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Loud vibration from unvented hot water tank.

Discussion in 'Plumbing and Central Heating' started by Mark_Hooker, 9 Apr 2020.

  1. Mark_Hooker

    Mark_Hooker

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    Hi. Over the past couple of months the hot water system in my house has started making an incredibly loud reverberation when a hot water tap is opened. I would liken it to the sound of a ruler being twanged on a desk. It is very loud and fades to silence over a period of about 3 or 4 seconds when a hot water tap is opened. It does not do it every time but usually when no hot water has been used for a couple of hours and a bit of pressure has built up in the unvented system. I can feel the vibration in the pipe that leads to the pressure vessel and I can also feel the vibration in the outlet pipe at the top of the tank. I have searched other posts within these forums where a ballcock was identified as the likely source of the problem. This being an unvented system it has no cold water storage. I would be grateful for any suggestions as to what the problem might be or any diagnostics I could try which would be safe for a DIYer on this type of system.
     
    Last edited: 9 Apr 2020
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  3. lightning

    lightning

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    Check the expansion vessel , also where is the expansion vessel sited
     
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  4. Mark_Hooker

    Mark_Hooker

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    Thank you. I attach a picture. It is the white expansion vessel through which i can feel vibration. I cannot be certain that this is the source of the sound. The tank and both red and white expansion vessels were replaced in July 2019. The previous tank had the vessel built in which had failed after 20 years service.
     

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  5. Nozzle

    Nozzle

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    The vessel needs repressurising to ~3.5bar.

    Isolate the waterside inlet.
    Open the hot tap and allow to reduce to a dribble.
    Attach a bicycle pump to the air valve.
    Pump.
    Measure pressure.
    Once satisfactory, shutoff hot tap, re-open inlet valve

    Edit - looks like the air valve will be hard to access though (on top)

    Nozzle
     
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  6. Mark_Hooker

    Mark_Hooker

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    Thank you for your response. There is about a 2 inch clearance at the top where I can feel a plastic cap. I take it this will unscrew? I have an electric pump for the car with a right angled connector which will hopefully fit. It can also be set to cut off at the desired pressure. Is it common for these vessels to lose their pressure? I will update this thread with the result over the weekend. Best wishes and thanks again.
     
  7. Nozzle

    Nozzle

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    They are supposed to be repressurised when your boiler is serviced, but it's easy to check yourself so no need to wait an entire 12 months. There's a rubber bladder inside which pushes against the water and being as the air is compressible it can expand and contract as required when the system goes hot/cool. An added bonus is that it can absorb the energy from a water hammer event, which is what you're describing.

    When the bladder is collapsed though, the expansion vessel is "water solid" and therefore there is no room for expansion. You might also see water dripping inside the tundish and out through a pipe in an external wall (low down, not over head)

    Nozzle
     
  8. Mark_Hooker

    Mark_Hooker

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    Thank you for the advice. Due to the height of the pressure vessel installation it was not possible for me to check the pressure without it being removed. The installing plumber came back and removed it. The pressure was checked and found to be correct. Plumber took technical advice from the manufacturer of the unvented system and assessed that the problem might be with a defective mixer tap in the house allowing a flow of water between hot and cold water circuits. This in turn was causing the pressure balancing valve to oscillate in an attempt to balance the pressures. In order to test this we isolated the cold water feed to the bathroom basin mixer tap (one of many mixer taps in the house) and the vibration stopped and did not return for the 48 hours during which the cold water to the mixer tap was isolated. This is not a conclusive diagnosis as in fact when the cold water feed was switched back on again, the vibration remained absent. Some days later I noticed that the pressure balancing valve was now venting water to the outlet shared with the tundish. Plumber thought that weeks of violent vibration may have damaged the pressure balancing and so it was replaced under warranty. It has been about 3 weeks now and the noise has not returned. If anyone reads this and thinks they may have the same problem as me, consider the possibility that the fault may not be the source of the noise and set about isolating cold water to each of your mixer taps. Or the problem might be that the pressure balancing valve is broken. Good luck and thanks again for advice given in the thread.
     
  9. DIYnot Local

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