What It Is
A water hammer arrestor is a sealed cylinder containing a spring-loaded piston backed by compressed air or nitrogen. Installed near a valve or fixture, it absorbs the hydraulic shockwave when water flow suddenly stops. Unlike old-style air chambers (capped vertical pipe sections), modern arrestors are hermetically sealed so they cannot waterlog over time and lose their cushioning ability.
What It Does
When a valve closes and the water column slams to a stop, the pressure spike travels back through the pipe. The arrestor sits adjacent to that flow path. The surge pressure compresses the piston against the gas chamber, absorbing the energy of the shockwave. The piston then resets as pressure normalizes. The result is that the shockwave is dampened before it reaches the rest of the piping system.
Where It Is Located
Arrestors are installed as close as possible to the source of the hammer - typically at the supply connection for the washing machine, dishwasher, ice maker, or any solenoid valve that closes quickly. They screw onto a standard fitting in the supply line, either inline or on a tee fitting. The orientation (vertical, horizontal, or angled) does not affect their function.
How It Works
Inside the cylinder, a sealed piston divides the device into two chambers. One side connects to the plumbing supply line. The other side contains compressed air or nitrogen sealed permanently at the factory. When a pressure surge enters the device, it pushes the piston, compressing the gas. The compressed gas acts like a spring, pushing back as soon as the surge passes, resetting the piston. This cycle happens in milliseconds with each valve closure.
Sizing matters - arrestors are rated by flow rate and water pressure. Installing an undersized arrestor for a high-flow appliance won't fully absorb the shockwave. Plumbing codes specify arrestor sizing based on fixture units.
Common Problems or Signs of Failure
- Water hammer returns after the arrestor was installed - arrestor may be undersized or in the wrong location
- Arrestor becomes waterlogged - rare with modern sealed units, but the gas charge can leak over many years
- Thread seal fails and the arrestor leaks at the connection
- New appliance installed (faster-closing valve) overwhelms an existing arrestor sized for lower flow
Can a Homeowner Check It?
- Listen for water hammer after a specific fixture is used - if it continues after arrestor installation, check the location
- Confirm the arrestor is installed as close to the solenoid valve as possible, not further away on the line
- Check the connection for drips or moisture - a leaking thread seal is a simple fix
- Shake a suspect arrestor - if it feels completely solid (no movement), the piston may be stuck
When to Call a Plumber
Call a plumber to properly size and install water hammer arrestors, particularly for washing machines and dishwashers. A plumber can also determine whether the root cause is actually high system pressure (better addressed with a pressure-reducing valve) versus a fast-acting valve that needs a local arrestor. Sometimes the right solution is both.
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