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Thread: New home, mammoth Evohome install

  1. #31
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    Some other updates.

    I spoke with Vokera about disabling the DHW preheat that was happening every hour over night even though I am using as a system boiler. Moving Jumper tag 5 to 6 disabled this.

    I’m experimenting with overriding the Evohome low load control algorithm on the basis that the HCC80s employ TPI anyway.
    Short term 10C requests are now substituted with a minimum control setpoint. That’s 40C on my boiler.

    Updated:

    But just overriding the 10C request is not enough. The maximum relative modulation is set to 0 in some cases stopping the burner. I’m overriding this too.

    Also Evohome disables central heating along with the 10C request, that needs to be overridden for the short term low load 10C setpoints.
    Last edited by kevinsmart; 30th January 2021 at 08:02 PM.

  2. #32
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    My low load control workaround appears to be an improvement, less burner on/off cycles.

    Yes, eventually the boiler will anti-cycle but less than before, so I think itís more efficient.

  3. #33
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    Quote Originally Posted by kevinsmart View Post
    My low load control workaround appears to be an improvement, less burner on/off cycles.

    Yes, eventually the boiler will anti-cycle but less than before, so I think it’s more efficient.
    I implemented something similar, inspired by your comments in the 'Evohome & Opentherm' thread.

    IMG_6077.jpg

    Blue line is the Evohome setpoint, complete with low load control CS=10 requests. Red line is the modified request that the boiler receives via the OTGW. I'm letting the boiler work out what it needs to do.

    As you say, Evohome seems to send some max modulation commands as well as the setpoint requests. I pretty sure by boiler (ATAG iC27) ignores them, which is just as well since they don't seem to coincide much with the heating demands. Hey ho. I overrode them anyway.

  4. #34
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    That’s great! How long has your tweak been running, are you happy with the results? Any overshoots?

  5. #35
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    Well done. You guys will soon be writing your own stand alone OTB code. In fact this was something I had planned to do. But I wanted to implement an OT to eBUS interface, which didn't need the VR33 and was a better implementation of the OTB. The sudden drops to 10C followed by the sudden rise to 90C make no sense to me at all. And they don't even appear to match a sudden heat demand surge too.

  6. #36
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    Quote Originally Posted by kevinsmart View Post
    That’s great! How long has your tweak been running, are you happy with the results? Any overshoots?
    Thanks. I’ve only had it working for a couple of days but so far so good. It took me a while to work out what the otgw was doing so I had a few false starts. I'm using a modified py-otgw-mqtt rather than the Home Assistant otgw integration. More moving parts but it’s allowed me to work out what’s going on.

    Regard overshoot .. no, no appreciable difference that I can see.

  7. #37
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    Quote Originally Posted by bruce_miranda View Post
    Well done. You guys will soon be writing your own stand alone OTB code. In fact this was something I had planned to do. But I wanted to implement an OT to eBUS interface, which didn't need the VR33 and was a better implementation of the OTB. The sudden drops to 10C followed by the sudden rise to 90C make no sense to me at all. And they don't even appear to match a sudden heat demand surge too.
    Thanks. Agreed, those CS=10 dips were crying out to be squashed. My only concern now is that I need otgw / py-otgw-mqtt & Home Assistant (all on raspi) to run flawlessly 24/7. Not sure how I feel about that, although the setup's been working very reliably for at least a year in a monitor only mode.

  8. #38
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    Quote Originally Posted by PJB View Post
    Thanks. Agreed, those CS=10 dips were crying out to be squashed. My only concern now is that I need otgw / py-otgw-mqtt & Home Assistant (all on raspi) to run flawlessly 24/7. Not sure how I feel about that, although the setup's been working very reliably for at least a year in a monitor only mode.
    Yes, that’s my concern now too, I am reliant on the OTGW for OpenTherm to work properly. Without the OTGW, Evohome would be attempting to heat my UFH to 80C, as that is what my boiler reports as the Max CH Setpoint.

    Should be a simple fix, and it has been requested before, for Evohome to specify the max DHW and CH temperatures.

    I might order another OTGW, just in case it fails.

  9. #39
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    I have never liked the idea of my DIY OTGW module sitting in the critical path. The eBUS monitor on the other hand sits to the side and even if the power to it fails, nothing stops.

  10. #40
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    I can now see how sub-optimum Evohome’s OpenTherm low load control is.

    Basically it appears to be invoked if the burner is observed to go out.

    But this can naturally happen when, through falling demand, the control set point temperature is reduced.

    Example with my boiler, that has +/-5C set point hysteresis:

    Current 48C setpoint (70% demand) reduces to 40C (50% demand).

    Current 48C flow temperature is above 40C demand + 5C upper limit so burner off.

    Burner off observed, Evohome thinks the boiler can’t modulate lower so switches to periodic 10C off/40C on behaviour.

    The reality that I can see with my workaround, the boiler is able to easily maintain the 40C request after one 3-minute anti-cycle delay.

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