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Thread: Evohome Opentherm - tearing my hair out!

  1. #11
    Automated Home Legend paulockenden's Avatar
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    Evohome will always call for 100% unless all zones are within the proportional band (I.e within 1.5 degrees of the set-point).

  2. #12
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    There is only the one zone during these hours calling for heat and it's at the setpoint, and keeping there.

    I don't understand however as this is they case why is there such massive variances in flow rates every 5 mins, I thought the point of Opentherm is that if it wants 30deg, it pulls it?

    Could it be I've set my vallient partial load to be 35kw? This still wouldnt explain the call for heat when none needed, but may explain the highs/lows frequency?

    thx

  3. #13
    Automated Home Legend paulockenden's Avatar
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    None needed? If the zone is at setpoint there IS a demand needed to keep it there, else it would cool down. That's one of the fundamental things about how Evohome works.

    Are you saying that you're artificially configured your boiler so that it can’t fully modulate down to a low output?

  4. #14
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    Hi Paul,
    as per my first post in this chain the boiler is showing a demand when Evohome says there isn't one, this may be explained however by the 1.5+/- you've said where it may pull.

    However the point of opentherm is that it shoudl say 'give me 30 degrees flow' and the boiler will do that, however the graph shows that the boiler output is modulating at 60 degrees every 5 mins, which negates the whole point of opentherm, I might as well go back to the BDR91 and let my boiler work out what do do.

    On the valient you can set max flow rate (currently 70deg) and partial load output, either auto, or anything between 10kw >

  5. #15
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    The way a Vaillant boiler works is that any partial load set is the max the boiler can modulate to. However the boiler will modulate below this number. Most newer Vaillant have an Auto range rate setting, why not use that? My 438 boiler doesn't have that.
    With the current implementation of OT any set point that is within 1.5C of the temp will cause a Heat Demand. And any temperature that is lower than 1.5C of the Set point will cause maximum demand.
    The only bit I cannot explain without some data on your system is why when there is 0% Heat Demand from everything the boiler is still firing. Are you sure the boilers pump over run time is not bridging two actual brief demands thus making the boiler appear on all the time? Do you have any means of capturing the real time heat demand and boiler target temperatures?

  6. #16
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    Quote Originally Posted by bruce_miranda View Post
    The way a Vaillant boiler works is that any partial load set is the max the boiler can modulate to. However the boiler will modulate below this number. Most newer Vaillant have an Auto range rate setting, why not use that? My 438 boiler doesn't have that.
    With the current implementation of OT any set point that is within 1.5C of the temp will cause a Heat Demand. And any temperature that is lower than 1.5C of the Set point will cause maximum demand.
    The only bit I cannot explain without some data on your system is why when there is 0% Heat Demand from everything the boiler is still firing. Are you sure the boilers pump over run time is not bridging two actual brief demands thus making the boiler appear on all the time? Do you have any means of capturing the real time heat demand and boiler target temperatures?
    Ok, I get the first bit, however should evohome be asking for say 40 degrees and the boiler giving that? I don't see how I'm getting burst of 60 degrees requested + delivered with opentherm. The required flow temp should be much lower and only high 60+ when at max (ie DHW demand or when rads need more than 1.5deg from their setpoint).

    On the second point I can see on the boiler that the opentherm is calling for heat, rather than it just being the overrun. Sadly I cant see in realtime the head demand call, other than be keeping my head in the cupboard.

    Could resetting the system possibly solve this? Although I'd be annoyed as I'd lose the historical data in Domoticz.

    Finally, I've raised a ticket with Honeywell, can they see more diagnostic info that we can?

  7. #17
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    Yes Honeywell can see more data if you give them permission to suck that from your controller. Sadly the Heat Demand isnt on the app app, but atleast it's visible these days.
    Try this. For a 2 hour period make ALL your setpoints for every zone low, much lower that 3C from the room temperature but higher than the 5C OFF. Then within that time keep an eye on the Heat Demand screen, I think it updates and will show you refreshed values. Check if you have the boiler fire with a confirmed 0 Heat Demand. For me that is your issue.
    If you boiler flow is ramping up beyond the Target flow set in D.9 then you have a different issue.

  8. #18
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    Do you ever have a period where the boiler shows no demand? (as in the little radiator goes off the screen on the boiler).

  9. #19
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    Quote Originally Posted by billytkid View Post
    I did just try again a signal test and all looks fine, however one thing which it'd be good for other to check.
    When checking DHW both Opentherm module and BDR91 flash 5 times (full signal), and likewise when checking boiler CH call both again flash.

    Is this what you guys see?
    Yes, on a stored hot water system it seems to be normal for all the Relay lights to flash no matter whether you choose to test hot water or heating.

    Also, here is a bit more data on last night. At the bottom is the only room in the house calling for heat, so I can see why the boiler comes on, but what the hell is the massive varience in pull temp for a 17.5deg setpoint?! Surely it shoudl be calling for nearer to 30 to keep that one room constant?

    Attachment 1264
    That may be a sign that that particular zone suffers from overshooting and oscillation - if you're able to graph the heat demand from the HR92 (Domoticz with an HGI80) you'll probably find that it is oscillating up and down. Also check the measured temperature for a 1-2C oscillation in temperature.

    When only a single zone is calling for heat, the requested Opentherm temperature is more or less dictated by the call for heat of that one HR92. If for some reason it is struggling to keep the temperature steady and ends up opening and closing a lot, this will naturally cause the call for heat to go up and down and so the flow temperature request.

    This will not tend to happen when there are multiple zones calling for heat at the same time because the call for heat is dictated by the zone with the highest demand, so any oscillation of lower demand zones won't have much effect on the heat demand sent to the boiler.

    Something that may cause an unusually high heat demand from the boiler from a single zone at 17C, is if the valve is not calibrating the valve body properly. If you check the valve position (option 10 in the HR92 menu) it goes from 0 to 100 with 0 representing the pin pushed fully down and 100 fully up.

    Ideally the valve should just start allowing water to flow at a pin travel of 30. This is also the point where the HR92 starts calling for heat from the boiler.

    If due to a sticky or incompatible valve body the water doesn't start to flow until say 40 or 50% pin travel, it will actually be calling for heat from the boiler before the valve opens enough to allow water flow through the radiator. Which means by the time it finds the point where the valve opens and it can maintain the room temperature correctly (which your graph shows is happening) the flow temperature will be unnecessarily high.

    You could try forcing the HR92 in that zone to recalibrate - remove it from the radiator, turn the black wheel full anti-clockwise (open) and then refit the HR92. After about a minute it will say cycle.

    Whenever fitting or refitting HR92's you should always fully wind the black wheel anti-clockwise to allow it to calibrate properly.
    Last edited by DBMandrake; 29th April 2018 at 07:06 PM.

  10. #20
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    Thanks DBMandrake,

    I've reset the head to see if this'll solve it, but the graph below shows flow/return, and this one zone in particular which tracks it setpoint almost exactly.

    chart.jpg

    I'll wait for Honeywell to look at the back end data to get their heads around it, and in the meantime I'll set the boiler partial load to be auto (although i only put this on max from other posts saying to handovre all control to opentherm).

    thx

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