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Combi Boiler.

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Old Alpha HE CB 25.


 I have checked the pump, all o.k. Fan o.k. New hot water diaphragm- switch working o.k.

Circuit board o.k. Boiler tries to light but shuts off after a few seconds. No hot water. No central heating.


The strange thing is that when a hot tap is opened the central heating water pressure goes down. Is this a heat exchanger problem?


Thanks,


Z.
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  • 0
    When the hot tap is opened at the sink, the closed heating (radiator) system de-pressurises to zero.

    And stays at zero when the tap is closed again?


    That's odd - even with the hot tap open the DHW pressure should be significantly higher than zero.


    Is there any sign of water escaping from the pressure relief valve? (usually through a copper pipe poking through the outside wall) - if the expansion vessel has failed a small increase in pressure can result in the PRV opening and so dropping the pressure. Of course the PRV should close again as soon as the pressure drops towards normal - but they're often a bit sticky and don't always reclose when they should.


       - Andy.
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  • 0
    When the hot tap is opened at the sink, the closed heating (radiator) system de-pressurises to zero.

    And stays at zero when the tap is closed again?


    That's odd - even with the hot tap open the DHW pressure should be significantly higher than zero.


    Is there any sign of water escaping from the pressure relief valve? (usually through a copper pipe poking through the outside wall) - if the expansion vessel has failed a small increase in pressure can result in the PRV opening and so dropping the pressure. Of course the PRV should close again as soon as the pressure drops towards normal - but they're often a bit sticky and don't always reclose when they should.


       - Andy.
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