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Thread: vailllant eco-therm pro 937 vr33 and evohome opentherm

  1. #1
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    Default vailllant eco-therm pro 937 vr33 and evohome opentherm

    Hi everyone
    I have tried looking around and cant find 100% acurate instructions on what i need to do what needs bridging what doesnt etc

    i have only just bought all the kit so havent tried anything yet

    but what i gather is
    1. install vr33
    2. connect x32 to x31
    3. connect cable that was connected to x31 to the other connector on the vr33
    4. disconnect 230v control
    5. bridge the + and - for 230 and 24V control
    6. wire the r8810 to the T0 terminals on the vr33


    do i need to do anything else how can i tell if its all working?

    many thanks for any help you can give!

  2. #2
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    well its installed and seems to work fine but whats the best way to tell if open therm is working properly? and what do i set the heating temp to on the front of the boiler?

  3. #3
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    With OpenTherm the temperature you set on the front of the boiler for heating sets the maximum flow temperature that the Evohome is allowed to ask the boiler for. The Evohome can and will ask for lower flow temperatures if it deems that the demand is low enough to be satisfied by a lower than maximum flow temperature. (I think boilers also have a pre-set minimum flow temperature somewhere on the order of 40 degrees that you can't adjust - below this it will just turn off)

    So for example if you set your boiler to 70 and you set at least one zone at least 2 degrees higher than the current temperature you should see your boiler flow temperature ramp up to 70 degrees. However when your zones are all at or near their desired temperature you should see your boiler running but with the flow temperature significantly lower - say 50 degrees.

    If you see this kind of behaviour, OpenTherm is working. It's up to you to decide what you set the maximum to - in winter you will obviously need to set it higher than summer just like you always have, however the beauty of using OpenTherm is that even in winter with it set to a high maximum flow temperature, once your rooms are up to temperature and the heat demand to maintain a constant temperature is less than during the warm up period, the system will automatically reduce the flow temperature to increase efficiency and provide more accurate temperature control of the rooms. (Reduced flow temperature reduces any tendencies for rooms to overshoot their set temperatures and also makes the boiler more likely to be in condensing mode and thus more efficient, as well as reducing heat loss from the boiler and pipe work)
    Last edited by DBMandrake; 12th October 2016 at 10:33 AM.

  4. #4
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    What I have noticed is that my Opentherm will only ever request the maximum flow temperature when there is HW demand. So in this condition the OT bridge will set the max requested to 90C but the boiler will give it as much as you set it on the front of the boiler. For CH demand it always seems to request lower than 60C, so it never gets close to the boiler max set.

  5. #5
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    Doesn't sound like it's working properly then. Is it always exactly 60 or just around 60 ? How do you know it's 60 - are you just going by the number on the boiler ?

  6. #6
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    Sorry my mistake.

    When I connect the Evohome Opentherm bridge the d.09 changes based on heat demand. Values I saw were 90 when there is HW demand, 53 when there is normal CH demand, 74 when there is a lot of CH demand, 40 when there is low CH demand. Pressing the Evohome Opentherm bridge button toggles the target between 90 (on and full) to 10 (off).

  7. #7
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    thank you mine seems to be working fine i seem to be getting different temps reported depending on demand etc connecting 4 zones tonight so fingers crossed it continues to behave

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