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Nest Thermostat not firing up CH

Discussion in 'Plumbing and Central Heating' started by _Jay_, 3 Apr 2021.

  1. CBW

    CBW

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    S4 is burner on for heating yes, so in essence, heating should be on.
     
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  3. DP

    DP

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    Switch the boiler on, let it complete self check, then try putting the demand on
     
  4. _Jay_

    _Jay_

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    Ok, so I set a schedule last nigh and the boiler did fire up at the desired time, so something is working as expected. It then fired up this morning when i manually set the the desired temperature well above the room temperature.
    So this appears to demonstrate the call for heat is being read by the boiler.
    I still have the problem where turning off a call for heat and then a turn on for call for heat in quick succession does not have an effect at the boiler. I am not sure if there may be some kind of lockout period at the boiler.
    Whilst I should not need to do this on off on off under normal usage i was only doing i to to test the thermostat was connected correctly. So perhaps there is nothing wrong and this is how it should work?

    But I do appreciate all the replies. ]

    One more question - and perhaps I need to post this as a new thread. On our old Honeywell Stat (round dial type) when the heating kicked in the boiler would fire and remain on (orange light on indicating burner on) until the target temp was reached and then go out.
    With the nest I have noticed that even if the target temp is not reached, the orange burner light will go out for some time and then come back on again. I observed at multiple times this morning that even though the target temp was say 2 degrees above current room temp, the burner would be off and then come back on, and this would repeat.
    Am I misunderstanding how these thermostats work? I would have thought this specific operation (heat until specified target is reached) would be the same as the old honeywell ?
     
  5. ianmcd

    ianmcd

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    That is not how Nest works, your old thermostat could do that, but the nest isnt in constant communication with the heat link, it can take between 30 seconds to a min to send the signal, that is normal
     
  6. _Jay_

    _Jay_

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    But I can hear the ‘click’ in the heatlink almost immediately after turning up the temperature on the thermostat; this indicates the heatlink has picked up the request and from there that should cause the boiler to fire up.
    Even after I hear the ‘click’ in the heatlink , 3 minutes later The boiler still did not start heating.
    As I say it seems to work on a schedule so far so i am not so concerned now.

    Just trying to work out the on off on off cycle even though the target temp has not been reached .
    Tempted to go back to the Honeywell!
     
  7. fezster

    fezster

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    If I'm reading your photo correctly, you have the heatlink live/neutral wired to boiler terminals 3 and 5?

    You should have a direct mains feed via a fused spur to the heatlink.
     
  8. MJN

    MJN

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    Might your boiler have an anti-cycling timer feature that prevents too many fire-ups in a given period (by ignoring repetitive calls for heat)?
     
    Last edited: 5 Apr 2021
  9. Jajodu

    Jajodu

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  11. CBW

    CBW

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    3 and 5 are permanent live and permanent neutral from the boiler, so in effect, no different from a separate source
     
  12. CBW

    CBW

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    Perhaps the Nest is reaching the target temperature as requested, rather than the Honeywell, which is probably overshooting. If the Honeywell reaches a set temperature of 20 degrees Celsius, then the boiler is still producing heat, so in effect, could be getting as high as 23 degrees Celsius. With the Nest, it’s trying to load compensate, so basically firing up on short bursts to reach the 20 degrees which was set, so will probably stop boiler firing at say 18 degrees, and eventually reach the 20 degrees with the pump overrun.
     
  13. _Jay_

    _Jay_

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    This makes sense, let me investigate if there is a setting that controls anti cycling frequency. Didn't think of that but valid point. Thank you.
     
  14. _Jay_

    _Jay_

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    hey Chris, what you say seems reasonable and without any further info would repeated stop starts of the burner not end up being less efficient ? Perhaps even more so with a non-condensing boiler that we have although to be fair I do like the vaillant, it has served us very well.
    Also, heating has come on every time so far when set from schedule, so wiring appears correct.
     
  15. ianmcd

    ianmcd

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    with the nest calling for heat, and the burner on, dont touch the nest, turn the boilers CH thermostat to minimim, wait till burner goes out, turn boiler thermostat back up and if you have a timer delay(very likely) the burner wont come back on straight away, some displays show an egg timer to show it is in timer delay, some are time set some are temp set
     
  16. _Jay_

    _Jay_

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    Ok so I tried this. Burner on, turned temp on CH right down. Burner went off. immediately put CH temp back to where it was, burner did not fire. Hung around for a few mins watched the temp display on boiler drop to approx 33 degrees and nothing.
    Mrs then went and turned on hot tap so immediately burner came back on. Therefore didnt get to complete test as I would have liked as I was timing it.
    So pretty confident that it is the anti cycle that was kicking in when I was first testing the nest after install - my turbo max display only has the temp so doesn't give any further info.
     
  17. ianmcd

    ianmcd

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    yes that is exactly what is happening, the DHW demand over rides the timer delay
     
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