Raspberry Pi OpenTherm Weather compensation project

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  • kevinsmart
    Automated Home Ninja
    • Sep 2018
    • 257

    #16
    I’m looking at adding weather compensation to my Evohome Intergas OpenTherm setup by customising the OTGW firmware.

    The good thing is that the OTGW can read the Outside temperature from the external sensor connected to the boiler.

    The R8810 OpenTherm bridge calculates the control setpoint from percentage demand based on the Max CH water setpoint.

    The latter defaults to 90C with Intergas even if Tmax is manually set lower via the boiler temperature display. The boiler will clamp the maximum to the boiler setting though e.g. 60C.

    It is possible to set Tmax lower from the OTGW. Immediately the control setpoint is seen to reduce proportionally.

    I’m looking for a recommendation on the best algorithm for weather compensation.

    1. Should I reduce the Max CH water setpoint based on the outside temperature according to a heating line.

    2. Or should I clamp the control setpoint temperature, based on a maximum calculated from the outside temperature, leaving the Max CH water setpoint at 90C?

    The difference will be that for the latter a % demand will be mapped to a consistent temperature as long as that temperature is below a weather compensated maximum.

    But for the former the % demand will map to a different temperature based on the outside temperature, lowering as the outside temperature increases?

    Comment

    • DBMandrake
      Automated Home Legend
      • Sep 2014
      • 2361

      #17
      Originally posted by kevinsmart View Post
      It is possible to set Tmax lower from the OTGW. Immediately the control setpoint is seen to reduce proportionally.

      I’m looking for a recommendation on the best algorithm for weather compensation.

      1. Should I reduce the Max CH water setpoint based on the outside temperature according to a heating line.

      2. Or should I clamp the control setpoint temperature, based on a maximum calculated from the outside temperature, leaving the Max CH water setpoint at 90C?

      The difference will be that for the latter a % demand will be mapped to a consistent temperature as long as that temperature is below a weather compensated maximum.

      But for the former the % demand will map to a different temperature based on the outside temperature, lowering as the outside temperature increases?
      This is just my opinion based on analyzing it with thought experiments and no actual testing...

      I think the best algorithm would be to use the weather compensation system to calculate a maximum allowed CH temperature based on outside temperature and the weather compensation slope/offset adjustment, and then use that maximum to proportionally scale the evohome requested temperature rather than clamping/clipping it.

      The reason is that if you simply clamp the Evohome's requested flow temperature (which I believe is 90C for a 100% heat demand ?) then the lower the weather compensation CH max the more of the proportional control band from the Evohome you lose when clipping the requested temperature.

      However if you linearly scale it you maintain the full proportional control from the Evohome. Also in theory this means that if you have your weather compensation slope and offset set correctly for your house then the required heat demand sent from Evohome will be the same regardless of whether the outside is cold or warm.

      This should in theory remove the necessity of the Evohome to constantly "re-learn" the weather conditions by inference. When the weather gets colder you get an increased flow temperature from the same heat demand sent by the Evohome and essentially the Evohome no longer has to worry about adapting to changing heat loss through the walls as the change in flow temperature will compensate for it. This should be beneficial on days/weeks when there are large swings in outdoor conditions within the same day or few days.

      Comment

      • kevinsmart
        Automated Home Ninja
        • Sep 2018
        • 257

        #18
        I ran a short experiment, reducing the Max CH Setpoint and found with it set at 60C, for an outdoor temperature of 5C, the proportionally lower requested temperatures via OpenTherm resulted in my HM80 hunting around more.

        This was probably due to the boiler being unable to maintain the lower control setpoint and cycling.

        So my solution is currently capping the control setpoint based on the heat curve. Since my boiler Max CH temp was set at 60C at the front panel my setup was already sort of working this way. The difference now will be that the Max flow temp will reduce from 60C as the outside temperature increases above 5C.

        I’ll monitor it to see how it goes.

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          #19
          Much of the Evohome 'logic' is contained in the TRVs.

          I suspect that artificially messing with the boiler flow temperature will cause havoc with that learned data.

          I've never had cause to think that Evohome wasn't hoping well because of the outside temperature, so I'd be nervous about fiddling.

          P.

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