Water hammer is usually caused by the rapid shut-off of water which is flowing in pipework. Water is heavy and builds up momentum as it travels and suddenly stopping the flow causes the moving water to 'pile up' at the obstruction (valve) causing a local sharp pressure spike. As the moving water slows to a stop, the high-pressure region at the closed valve now bounces back up the water in the pipe, traveling considerable distances throughout the house pipework as a pressure wave. It can bounce back and forth several times before finally decaying and this is why you can hear it in many places within the property, not just at the origin, and the pipes can often move about as if someone was hitting them. It's worse in rigid pipework (copper) and can be caused by quarter-turn ceramic insert taps being closed too quickly, washing machine solenoid valves, but most commonly by old style (BS1212 Part 2 &3) direct-acting float valves in either toilet cisterns or in storage cisterns in the loft/attic. Even modern equilibrium float valves (BS1212 Part 4) can cause water hammer in certain unfavourable circumstances.
If you have water hammer, first check for any float valves you have in the house. Work out when it happens - if it's after you flush, check the WC valve. If it's after you draw off hot water (or cold water in the bathroom) check the attic for a cold water storage cistern. If the valve is old, and the water supply has a decent pressure (more than 1 bar) and is clean (no regular grit or other debris) then replace the valve with a modern BS1212 Part 4 equilibrium valve (e.g. Fluidmaster, Torbeck). Check that the discharge gap between the valve outlet and the final water level in the cistern isn't too large (must be at least 20mm by regulation, for 15mm pipework) since too great a gap can cause waves during filling, which tend to oscillate the valve's float, inducing continuous water hammer as the cistern approaches full and the valve begins to close.
If it happens after the washing machine stops filling, try replacing the machine's cold water inlet hose for a new one (they're cheap) since they are intended to absorb some of the pressure spike that occurs whenever the fast-closing solenoid inlet valve shuts off. If that doesn't fix it, you can install a 'mini expansion vessel/arrestor' close to the end of the pipework supplying the washing machine. If you do so, make sure it's the proper WRAS approved type, suitable for drinking water (it should be blue or silver, and not red which is only for central heating).
You can also easily check your cold water supply pressure, since the test gauges are cheap these days. If it is over 3bar, you might well consider fitting a decent pressure reducing valve to the incoming supply, and reduce your supply pressure to 2-3bar which will will be much kinder to your valves and fittings and will help reduce the risk of water hammer. Avoid the cheapest ones since the performance will be correspondingly poor - the Honeywell one (black stem, green knob) works well.
Finally, be thorough in your investigation. In an old property I worked on, all the ball valves were of a similar vintage and they all had an interaction with each other. When I replaced one noisy one, immediately another one (on the same pipework but upstairs) started to hammer, and turned out to be the actual culprit which was inducing the first one to hammer by means of a travelling pressure spike! So in this scenario, don't give up if you don't fix it first time.
Sparks (the Plumber), May 2018
