of sputtering.
- Flush toilets and run showers briefly to move air pockets out of the lines.
4. Install arrestors or cushions
- Add water hammer arrestors near appliances or valves that cause repeated shocks (dishwashers, valves, pumps). These are designed to absorb the pressure spike.
- For older systems, consider adding arrestors at several strategic points rather than just one location.
5. Test and observe
- After purging and adding arrestors (if installed), operate the valve slowly a few times to confirm the noise is gone or reduced.
- If the hammer still occurs only after valve movement, note the pattern and proceed to the next checks or call a pro.
What Not to Do
- Do not slam the valve closed. That makes pressure spikes worse and can damage the valve and piping.
- Do not force a stuck valve—forcing can break the stem or cause a sudden uncontrolled release of pressure.
- Do not ignore repeated loud bangs; ongoing stress can loosen pipe hangers and fittings, creating bigger problems.
When to Call a Professional
- Call a plumber if the hammer continues after you reopen slowly and purge air, or if you cannot operate the main valve safely.
- Call a professional if you notice leaks, damaged pipe hangers, or persistent pressure problems.
- If the valve movement causes other unusual symptoms—like hot water shutoff only affecting one set of fixtures—see main shutoff only stops hot water and mention this behavior to the plumber.
Safety Notes
- Wear eye protection and gloves when working on valves or bleeding lines. Water can be hot and pipes may be under pressure.
- Shut off nearby electrical panels or appliances if water is likely to spray near them.
- If you are unsure about depressurizing the system or replacing parts, stop and call a licensed plumber.
Common Homeowner Questions
- Why did it start after I touched the valve? A sudden change in flow triggers pressure waves; moving the valve even a little can create that change.
- Will arrestors fix it permanently? Often they reduce or eliminate hammer, but correct sizing and placement matter for a lasting fix.
- Can I do this repair myself? Simple purging and slow valve operation you can do, but installation of arrestors or any pipe repairs may be best handled by a plumber.