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Water hammer

‘Water hammer’ refers to the banging noises which sometimes come from plumbing pipework, it's normally one or a series of banging noises coming from the pipework usually when a tap or valve is turned off.

It is usually caused in high pressure (i.e. mains pressure) water systems either when a tap is turned off quickly, or by fast-acting valves on appliances, which suddenly stop the water moving through the pipes and sets up a shock wave through the water which causes the pipes to vibrate and ‘shudder’.

‘Water hammer’ will be made worse by having worn valves and pipework inadequately supported.

Not only is the noise annoying, but it can lead to damage to joints, so ‘water hammer’ should be cured rather than tolerated.

Causes and cures of 'water hammer'

There are four probable areas to look at for the cause of ‘water hammer’, and a number of possible cures - the appropriate cure will depend upon the actual cause and the installation.  If the ‘water hammer’ only started after some work was done on the plumbing system or a new appliance installed, start by looking at those areas; however, any change to a plumbing system may start ‘water hammer’ in another area.

Probable sources of ‘water hammer’

  1. Inadequately secured pipework – more likely to cause ‘water hammer’ after new work has been done; if ‘water hammer’ first occurs without new work having been done, check the other possible cures first.
  2. Ball and float valves.
  3. Washing machines and Dish washer.
  4. Worn stop valves.

Unsecured pipework

Mains plumbing pipework needs to be adequately clipped to the surface it runs along, so check along the pipework that it is tightly secured at no greater spacing than every 2 metres.

Don’t forget the pipe runs under the floorboards and any boxed in pipework – in fact, these are possibly more likely to have been left inadequately secured than visible pipes, so you may want to look at these first despite the additional work involved in lifting floorboards etc. Ensure that pipework is clipped, secured and supported at regular intervals using pipe clips of the appropriate size.

Ball /float valves

‘Water hammer’ can result from ripples inside open water tanks where the water level is controlled by a ball/float valve – the ripples being caused by the inflow of water with the result that the valve float ‘bobs’ up and down thus repeatedly opening and closing the valve. This repeated opening and closing of the valve sets up shock waves which reverberate along the pipework causing the ‘water hammer' effect.

If the water tank is plastic, check that a metal reinforcing plate was fitted on the side of the tank when the float valve was installed (see illustration below) - this reduces the flexing of the tank.

Other than the above, ripples in the water will usually only cause problems if the ball/float valve is worn or if the valve has a low pressure valve nozzle fitted when the water supply is high pressure.

Ensure that the washers and diaphragms in ball/float valves are not worn and that the appropriate valve nozzle is fitted – the latter shouldn't be the problem if the ‘water hammer’ started some time after the valve was fitted.

If checking and repairing the valves don’t cure the problem, one solution may be to fit an ‘equilibrium valve’ instead of the usual valve in the tank or cistern. An ‘equilibrium valve’ uses the inlet water pressure to help close the valve, however they are not all that easy to purchase in the UK.

water tankOne way to dampen the movement of the float arm is to attach a small plastic yogurt pot to the float arm using a piece of fairly stiff stainless steel or galvanised wire. Suspend the carton from near the ball so that the top of it is just under the water level. The pot full of water should act as a damper to stop the arm ‘bobbing about’ without interfering with the operation of the valve.


Fitting a Torbeck cistern valve (they are available as either bottom or side entry to suit most types of cisterns) is another possible cure, it will prevent ripples on the water surface so the valve will close cleanly.

Washing machines and Dish washer

The water supply to both of these machines will be controlled by electrically operated solenoid valves which stops the flow of water instantaneously, so possibly setting up the shock wave through the pipework to cause the ‘water hammer’ – however, in most cases the flexible hoses attaching the appliance to the water supply are flexible enough to absorb any shock in the pipework.

Stop valve

Stop valves (stopcocks) and taps can cause ‘water hammer’ if they have loose gland packing and/or worn washer jumpers – taps are actually unlikely to be a problem as they would be closed when the noise occurs.

However, stop valves will generally be open when the ‘water hammer’ shock wave travels through the pipework and the shock wave could well ‘rattle’ the valve handle and a loose jumper.

While it is possible to tighten the gland packing and replace a loose jumper, the easiest way to cure the problem is just a case of replacing stop valve.

When all else fails.

There are a number of Water Shock Arrestor on the market – these are relatively simple devices which trap air within the water pipes so that the shock wave in the pipework is absorbed. The unit needs to be installed as close to the source of the problem as possible. Some arrestors include a valve where a tyre pump can be connected to add air as necessary. Water Shock Arrestors are not that common in the UK, you'll probable need to go to a Plumbers Merchant rather than the local DIY store.

It is possible to make a home made Water Shock Arrestor:

  • Take a 30cm (12in) length of 22mm copper tube and cap one end.
  • Connect this vertically into the existing pipework using a compression a T-joint as close to the source of the ‘water hammer’ as possible so that air is trapped in it.


This will act as a shock absorber in the pipework, however, overtime the air will be absorbed into the water so the system will periodically need to be drained down and capped pipe removed so it can be refilled with air.