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Baxi 836 boiler

Discussion in 'Plumbing and Central Heating' started by Jenny3, 26 Aug 2021.

  1. Jenny3

    Jenny3

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    i have a Baxi 836 boiler.
    With a nest link fitted to it.
    We have had 2 power cuts in the last 3 months & each time it has blown the nest link. We had a new link attached to the power the first time thinking it was just a faulty one but it has blown again on Monday when we had a power cut. We have a new consumer unit with a surge breaker, any ideas why it’s happening out electrician & heating engineer just can’t work out why.
    Also can we over ride so we can use the heating manually until this is sorted
    Thanks Jenny
     
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  3. DP

    DP

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    Post power cut, on power resumption I wonder if surge is the issue. Some equipment comes fitted with surge protection, difficult to say why the heating control goes belly up as your other electrical units will be subjected to same fluctuations especially the boiler that will be on the same circuit as the Nest.

    I would try another brand eg Honeywell Lyric 6 if you keep having issues with nest else talk with nest technical.

    Lightening struck can cause a lot of damage, Varistors are often fitted on pcbs to short circuit or limit the excessive voltage.
     
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  4. muggles

    muggles

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    What size fuse is protecting it? Should be 3 amp.
     
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  5. CBW

    CBW

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    Customer stating surge protection on the consumer unit though?
     
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  6. Harry Bloomfield

    Harry Bloomfield

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    I read it as blowing a fuse, but I am sure from the description which fuse or the type that is blowing. If it is a circuit board fuse of the glass type, sometimes these need to be anti-surge types, to delay them blowing when first powered up.
     
  7. DP

    DP

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    I suppose in that case it will not be an issue any more
    It is strange it is the nest that is dying yet boiler is ok.
     
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  8. Jenny3

    Jenny3

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    It is not blowing a fuse, the nest link is actually blowing. It doesn’t trip the fuse board.
    The boiler etc all come straight back on but the heat link won’t work, you are suppose to be able to reset them but you can’t do nothing, no lights. Had a electrician look he has check & there’s on fault with the electrics
     
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  9. DP

    DP

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    doubt he will see anything as mains voltage when it is functioning, is quite stable. A few volts plus or minus will not cause a defect. Electrician will check the voltage and confirm all is well.

    As said previously, fuse for the boiler/ nest should be 3 amps. Often see 13 amp that does not blow but kills the pcb that is not rated for that power.
     
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  11. Jenny3

    Jenny3

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    It is a 3amp fuse & hasn’t blown
     
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  12. Jenny3

    Jenny3

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    We have come to the conclusion that it is a nest fault. There are other people with this happening on goggle forum. Been in touch with Nest they are sending me another one out, free of charge, this will be my third one. They don’t know why it’s happening to people
    Thanks for trying to help me
    Jenny
     
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  13. bernardgreen

    bernardgreen

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    This may be a result of the Nest's 24 V (?) power supply turning on faster than the 24 V (?) supply in the boiler.

    This diagram was drawn for a case where sensors ( VCC 1 ) connected to a data processing system ( VCC 2 ) were failing. ( modified for this post )
    The transistors T1 in the sensors was going open circuit after a loss of mains.

    It might be applicable to some of the cases of a Nest connected to a boiler

    Consider VCC 1 as the Nest and VCC 2 as the boiler.

    If the supply in the Nest reaches 24V before the power supply in the boiler reaches 24 V then the output(s) from the Nest might be powering the boiler's circuitry via protection diodes in the boilers PCB. This could damage the output devices in the Nest, ( T1 )



    protection diodes destroy driver device.jpg
     
  14. DP

    DP

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    Bernard, checked before I replied to you post. Makes no sense on many levels when boiler and nest are concerned.

    nest receiver supples power to the the thermostat. Receiver will only talk with the boiler by means of a relay contact that makes or breaks to start the boiler or disconnect the demand

    nest will almost certainly not have a transformer power supply but a SMPS that is stable over a wide range of input voltages providing stable output voltage with feedback to the input for control of DC output which in many cases is voltage and current strapped.

    A good SMPS should have all sorts of input voltage filtering and protection and would pop the fuse if something is not right. Check out this type of power supply, you will see what I am about.

    There is no T1 but two relays instead
     
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  15. bernardgreen

    bernardgreen

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    DP

    It was a shot in the dark.

    This
    suggests there may be something that needs to be looked at in more detail.
     
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  16. DP

    DP

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    If the same component fails every time, prudent to replace that component for another make if possible as item is ancillary and not boiler component ( which has to be the same else boiler would be modified)
     
  17. bernardgreen

    bernardgreen

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    Changing to a different type of component may be necessary.

    Changing from a simple ON-OFF mechanical thermostat to an "intelligent" multi-function timer/thermostat/remote controller is not a case of remove old and connect new.

    The old thermostat would have switched the control line from the boiler with a robust mechanically operated switch. The new multi-function controller may have a small compact relay whose contacts may be operating at their limits of voltage and/or current switching capacity.

    Do boiler manufacturer's data sheets include any details of the current that flows through the boiler's control inputs ? This is the current that the relay in the controller has to switch ON and OFF.

    Do boiler manufacturer's data sheets include any details of the boiler's input circuitry ?

    Control inputs can be mainly :-

    Resistive......no transient effects on the contacts in the controller
    Capacitive....high inrush current when the contacts in the controller close, transient current overload and heating damaging contact surface coating
    Inductive.....high voltage spikes when the contacts in the controller open, transient arcing and erosion of contact surfaces

    Do manufacturers of multi-function controllers provide details of what the contacts in the relays can actually switch without creating excessive wear to the contact surfaces ?

    If that data is available then does the person selecting the new multi-function controller have the technical skill to determine if the relay in the multi-function controller is compatible with the load that it has to switch ?.
     
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