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Pro Combi 85 HE(Vokera) lockout problem

Discussion in 'Plumbing and Central Heating' started by peterfb, 20 Mar 2011.

  1. peterfb

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    Hi there


    We have pro combi 85 HE combi boiler which suddenly started going into lock out with solid red light.
    When this happens the only way to get it working is to turn the main electricity switch off for a while then the boiler will reset and will work for short period of time before it goes to lock out again.
    Accidently I find out that if I increase the temp from the controls the boiler will work fine for weeks without any obvious problems.

    any ideas as to what can be causing this?

    I've added a boiler diagram:
     
  2. stef99

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    Im only offering help here as no-one else has, Your problem seems to be the temperature sensor inside the boiler, I know this is vague, but you could try looking at that first, There maybe some info about it in the installers instructions, failing that, remove it and give it a good clean?
     
  3. Mrmethane

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    Hello

    When the boiler locks out is it in heating or hot water mode, or both. It does sound a little like a flow problem which would maybe cause the appliance to cut out on over temp. Have you restricted the flow at all, shut down some radiators or removed some, or are the TRVs shutting down in warmer weather. A bit of history may help identify the issue.
     
  4. peterfb

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    Hi
    I edited the original post as I meant that I have to turn the main electricity switch off for a while in order to get the boiler to reset.

    It does this in both modes and started suddenly. It's a four year old boiler never had any problems before
     
  5. peterfb

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    Hi there are two thermistors inside the boiler according to the schematic. If I attempt to replace them is there a chance for water leakage or they are not in direct contact with the water passing through the boiler so no need to drain the system in order to replace?
    Thanks
     
  6. Mrmethane

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    I had a quick look online to see if I could find a user guide, I found one for the Vokera if its your model I am not sure, but it does say if the light is red and permanently on then it has a over temperature fault. This can be something has simple has a faulty sensor, a problem with the pump or a main pcb fault.
    The over temperature sensor isolates the boiler to protect it from overheating, when the burner ignites for example when you turn on your hot tap the burner usually goes into high burn to generate hot water flow. If this flow is insufficient to remove the heat and the temperature within the heat exchanger rises to unacceptable levels the over temp sensor will shut the unit down.
    This is why you have to wait a little while before it will reset, it has to cool down a little first.
    Sensors are the first thing to look at, however there are a number of things that could generate that fault, the pump being another. I am not familiar with the Vokera so you should find yourself a decent service engineer and have it checked out fitting the wrong part could prove costly.
     
  7. Mrmethane

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    Usually thermisitors clip on to pipe work and do not require water isolation to replace
     
  8. Mrmethane

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    On the left is the over temperature sensor, is it 240V or low voltage, do you have a multimeter?
     
  9. peterfb

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    Thanks Mrmethane,

    When I turn the hot water temp knob to a lower temp setting the produced hot water is not very hot but boiler lockouts very quickly. when I turn it to a high temp setting(almost to the end of the scale) the boiler works fine(at least worked for the last 2 weeks) and the produced hot water is very hot but it works.

    I had an engineer to look at it who scratched his head for a while, checked the air pressure switch and the venturi, checked the radiator circulating water for impurities (it turned out there is a chemical inside which apparently keeps the system clean) then went away unable to diagnose the exact cause. He did mention the PCB but did not check the board itself.
    This is the reason I'm looking for some help on here before another expensive visit of a "specialist".
     
  10. peterfb

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    Yes I do. Everything inside the boiler attached to the PCB works on 24DC.
    Do you mean 26. The hot water sensor?
     
  11. Mrmethane

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    No the high limit stat number 20, test this when the boiler next locks out and see if this has tripped, if it has then you have to determine if it is tripping at too low a temperature, or is the water in the boiler too hot, and if so why.
    Yo stated earlier that if you increased the water temperature on the appliance it worked longer, did you mean the heating temperature or the domestic water temperature?
     
  12. peterfb

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    Thanks Mrmethane,

    I'll measure it next time it lock out. will the boiler require to be drained in order to replace the thermostat?
    I did indeed increased both hot water and heating on the boiler. I haven't experimented anymore once i got it in sort of stable state until I have a solid idea what can be the culprit
     
  13. Mrmethane

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    Try to determine through a process of elimination what the increase in temperature does to your appliance, does it increase the pump speed, does a bypass valve shut, what? Most boiler service instructions include a flow chart showing the operation.
    I am still convinced that it is a over heat problem but what is causing it is the mystery, also all the information you have and can think of will help you get to the source of the problem. Think what is happening within the appliance, only with regard to the flow of water through it. It comes on following a call for heat, i.e a time control or thermostat, the pump starts and water flows through the heat exchanger, the burner fires and heat transfer occurs.
    Simple calculations will show you what the flow rate should be through the appliance at various burner rates. As you can imagine if the flow rate is too slow the water increases in temperature too quickly and becomes dangerous, thats when the over heat cuts in and protects the appliance. By turning the water temperature up you have altered the way it operates and therefore overcome the problem. The trick is now, what does turning the temperature up do to the appliance?

    Good luck
     
  14. peterfb

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    Thanks Mrmethane.
     
  15. callumk1

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    hi. it sounds like your plated heat exchange is clogged
    needs replacing or descaling.
     

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