When are water hammer arrestors required?
Summary (TL;DR)
AS/NZS 3500.1 does not strictly mandate water hammer arrestors on every job, but it does require that plumbing systems be designed and installed to avoid excessive water hammer. If hammer occurs due to quick-closing valves (like washing machines or flick mixers), an arrestor should be installed as close to the offending valve as possible.
Preventing Water Hammer (AS/NZS 3500.1)
Water hammer (hydraulic shock) occurs when flowing water is suddenly stopped by a quick-closing valve. The shockwave rattles the pipes, causing noise and potential damage to fittings.
Primary Prevention Methods
Before installing an arrestor, AS/NZS 3500.1 requires you to address the root causes of water hammer:
- Control the Pressure: Ensure the static pressure does not exceed 500 kPa. High pressure exacerbates water hammer. Install a PLV if necessary.
- Secure the Pipes: Ensure all pipes are clipped at the correct intervals according to the standard. Loose pipes will bang against framing.
- Size Pipes Correctly: Keep water velocity below the maximum limits (usually 3.0 m/s) by using adequately sized pipes.
When to Install an Arrestor
If pressure is controlled and pipes are secured, but hammer still occurs (often caused by washing machine solenoids, dishwasher valves, or ceramic disc flick mixers), a water hammer arrestor should be installed.
Installation Rule: The arrestor must be installed as close to the quick-closing valve as possible to absorb the shockwave before it travels through the pipework.
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