Further expansion vessel issues

[Tim+]

I recharged the expansion vessel on my unvented HW system the other day and since then we?ve had occasions when we?re getting a ?water 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 I?m 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.

I?ve checked to see there was air in the pipe up to the vessel but there doesn?t appear to be any. I?ve tried dropping the pressure a bit in the vessel but it hasn?t made any difference.

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

Tim

[Chris B]

Does it sound like this (sound starts at 14 seconds after I have walked to the bathroom and turned on the bath tap)

I can tell you what caused it in my case, the expansion vessel had been over pressured on the water side.

Full story professional plumbing firm completely redid the water/heating system including pressurised hot water.

12 months later same firm redid bathroom

after a couple of months I noticed water coming from the tank vent valves.

Called plumber who said it would be a tank warranty claim and I would have to get in the tanks rep to assess it.

Did that and the tanks rep asked what the cold mains pressure was (ask me one on sport) - so he tested it 7 Bar. Said that modern thermostatic shower mixers (as fitted in the bathroom upgrade) are only good to about

5 bar, and it would have been blowing through there and pressurising the HW and exp tank to 6 bar(the blow off pressure). He said that the HW tank would be fine but the diaphragm in the exp tank would have stretched and it would need to be replaced (along with the tank blow off valve and a pressure regulator fitting to the cold supply).

Original plumber came and fitted the pressure regulator valve and "cleaned up" the blow off valve and checked the pressure in the exp tank

- he said that they would be fine and didn't need replacing despite advice of tank rep.

About 3 months later this noise started and I called the plumber back

To his credit after a bit of complaining he replaced first the blow off valve (as it was cheaper than the exp tank) which had no effect so he also replaced the exp tank. The noise has not returned.

[Tim+]

Cheers. Mine starts at a lower frequency but otherwise very similar. I?ll try dropping the pressure a bit more.

Tim

[Tim+]

Took 4 goes but reducing the pressure in stages seems to have done the job.

Logically I guess the filling pressure was the only thing that I had changed but I just recharged the tank to its factory setting and assumed that had to be right.

Many thanks for the tip.

Tim