What is a water hammer arrestor?
A water hammer arrestor is a small device in the water supply that absorbs the pressure surge of water hammer. Inside is a compressible cushion of air or gas, separated from the water by a piston or diaphragm. When a surge arrives, the water briefly compresses the cushion, flattening the pressure peak so the bang disappears or is much reduced.
Types
- Piston or diaphragm arrestor: a sealed gas charge behind a piston or diaphragm. This is the standard type and lasts, because the gas cannot dissolve into the water.
- Air chamber: a simple capped length of pipe filled with air. It works at first, but the air gradually dissolves into the water, the chamber fills up and stops working.
- Appliance arrestors: made as an adapter between the appliance valve and the hose of a washing machine or dishwasher, so no pipework changes are needed.
Where to fit it
As close as possible to the appliance or tap causing the hammer, because that is where the pressure wave starts. Typical locations are the washing machine and dishwasher connections, under a single-lever mixer tap, or at solenoid valves on irrigation systems and water softeners.
Installation tips
- Follow the orientation given by the manufacturer; many arrestors must be fitted upright.
- Correct size and pressure rating for the pipe diameter and supply pressure.
- Clip the pipes too: an arrestor does not help if the pipe itself is loose and slaps the wall.
- Keep it accessible so it can be checked or replaced later.
- Check the cause: if banging continues, have the supply pressure measured; a pressure reducing valve may be needed.
Related terms
- Water hammer
- Expansion vessel
- Compression fitting
- Union nut
- Pressure pipe
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