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Mains water stop cock affects pressure on combi boiler

Discussion in 'Plumbing and Central Heating' started by Jupiter01, 15 May 2020.

  1. Jupiter01

    Jupiter01

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    Hi All,

    I was helping may parents by replacing a basin tap. I shut off the mains water tap inside the house to carry out the work. I then turned the water back on.

    I then noticed that the water pressure on the combi boiler was almost down to zero. Is this expected? I thought the combi boiler had its own closed circuit of pipes and am unclear on how this could be affected by the mains water.

    I'd appreciate your insight on this. Thanks
     
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  3. denso13

    denso13

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    Do you mean the pressure gauge on the boiler?
     
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  4. Jupiter01

    Jupiter01

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    Yes, pressure gauge on the boiler. Since topping it up again, the pressure has been fine for a couple of weeks now.
     
  5. denso13

    denso13

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    Just coincidence, nothing to do with the basin tap.
     
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  6. bernardgreen

    bernardgreen

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    If the filling loop is still connected then maybe in the absence of mains pressure at the closed filling loop valves these valves are letting by from pressurised boiler system to zero pressure mains.
     
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  8. denso13

    denso13

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    Then they would surely let by the other way when the mains is back on and over pressurise the heating system.
     
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  9. bernardgreen

    bernardgreen

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    It seems that when the valve is shut the ball is pressed against the seal by mains pressure. When boiler pressure exceeds mains pressure the ball is pressed away from the seal. Hence the arrow indication on the valve indicating which way the valve should be fitted
     
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  10. denso13

    denso13

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    I have never had an instance of turning the mains off and finding that the heating system pressure has dropped to zero because both filling loop valves are passing. Have you?
     
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  11. bernardgreen

    bernardgreen

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    I have seen where putting pressure on a ball type valve in the wrong flow direction can result in the valve "weeping".

    Vaguely recall this weeping was ( is ) one of the reason why the filling loop should be removed when not re-pressuring the system
     
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