That loud thudding or banging noise when you turn off a tap or your washing machine finishes filling — that's water hammer. It's more than just annoying; over time it can stress your pipes, joints, and appliances. The good news is there's a straightforward fix: a water hammer arrestor. Here's what you need to know as a New Zealand homeowner.
What Causes Water Hammer?
Water hammer happens when fast-moving water is suddenly stopped or forced to change direction. Modern appliances like washing machines and dishwashers use solenoid valves that snap shut almost instantly, sending a pressure shockwave back through your pipes. That shockwave is what you hear as a bang or series of knocks.
Older NZ homes often have copper or galvanised steel pipes that are rigidly fixed, which makes the noise worse. High mains water pressure — common in many parts of New Zealand — also increases the severity of water hammer.
What Is a Water Hammer Arrestor?
A water hammer arrestor (sometimes called a shock arrestor) is a small device fitted to your water supply line. It contains a sealed air chamber with a piston or bladder inside. When a valve snaps shut and sends that pressure spike through the pipes, the arrestor absorbs the shock by compressing the air inside — a bit like a mini suspension system for your plumbing.
This is different from the old-fashioned air chamber (a capped length of pipe), which was once the standard fix. Air chambers work initially but gradually waterlog over time and lose their effectiveness. A proper arrestor with a sealed piston or bladder maintains its performance long-term.
Do You Need One?
You likely need a water hammer arrestor if you notice any of the following:
- Banging or knocking when a tap, toilet, or appliance valve closes
- Rattling pipes behind walls or under floors
- Vibration felt through walls near the laundry or kitchen
- Repeated joint or fitting failures that your plumber can't otherwise explain
Even if the noise seems minor, repeated pressure spikes can fatigue pipe joints and fittings over time, potentially leading to leaks. It's worth addressing sooner rather than later.
High-Risk Spots in NZ Homes
The most common locations where water hammer occurs in New Zealand homes include:
- Washing machine connections — solenoid valves are the number one culprit
- Dishwasher supply lines
- Toilet fill valves
- Single-lever mixer taps that are turned off quickly
- Any tap or valve on a high-pressure mains supply
How Is a Water Hammer Arrestor Installed?
Arrestors are typically screwed onto the water supply line as close as possible to the offending valve — for example, directly behind the washing machine taps. They come in various sizes and connection types to suit different fittings.
Installation is plumbing work and in New Zealand it must be carried out by a licensed plumber. Under the Plumbers, Gasfitters and Drainlayers Act 2006, connecting or modifying any part of your home's water supply pipework is restricted work. This means it is illegal for an unlicensed person to do it, and uncertified work can affect your home insurance and LIM report.
A licensed plumber will also check your mains water pressure at the same time. BRANZ and the NZ Building Code (clause G12 — Water Supplies) specify acceptable pressure ranges for household plumbing. If your pressure is too high, a pressure-limiting valve (PLV) may be the better or complementary solution.
Other Solutions Worth Knowing About
| Solution | How It Helps | Best For |
|---|---|---|
| Water hammer arrestor | Absorbs pressure spikes at the source | Localised banging at one appliance or tap |
| Pressure-limiting valve (PLV) | Reduces overall mains pressure | High pressure throughout the whole house |
| Pipe securing/re-clipping | Stops pipes rattling in wall cavities | Loose pipes amplifying the noise |
| Slow-close valves | Reduces the speed of valve closure | Toilets, some taps |
Sometimes a combination of fixes is needed, especially in older homes.
What Will It Cost?
As a rough guide only, the cost of having a plumber supply and fit a water hammer arrestor at a single location is generally modest — think in the range of one to two hours' labour plus the cost of the fitting. If a pressure-limiting valve is also required, that's a larger job. Get a few quotes from licensed plumbers to compare; you can find qualified local plumbers on tradedirectory.co.nz.
When to Call a Professional
Always call a licensed plumber for:
- Installing or replacing a water hammer arrestor
- Checking and adjusting mains water pressure
- Installing a pressure-limiting valve
- Investigating repeated pipe joint failures or leaks
Do not attempt to modify water supply pipework yourself. Beyond the legal issue, incorrect work can cause leaks inside wall cavities — leading to costly damage and potential mould problems that affect your home's compliance with the Healthy Homes Standards.
Safety note: If you ever hear a sudden loud bang followed by reduced water pressure or visible dampness, turn off your water at the toby (street valve) and call a plumber immediately — this may indicate a burst pipe rather than simple water hammer.