Quick Answer:
If you hear a hammering or banging right after a backflow preventer was installed, that’s usually water hammer: a pressure shock when flow stops suddenly. A common fix is to install arrestors or air chambers and to reopen the supply slowly to let the system stabilize. Start with the simple checks below before calling a pro.
Why This Happens
- Backflow devices add resistance and change how valves and fixtures react. When a valve closes quickly, the moving water has nowhere to go and creates a pressure spike — the hammering sound.
- Trapped air pockets or missing arrestors make the noise worse. Over time trapped air can compress and cause repeated knocking.
- If you notice reduced flow along with the noise, it can be linked to changes in system pressure — for example, water pressure low after a new device or adjustment.
- Intermittent closing of a check or relief inside the backflow can also cause pulses you might notice as intermittent pressure loss or banging.
Step-by-Step What to Do
1. Quietly observe where and when the noise happens
- Note if it happens after a specific fixture turns off (shower, washing machine) or right after the main was opened following the installation.
- Listen along pipe runs to identify approximate location — knocks close to a fixture point to localized plumbing; knocks near the meter suggest main-line effects.
2. Close and reopen the supply slowly
- If the installer shut the main quickly, slowly reopen the supply to let pressure equalize. Rapid opening or closing can trap air and create shocks.
- When you next need to operate valves, turn them gently and pause to let flow settle before moving further.
3. Check for and install arrestors
- Look for existing small piston or chamber-style arrestors near quick-closing valves (dishwashers, washing machines) and at branch lines. If missing, install approved arrestors near the fixtures causing the noise.
- For whole-house banging, add water hammer arrestors at strategic points or have a plumber fit a dedicated arrestor on the supply near the backflow device.
4. Secure loose pipes and fittings
- Tighten loose straps and clips — movement amplifies noise. Add cushioning where metal pipes contact wood.
- Check for newly loosened fittings around the installation and tighten to manufacturer torque specs if you are comfortable doing so.
5. Test fixtures one at a time
- Run fixtures individually to see if one appliance or valve triggers the hammer. That helps focus repair efforts (e.g., install an arrestor at that location).
- If the noise disappears after installing arrestors and slowly reopening the supply, the issue is usually resolved.
What Not to Do
- Do not slam isolation valves — rapid valve movement can make water hammer worse and may damage the backflow device or piping.
- Do not remove the backflow preventer yourself to stop the noise. Removing it can violate code and allow contamination of your water supply.
- Do not ignore persistent banging; it can loosen joints or cause leaks over time.
When to Call a Professional
- Call a licensed plumber if the noise continues after installing arrestors and securing pipes, or if you cannot safely access the backflow device area.
- Hire a pro if you detect leaks, visible pipe damage, or if the backflow device needs to be modified to meet local code. Some backflow setups require specific arrestors or pressure-reducing adjustments only a technician should make.
- Contact your water utility or the installer if the device was recently fitted and the problem started immediately after installation — they may need to inspect their work.
Safety Notes
- Turn off water slowly when isolating sections to avoid sudden pressure changes. Use a second person if needed to control valves safely.
- Be cautious of hot water when bleeding lines — open hot taps gradually to avoid scalding.
- Do not attempt to modify or bypass a backflow preventer yourself. These are safety devices intended to protect potable water and are often regulated.
Common Homeowner Questions
- Will the noise damage my pipes? Short answer: yes over time; repeated hammering can stress joints and fittings, so address it promptly.
- Can I install arrestors myself? If you are comfortable with basic plumbing and shutoffs, you can fit many fixture-level arrestors yourself, but larger or code-required installations should be left to a plumber.
- How long after installing arrestors should I expect quiet? Often immediate or within a few cycles of use as trapped air dissipates; persistent noise after installation means further diagnosis is needed.