Water Hammer
Updated February 20, 2026
Loud banging when a valve closes suddenly. Shockwave can damage pipes, loosen fittings, and stress connections. Fix: hammer arrestors, recharge air chambers, or reduce pressure.
Overview
Water is incompressible. Flowing at speed and suddenly stopped, kinetic energy becomes a pressure wave -- up to 600 PSI in severe cases. Far more than residential pipes handle. Single bang unlikely to damage, but repeated hammer weakens joints, loosens fittings, leads to leaks or bursts. Well-established fixes, mostly inexpensive and DIY.
Symptoms
- Loud bang when a faucet, washer, or dishwasher valve closes
- Happens when flow stops suddenly -- not while running
- Pipes visibly shake in the basement or crawl space
- Worse at certain fixtures, may not happen at every faucet
- Drips or moisture appearing at joints that were previously dry
- Started or worsened after water was shut off and restored (repair, vacation)
Common Causes
- Missing or failed arrestors -- code requires them at quick-closing valves. Many older homes have none. Existing ones fail when the internal bladder wears out.
- Waterlogged air chambers -- pre-1990s homes use pipe stubs instead of arrestors. Air pocket cushions the shock, but gets absorbed over time. Chamber fills with water, no cushioning.
- High pressure (over 80 PSI) -- more kinetic energy, more violent hammer, more damage.
- Quick-closing solenoid valves -- washers, dishwashers, ice makers snap shut in a fraction of a second. Much more severe than a hand-operated faucet.
- Loose pipes -- not strapped to framing, they move when the wave hits. Amplifies the bang and stresses joints.
What You'll Need
How to Fix It
-
Identify Which Fixture or Appliance Triggers the Hammer
Run each fixture and appliance one at a time, listen. Washing machine (especially between fill cycles), dishwasher, single-lever faucets shut off quickly, toilet fill valves. Source tells you where to install the fix.
Tip: Only during the washing machine? Solenoid valves on the supply hoses. Single most common source of residential water hammer. -
Recharge Air Chambers (Older Homes)
Main water off. Open highest and lowest faucets to drain the system. Air enters as water drains. All fixtures stop flowing? Close everything, main back on. Pipes refill, air chambers retain a pocket at the top. Cushioning restored. Test.
Tip: Free, 15 minutes. But air chambers waterlog again eventually. Returns repeatedly? Mechanical arrestors are the permanent fix. -
Install Water Hammer Arrestors
Sealed chambers with a piston or bladder that absorbs the shockwave. Industry-standard permanent fix. Washing machine: pair of screw-on arrestors between the supply valves and hoses, under $15 each. Other fixtures: closest accessible point on the supply line. Size to pipe diameter (1/2 or 3/4 inch).
Warning: ASSE 1010 certified (AA rated). Cheap unrated devices may not work. Sioux Chief and Watts are reliable brands. -
Check and Reduce Water Pressure
Pressure gauge on a hose bib. Above 80 PSI? PRV needed or existing one needs adjustment. Counterclockwise on the adjustment screw reduces pressure. Aim for 50-60 PSI. No PRV? Plumber installs on the main line.
Tip: Test at different times. Municipal pressure fluctuates -- acceptable during peak but may spike late at night. -
Secure Loose Pipes
Exposed pipes: straps or clamps every 6-8 feet on horizontal runs, every direction change. Cushioned clamps (not bare metal). Foam insulation where pipes pass through framing holes. Long horizontal runs are the priority.
Tip: Banging inside a finished wall? Arrestors at the nearest accessible point upstream is more practical than opening the wall. -
Verify the Fix
Test every trigger. Washing machine through a full cycle, faucets opened and closed quickly, dishwasher. Noise gone? Done. Some hammer remains? Additional arrestors at other locations, or address both pressure and arrestors together.
When to Call a Pro
Call a plumber for PRV installation, if hammer persists after arrestors and pressure reduction, if banging is inside finished walls, if joints are already dripping from hammer damage, or for complex manifold systems. Pressure analysis can determine exact severity.
Prevention Tips
- Arrestors at every quick-closing valve. Washer, dishwasher, ice maker are the priority.
- Pressure at 50-60 PSI. Single best way to reduce hammer severity system-wide.
- Close faucets gradually. Single-lever faucets go full to off in one motion -- slow down.
- Strap all accessible pipes with cushioned clamps. 6-8 feet on horizontal, every direction change.
- Replacing appliances? Models with slow-closing solenoid valves reduce hammer.
- Drained the system? Air chambers need recharging before normal use.