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Thread: Evohome and Opentherm (S-plan)

  1. #1
    Automated Home Jr Member
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    Default Evohome and Opentherm (S-plan)

    HI

    I have installed evohome this weekend and on the whole it has gone to plan. I have an Vaillant ecotec pro 28 combi boiler. I have two zones of heating ( Zone 1- Radiators, external stat , Zone 2- Single zone UFH- using the controllers internal sensor). These zones are currently controlled by zone valves ( i do plan on installing evohome TRVs for zone 2 but cant afford that at the moment). The boiler is controlled by opentherm.

    At the moment the boiler is receiving a request for heat (54 degrees) the zone valve for Zone 2 opens correctly. (zone 1 is turned off at the controller) After a few minutes the zone valve closes but there is still a request for heat. The ABV kicks in and the boiler turns it self off as it is now at temperature, yet it is still receiving a request for a heat. After this the boiler can get through its anti cycling time, and then fire back up with the zone valve still shut. The controller then increase the boiler temperature to maximum.

    I have a potential fix. The boiler has a link (between 3 and 4). This is required for the boiler to fire with the opentherm. Instead of the link i can connect these through the zone valves meaning that the boiler will only fire if the zone valves are open. My concern is that if the controller is requesting heat yet the boiler is unable to respond am I limiting/hampering the system.

    Is there a better way i can set this up (without spending the money on 12 TRVs) or have i missed something in the setup/settings which will fix my issue.

    If it is all right with my set up will my potential fix cause any issues.

    Many thanks

    Mat

  2. #2
    Automated Home Legend
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    What external stat are you using for Zone 1? And do you have a BDR91 opening that zone valve?
    On zone 2, is a BDR91 opening that zone valve?
    How have you configured this basic 2 zone system? using the controllers s plan configuration or as simple zones?

    Also, please confirm that you have the OT bridge + VR33 to make your Vaillant boiler OT ?

  3. #3
    Automated Home Jr Member
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    Hi Bruce

    Zone 1 has a Y87RF stat, Zone 2 using the internal controller stat. Each zone valve has a separate BDR91 controlling it. These are configured as two zone valve zones, these were created manually not via the wizard.

    I have the OT bridge configured in system devices and this is connected via a vr33 to the boiler.

  4. #4
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    Your BDR91s will do TPI and your OT Bridge won't. So there will be times when your BDR91s will be closed but there is still a heat demand. That's the issue. You can use my hybrid OT method to make sure the boiler only fires when the BDR91s are ON via the 3-4 terminals, while the OT Bridge modulates the temperature. But then you kindda defeating the purpose of OT.

  5. #5
    Automated Home Jr Member
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    Cheers Bruce,

    I came to this conclusion last night and have now set it up as you have suggested.

    This is working reasonably but i have noted that this seems to be setting the boiler to the max temperature after a few minutes of the zone valve being closed and the opentherm requesting heat.. I am guessing this is because the boiler is not responding to the request for heat. I am going to leave it for a few days and see how it settles down.

    In the long run, i think i am going to have to get rid of the zone valves and put trvs on all the rads. I will also need to put an inline trv in to control the single zone UFH. However this will require 12 hd92's so may have to wait for next year for that.

  6. #6
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    Since you are 'messing' with the boiler's demand the OT is responding more aggressively. I guess the OT is not detecting the flow temperature rising even though it is expecting it to, so steps up the demand.

  7. #7
    Automated Home Ninja
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    Quote Originally Posted by matjholden@yahoo.co.uk View Post
    However this will require 12 hd92's so may have to wait for next year for that.
    I have 24 of the buggers! I haven't told the wife that the rad heads alone cost 1,200!

    If you go for an all-HR92 setup with no CH zone value (which is Honeywell's preferred setup), you will need some form of bypass to prevent deadheading your pump if the valves all close but the pump overrun is still active. You may have one built in to your boiler, or you may not. Or you may have an external pump as well as your internal pump (like I do, connected via a low-loss header). Either way, if you decide to use an automatic bypass valve you won't be able to use the modulating settings on your pump - they're not compatible. An alternative is to leave a radiator open with no TRV (mechanical or electronic) attached so the water has somewhere to go even if all the other radiators are closed. You may or may not view this as a waste of energy - your call.

  8. #8
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    The boiler does have an inbulit ABV however i am looking at installing an external ABV as well hopefully this will interfere less with the opentherm heat request.

  9. #9
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    Unless you've got an external pump (which would then need hydraulic separation from the internal pump), I'm not sure that a second ABV will add any benefit. I'm not even sure that it won't do some harm!

  10. #10
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    I am trying to work round the issue with opentherm setting the boiler to max. When I go home I will have a look at where the senors are on the boiler and what they are reading. I am guessing that when my zone valves are shut all the flow is going through the bypass and that either the flow sensor or return temperatures are not changing. My thinking was that by putting an external ABV that the water will flow through this external ABV rather than the internal ABV and that the opentherm wouldn't have the need to raise its tempeature

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