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Strange behaviour Y plan system - help!

Discussion in 'Plumbing and Central Heating' started by johnsillitto, 28 Mar 2015.

  1. johnsillitto

    johnsillitto

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    Location:
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    I have a strange fault with my daughter's central heating that has been coming and going for over a year.

    Before I go changing expensive items like the boiler PCB I wondered if any of the experts on this forum would have any insight to the possible problem.

    It is a standard sealed Y plan system with 3 port mid-position MV and a Potterton Suprima 70 HE boiler.

    Starting up from cold the boiler starts and the whole system heats up and works fine.

    After several weeks or maybe a few days or sometimes even after 30 minutes or so the burners turn off, the pump stops instantly (no overrun), and the green, right hand, 'Mains' lamp on the bolier is flashing.

    When it has stopped there is still mains supply to the boiler, and also mains on the 'call heat' wire at the boiler. Neutral is correctly wired.

    There is no red alarm light.

    Turning the boiler control knob fully anticlockwise for 60 secs to reset the PCB makes no difference. Turning it up again and the boiler just sits there with the 'mains' light flashing.

    Turning the whole system off for an hour or so seems to allow something to reset itself and turning it on again and it all works fine - for a while.

    Most of the time, while it is working with the burner burning, turning the boiler control knob fully anticlockwise turns off the burner as you would expect and then turning it on again and the burner lights after a delay of maybe 3 mins and then everything works as normal. This would seem to indicate that the boiler PCB is not faulty, or at least if it is faulty it is an intermittent fault on the PCB.

    I have checked the MV and that works as it should - the hot water diverts in the right directions and supplies mains on the 'call heat' wire to the boiler as it should. The valve spindle is free to move with the fingers.

    Recently I noticed that the pump failed to start when the boiler turned on but it was easily freed with a screwdriver on the spindle. Just to make sure I have dismantled the pump head and cleaned and lubricated it. There was no sludge in it to speak of but it does spin more easily by hand now.

    Powering the pump on the workbench and it starts every time but if you hold the vane when you apply power, it doesn't seem very strong - it is quite easy to stop it starting to spin. I don't know if this is normal for this kind of motor? It is a Grundfos UPS 15-50.

    Putting the pump back into service and you can hear it churning the water so it is obviously spinning.

    If the pump was not working hard enough could this cause these symptoms? Has anybody come across a weak pump before?

    Otherwise I don't know where else to start looking.

    I've measured the two thermistors, one on the casting and one on the flow pipe and they both measure satisfactoryily. They were both almost exactly 11 kilohms which is just about the middle of the spec.

    Does anybody have any thoughts or ideas?
     
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  3. Hugh Jaleak

    Hugh Jaleak

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    What make is the MV?
     
  4. johnsillitto

    johnsillitto

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    Drayton MA1 - but I don't think it has anything to do with the MV because the boiler is getting mains on the 'Call Heat' but it just isn't lighting.
     
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