I recharged the expansion vessel on my unvented HW system the other day and
since then weve had occasions when were getting a €œwater hammer€� that
lasts for about 5-6 seconds. Starts off at a lowish frequency and the
frequency rises until the hammer disappears. Pretty loud and scary and I
really want to stop it happening.

All this only occurs when drawing off hot water quickly for a bath or
shower.

It certainly never used to do this so Im a bit puzzled. The expansion
vessel is linked by about 1.5 metres of pipe in a vertical run down to the
HW outlet from the tank.

Ive checked to see there was air in the pipe up to the vessel but there
doesnt appear to be any. Ive tried dropping the pressure a bit in the
vessel but it hasnt made any difference.

Any other suggestions (or explanations of why this is happening now)?

Tim

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Please don't feed the trolls