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Noise and air locks in pipework

How to prevent water hammer and air locks.

On this page:

  • water hammer
  • air locks in water supply pipework

Water hammer

Water hammer (or pressure surge) generally occurs in a high pressure system when the flow of water is suddenly stopped. A sudden fluctuation in flow velocity sets up shockwaves through the pipework, causing the pipe to vibrate making a ‘hammering’ sound. It mostly occurs in metal pipes, although it can occur in plastic pipes.

Fast-acting taps such as lever taps with ceramic disc washers, solenoid valves such as those on washing machines, spring-closing valves and pumps are often a cause of water hammer. It is related directly to the water velocity – the faster the water travels, the greater likelihood of water hammer.

It is better to prevent water hammer than trying to fix the problem once a building is complete.

To reduce the likelihood of water hammer:

  • avoid direct contact of pipes with the structure
  • clip pipes with rubber insulated clips or clip over the pipe insulation
  • fix pipework rigidly to prevent movement
  • provide relief bends or flexible sections of pipe to absorb shock
  • fit grommets or cushioned packers where pipes pass through structural members
  • get a licensed plumber to install a water hammer arrestor on the pipe upstream from the device (such as dishwasher or washing machine) that is causing the problem. These devices use a piston and air pressure to absorb the surge in pressure
  • if you suspect that too-high water pressure is contributing to the problem, ask a licensed plumber to install a 350 kPa pressure-limiting valve on the water supply. Check that this is compatible with your plumbing fixtures and appliances. (Mains water pressure can be up to 900 kPa or even more in a few locations).

Air locks in water supply pipework

If air enters a water supply system, it will accumulate at high points and can restrict the flow of water. If there is not enough pressure to push the air bubble through the pipe, the air lock will remain until the pipeline is manually purged.

Air may enter the system from:

  • a cylinder vent
  • the tank if it runs low
  • water as it is heated.

Low pressure pipes should be graded to allow air to exit from predetermined high points and to prevent air locks from occurring.


Updated: 26 November 2020