Hammer arrestor causing water hammer?

29 Dec 2018
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Pontypridd, Wales
United Kingdom
This may sound odd but I wonder if it rings any bells. On recent threads to solve a water hammer problem, I fitted a hammer arrestor on the attic feed pipe. Turns out it was not needed as it was a float valve in the tank.

All seemed to be well until I turned off the water to the attic and forgot to tell my wife. She mentioned that there was no hot water and I twigged what had happened. Turning on the attic feed and all appeared well until she put a load of washing in the machine. We now have a louder, more pronounced hammer each time the machine fills, or when I flush one particular toilet.

Retracing my steps, I wondered if there was some air trapped so I shut off the main house feed, ran all the taps, and then refilled. This helped briefly but the hammer came back again. I don't know why but I decided to turn off the attic feed and this seemed to cure the problem. No hammer from the washing machine nor the toilet in question. So that leads me to wonder if there is something wrong with the hammer arrestor I fitted:

This has only been an issue in the last couple of days and the arrestor has been on there for a week. Could it be trapped air somewhere that needs to get out? As a stopgap, I have turned on the attic feed, but only a fraction to stop the hammer. I will try and increase this over the next day or so to see if it makes the difference and this is going to slow the filling of the hot water tank dramatically. The other thing I thought was to take the hammer arrestor off completely as it is surplus to requirements?
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The toilet could be the float valve inside this. Washing machines do produce hammer, as they have solenoid valves that just stop the water dead.

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