Quick Answer:
If loud banging (water hammer) starts only after the curb stop is used, the most likely causes are trapped air and a rapid pressure change when the curb stop is reopened. First try to purge air from the system, then reopen the curb stop slowly and, if needed, have arrestors installed. For related issues see Air in pipes after city water shutoff and Meter valve squeals under load.
Why This Happens
When the curb stop is closed and then reopened, the flow of water and any trapped air can move suddenly. That sudden change in flow produces pressure waves that make pipes bang. Common contributing factors:
- Air pockets trapped in high points of the system.
- A valve opened too quickly that creates a pressure surge.
- Loose pipe supports or long runs that amplify the noise.
Step-by-Step What to Do
1. Confirm the pattern
Make sure the hammer happens only after the curb stop is used. Note whether it starts immediately when you open the curb stop or after certain fixtures are turned on.
2. Purge air from the lines
- Open the highest and lowest faucets in the house (or an outdoor hose spigot) to give trapped air a place to escape.
- Let water run until the flow is steady with no sputtering, which indicates air has been cleared.
- Work one branch of the system at a time if you have multiple zones.
3. Reopen the curb stop slowly
- When you turn the curb stop back on, do it gradually—open a little, wait for pressure to stabilize, then open more.
- Avoid snapping the valve fully open in one quick motion; a slow approach reduces pressure shocks.
4. Check pipe supports and visible fittings
- Look for loose straps, hangers, or fittings and tighten or secure them where safe to do so.
- Loose pipes make the noise louder and allow movement that can damage joints over time.
5. Install water hammer arrestors if needed
- If banging continues after purging and slow reopening, install arrestors near fixtures or at strategic points where the shock is strongest.
- Arrestors are available in different sizes; a plumber can match the right type to your system.
What Not to Do
- Do not snap curb stop shut. Doing so can create strong pressure surges and damage valves, the meter, or pipe joints.
- Do not force or hammer pipes into place yourself; that can cause leaks or fractures.
- Do not attempt major work on the curb stop or meter if you are not licensed—those are often the water company’s responsibility or require a plumber.
When to Call a Professional
- If the banging keeps coming back after you purge air and reopen slowly.
- If you see leaks, wet spots, bulging pipes, or hear ongoing knocks when no fixtures are running.
- If the curb stop, meter, or main service line needs adjustment or replacement—leave that to the water utility or a licensed plumber.
Safety Notes
- Work carefully when opening or closing the curb stop. Sudden moves can cause pressure spikes and damage.
- Turn off fixtures and relieve pressure before attempting any repairs. Wear eye and hand protection if you’re working near exposed pipes.
- If the curb stop is at the property line, contact your water utility before doing anything that affects the meter or service line.
Common Homeowner Questions
-
Why does the noise only start after I use the curb stop?
Because closing the curb stop lets air collect or changes flow conditions; reopening can create a surge that moves that air and causes banging. -
Will installing arrestors stop the problem?
Often yes for recurring water hammer, but first purge air and confirm the valve is being opened slowly—arrestors are a common permanent fix. -
Can I fix this myself?
You can try purging air and reopening the valve slowly. For persistent issues, meter/curb stop work, or leaks, call a licensed plumber or the water utility.