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Myson cylinder thermostat goosed?

Discussion in 'Plumbing and Central Heating' started by CBW, 7 Jun 2020.

  1. CBW

    CBW

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    As per title really. I haven't had much pleasure of working with conventional systems as I mainly work on combi's. My wife and kids are complaining that the hot water is too hot, set at 60 degrees.
    • I checked with thermometer at tap was getting about 64 degrees.
    • Turned stat down to 50, now measuring about 61 degrees.
    • Boiler thermostat isn't set high
    • s-plan system.
    • Immersion heater not on
    • Does it sound like I need a new one? Is there a way to test it? Could it be anything else?
    Thanks.
     
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  3. dilalio

    dilalio

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    Check seating on cylinder... Good contact to metal.

    Check mv's are working correctly and switching boiler off/on as stat is manipulated up and down.

    Has heating been on?
    Might be sticky microswitch on HW valve.
     
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  4. oilhead

    oilhead

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    Don't forget, a cylinder stat positioned 2/3rds the way down the tank is not sensing the hotter water that rises naturally to the top. The cold water that replaces that you have used stays low and switches on whilst the top of the tank is considerably hotter. In very warm weather, this situation is not unusual.
     
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  5. CBW

    CBW

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    • Good contact on cylinder
    • Mv’s appear to be working correctly, and the cylinder stat when turned down, does turn off signal to mv
    • Heating hasn’t been on
    • Might have to open up valve
    One thing I forgot to mention, when it’s on for dhw, it’ll fire up, then when I’m assuming it’s satisfied the boiler goes off, as does the mv, but then I will hear it come on again like maybe a minute or two later? Would this point to a sticking microswitch?
     
  6. dilalio

    dilalio

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    Could be.
    Multimeter will tell.
    Does your boiler have preheat?
    You could buy a couple of Regin 5pin cable connectors and fit these onto the cables of the motorised valves... Then you can easily swap the CH MV for the HW MV and see if it resolves the issue, you can then order a new power head and fit it easily using the regin connectors.
     
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  7. MeldrewsMate

    MeldrewsMate

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    It may do, but you're ASSUMING when you should be MEASURING. Instead of that try turning the Myson 'stat down until you hear the click - does both the boiler turn off AND the ZV close at the same time? Given those precise symptoms I could say it's highly likely the microswitch is sticking, rather than the valve being stuck mechanically, though the overly hot water would suggest that heat is bleeding into the cylinder (unless it's a 3-port valve, not an S-plan).
     
    Last edited: 7 Jun 2020
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  8. ianmcd

    ianmcd

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    Switch heating off and HW on , turn Cyl stat right down and see if the zone valve closes , it could be that the zone valve for the HW the synchron motor has seized in the on position
     
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  9. dilalio

    dilalio

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    Still worth that test though.
     
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  11. CBW

    CBW

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    @MeldrewsMate yes defo an S-plan system. I have measured some voltages, but have mislaid my piece of paper with them written down. System is approx 10 years old from new. Also, if ch water was leaking into cylinder, wouldn't the water be discoloured?

    @dilalio - boiler does not have preheat, it’s a potterton suprima he. (50 I think).
     
    Last edited: 7 Jun 2020
  12. CBW

    CBW

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    Does this mean with the head removed? Also just opened up cylinder cupboard and the dhw zone valve the plastic part of the manual lever is missing :eek:. It may have come off when I changed the pump.
     
  13. ianmcd

    ianmcd

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    doesnt matter if the head is still on or removed, but if you remove the head it is worth seeing if you can move the valve shaft with your fingers to make sure that is not seized , it only move s a very small bit, but if you can move it then the valve is fine,yes the levers snap all the time, just look at the underside of the actuator and see if there is movement between off and demand , it is not a large movement as said, but you will see the difference
     
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  14. CBW

    CBW

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    Ok, so I have found the following:

    • Myson head (MPE222) comes off, but can’t seem to get to synchron motor - do i unswcrew the star shaped bits underneath?
    • It wasn’t the manual leaver that snapped, it was the plastic latching part to keep it in place, with a crack on the bottom piece, which may result in it staying open for longer?
    • Cylinder thermostat not clicking currently?
    • Cylinder thermostat turned down to 20, valve head removed, dhw on at programmer, started the pump and to fire up boiler
    • Boiler went to lockout as zv closed, but boiler fired up at 20 degrees on cylinder stat?
    Voltages:

    • Earth to common 250 vac
    • Earth to terminal 2 on demand 250 vac
    • Earth to terminal 3 on demand 3.65 vac
     
    Last edited: 8 Jun 2020
  15. ianmcd

    ianmcd

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    you would be better replacing the actuator, I think they are £56 at screwfix I bought one a few months back and think that is what I paid
     
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  16. ianmcd

    ianmcd

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  17. CBW

    CBW

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    Had a look and screwfix don’t appear to sell Myson? Had a look and it’s dearer than the valve in some places :eek: . Anyway found one on amazon for just £40. I’ll also ask through work.
     
    Last edited: 8 Jun 2020
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