Unvented cylinder hot pressure drops

Discussion in 'Plumbers' Talk' started by mikeman, Nov 21, 2020 at 11:54 PM.

  1. mikeman

    mikeman New Member

    Hi guys.
    I have an old ariston direct unvented cylinder with a problem I have not seen before.
    The hot water at the taps, the pressure is good initially but drops to a trickle after a few minutes. I first assumed the filter was blocked, but the cold feed from the balanced output is not affected. I could not remove the filter anyway so I replaced the inlet group. I checked the expansion vessel (1.3B) and recharged it. No change.
    This cylinder has sacrificial anodes in and has probably never been serviced before it went wrong. I can only think the hot pipe being severely restricted causing the problem, anyone any other ideas?

    I should point out I am qualified G3, although not GSR. Thanks
  2. kiaora

    kiaora Screwfix Select

    Have a good look at the hot distribution tapping on the cylinder,

    could be scaled up or blocked,

    I have found old cylinders with sacrificial anodes and the black plastic disk has floated to the top of the cylinder, !

    good luck
  3. mikeman

    mikeman New Member

    Thanks. The tapping is on the side near the top, so unlikely the disk is the problem. I think scale is most likely. I will investigate further.
  4. Astramax

    Astramax Screwfix Select

    Same problem with mine that has now been rectified....it was the PRV (pressure reducing valve).
  5. Astramax

    Astramax Screwfix Select

    Mine is an indirect cylinder with water heated via the gas boiler and an immersion heater as a back up.

    Ariston advise the annodes are replaced annually, if they have never been replaced I doubt if you will get them out due to corrosion.
    Last edited: Nov 22, 2020 at 8:30 AM
  6. mikeman

    mikeman New Member

    Yes, that was one of the first thoughts, this was replaced as part of the inlet group. I am going to suggest it is replaced as I will never be able to remove the anodes anyway. Thanks.
    As an aside, I have been fitting limescale filters on all new installations recently after I went to one that was so full it covered the bottom element! I suppose that could be the problem here, preventing the cold feed getting in fast enough? Water flows from the t&p valve even when a trickle from the taps so I still think outlet most likely.

    thanks. Mike.

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