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Vaillant VR66 wiring control centre causing heating modulation woes?

Discussion in 'Plumbing and Central Heating' started by Matthew B, 16 Jan 2021.

  1. SoftTopGeek

    SoftTopGeek

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    That sounds exactly what mine does.
    I was really hoping that it would be different to this when being controlled using the ebus (not the 230 V input), by having a Vaillant thermostat / controller / wiring centre. I guess this is especially useful, when the weather is mild, and you only need a low flow temperature, as in this situation, during the first minute, the flow temperature could well rise too much. If it does rise too much, would the boiler cut off, or would it cope with that, and modulate down still?
     
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  3. wannocks

    wannocks

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    The weather compensation adjusts the flow temperature not the power output.
    The ignition sequence is controlled by the boiler PCB and cannot be overridden or adjusted. It is fixed according to the size of the burner/ heat exchanger.
    If the flow temperature overshoots the demand temperature the boiler will go into pump overrun and try again once the cycle is completed.
    The main reasons for this are inadequate flow/ heat dissipation and/ or an oversized boiler.
    Having more radiators without TRVs can help, the weather comp when working correctly should match the heat demand to the building losses and maintain stable temperatures without TRVs reducing the circulation. This also requires the radiators to be correctly sized for the room.
     
  4. rbranco

    rbranco

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    To add additional clarification, ebus does not control power output as stated, the boiler does that internally to maintain the target flow temp requested. Even with all the VRC stuff, the boiler is still non the wiser about room/outside temps, that part is handled by VRC.

    VRC is the commander and boiler does what it's told but has autonomy not blow itself up, but in turn it tells the VR66 what to do as subordinate, both putting information about what they are doing on ebus so that VRC can report that back to you (faults, on/off, temps etc).
     
  5. jackthom

    jackthom

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    It looks like the softer start up characteristic of DHW mode is built into the boiler programming.
    It doesn’t begin by running at 60% of full power in this mode but limits itself to the DHW equivalent of d0.
     
  6. SoftTopGeek

    SoftTopGeek

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    Sounds like I could do with getting mine to thinking it is in DHW mode then, that is just what I need!!!
     
  7. rbranco

    rbranco

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    You can't without VR66 but then you loose CH control.
    It has to do this to prevent temp differential across the heat exchanger which causes issues, once flow temp rises it goes full power until flow temp is 80c. It never does low flow temps like CH.
     
  8. SoftTopGeek

    SoftTopGeek

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    I intended to use the VR66 (or similar).
    That is meant to link to DHW and CH motorised valves isn't it?
     
  9. Harry Bloomfield

    Harry Bloomfield

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    It serves as the control for the valves, via the ebus. If it is located near the valves, it drastically cuts down the complexity of the wiring needed.
     
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  11. rbranco

    rbranco

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    You connect the 2 or 3 port valves to it and also the CylStat or NTC.
    You can use it without full eBUS controls (ie manual programmer) but it needs you to wire it in a non-standard way as they do not indicate in the manual how to do it on normal controls. I did it but now ive got full eBUS.
     
  12. jackthom

    jackthom

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    There is a setting d78 for the target flow temperature for DHW which I’ve changed to 75C. I have the partial load for DHW set to 12kW which is fine for my cylinder.

    I suppose you could redirect only the existing boiler demand signal to go via the VR66, triggering the boiler for DHW whenever there is demand from either the room or cylinder thermostat but it would then always be at a fixed compromise flow temp for both heating and the HW cylinder.
     
    Last edited: 4 Feb 2021
  13. rbranco

    rbranco

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    d78 does not do anything without VR66 on an regular 4xx boiler, if you have a system boiler then it has the hardware internally to switch between CH/HW and thus VR66 is not needed.
    The whole point of having HW priority is that there is no need for mixing valves or multiple flow temp sensors for each flow, this is something the VR70/71 etc are needed for multiple zones.

    You always need to trigger the boiler signal on as its a master go/nogo for heating call, but for HW it needs to be both the boiler and also cylstat else it will default to CH.
    When using full eBUS controls you would bridge the connection in the boiler so its on all the time. eBUS is a stop based protocol, it sends messages telling the boiler not to run by effectively setting the flow target temp below what the boiler thinks it is, if you were to remove the VRC controller it should fire up and keep running until it hits the flow temp limits set on the dials. Its a fail working mode so you do not loose all heating if the controller dies.
     
  14. jackthom

    jackthom

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    Interesting thanks. The way my VR66 is being used, with only the cylinder stat connected to it, it gets our 428 running for DHW with no conventional signals applied to pins 3/4 of the boiler.

    Only the Central Heating demand triggers the boiler in the usual way with no apparent assistance from the VR66.
    More accurately the VR66 isn't inhibiting conventional CH mode although I know there are commands (tried using a raspberry pi with ebusd etc) which will do that.

    Incidentally I went for the 428 around 13 years ago thinking it would supply heating and hot water running together in a Y plan system.
    I've only recently added the VR66 plus a relay to give priority to Hot Water and we could now easily make do with the 18kW version.
     
    Last edited: 5 Feb 2021
  15. rbranco

    rbranco

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    So you keep your HW tank at full temp 24/7 or do you switch the cyl stat wire?

    VR66 does not need to inhibit CH as it handles both valves, so it just overrides the valves, its an effective inhibit.
    Remember there is cross over of the VR66 and system boilers. The VR66 is invisible to VRC controls, it really only speaks to the boiler and then the boiler pushes out stuff on bus that the VRC needs.
    My VRC does not show the VR66 as a component on the network, according to vaillant that is expected.

    Ps. Dispite what the manual says on VR66 it is possable to use they cyl stat and NTC at the same time as backup and temp control. Odd but I did it as test and worked as I expected.
     
  16. SoftTopGeek

    SoftTopGeek

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    The boiler power requirements change according to the situation of each of the TRVs, so that has always a bit of a difficulty to calculate and set that required output power hasn't it. Not sure what can be done about that?
     
  17. SoftTopGeek

    SoftTopGeek

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    That sounds good. In my situation, I would have the VR66 near the valves.

    Mine is an open vent boiler.
    Do I take it that the switch wiring in the motorised valves stays as it is then, whereby if either is powered up and is then open, there is a 230 V input to the boiler?
    I see how if the VR66 has set the motorised valve open, it (VR66) would know what flow temperature it required from the boiler.
    Also, if the value failed to open, I guess the boiler wouldn't fire at all.
    Does that sound right?

    Thanks for all the tips everyone.
     
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