Evohome opentherm and weather compensation

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  • Speedychuck
    Automated Home Jr Member
    • Oct 2015
    • 19

    Evohome opentherm and weather compensation

    The last piece of my puzzle I'm yet to install is my weather compensation sensor. It's sheer laziness because I have to make quite a long cable run, then get up on a ladder to fit it.

    Does anybody know if the evohome controller will react to outside temperatures? I'm not totally sure weather the temperature variation is managed by the boiler- for example on a cold day if the controller asks for 45 degrees does the boiler provide 50? Or weather the temperature data is fed back to the controller and it does the work?

    On the other hand, if the controller asks for 45 degrees is that what the boiler gives regardless?

    If anybody has an insight into how opentherm and weather comp work with evohome that would be great.

    Thanks
  • emmeesse68
    Automated Home Guru
    • Dec 2014
    • 103

    #2
    Hi Speedychuck,

    I have OpenTherm up'n'running and I managed to set up both an outside temperature sensor attached to my Ferroli boiler and an OpenTherm gateway between my boiler and the Honeywell R8810 to monitor what's going on between them.

    Right now I have the weather compensation disabled at the boiler, but the sensor is still telling the outside temperature to the boiler, that reports it to the R8810. As far as I understand, my EvoHome doesn't even take note of it. Nor does it show an outside temperature on the controller screen.

    When I activate weather compensation on my boiler, I have to configure the min and max temp and the slope, that means the compensation curve. My boilre then reads the outside temperature and adjusts the maximum temperature accordingly.

    On the OpenTherm side, what I read on the wire is that my EvoHome sets a control setpoint (it also sets modulation levels, but I'm letting these alone for now, for sake of simplicity, this is going to be a long post anyway ) that's the temperature setpoint for the boiler to reach before turning the burner off.

    What happens then is that the external sensor would modify the maximum water temperature that the boiler would provide, so when the outside temperature rises, the boiler would decrease the maximum water temperature.

    Last year I played with this a bit, and I think sometimes the R8810 tried to send a control setpoint higher than the limit defined by the compensation curve at that outside temperature. Trying to reach the control setpoint, my boiler was reaching that limit and used to stop the flame, showing a control code (d2 on my boiler) that meant that water was hotter than the allowed maximum. After a few minutes water used to cool down enough for the boiler to fire again, that made temperature rise again to the limit and the cycle repeated. This didn't look too good to me, that's why I deactivated weather compensation at the boiler.

    Actually my boiler would pass its temperature limit (max control setpoint) to the R8810, but appearently that doesn't make my EvoHome immediately aware of it (maybe the EvoHome reads it once in a while... or just once), allowing for a control setpoint higher than the limits. I might try enabling compensation again to dig into it a little deeper and check some different set-up.

    I'd like to know Honeywell experts opinion anyway, that could save me a few pointless tests (I'm a bit lazy as well ) and maybe some stress for my boiler...

    Comment

    • Speedychuck
      Automated Home Jr Member
      • Oct 2015
      • 19

      #3
      Thanks for the very in depth reply emmeesse. It's pretty much exactly what I wanted to know. On what you've said, I'm gonna leave the weather compensation out.

      If the evohome controller "learns" warm up times for a given water temperature, I can't see that adding an extra variable in the system is going to help. Especially if the controller is unaware of it. If the info from the sensor was fed back to the controller and the controller then acted on it, I would use it.

      So my thoughts are if the controller asks for 50 degrees and the boiler only provides 45 then the controller will be expecting the room to heat up faster than it does. I don't know how the evohome learns, but presumably the next time it asks for the same 50 degrees and the boiler allows it, it will heat up faster and possibly overshoot the set point.


      I wonder if the evohome takes account of the local weather automatically given the fact it is location aware.

      Thanks again for the info.

      Comment

      • top brake
        Automated Home Legend
        • Feb 2015
        • 837

        #4
        Hello, what boiler do you have?

        Originally posted by Speedychuck View Post
        The last piece of my puzzle I'm yet to install is my weather compensation sensor. It's sheer laziness because I have to make quite a long cable run, then get up on a ladder to fit it.

        Does anybody know if the evohome controller will react to outside temperatures? I'm not totally sure weather the temperature variation is managed by the boiler- for example on a cold day if the controller asks for 45 degrees does the boiler provide 50? Or weather the temperature data is fed back to the controller and it does the work?

        On the other hand, if the controller asks for 45 degrees is that what the boiler gives regardless?

        If anybody has an insight into how opentherm and weather comp work with evohome that would be great.

        Thanks
        I work for Resideo, posts are personal and my own views.

        Comment

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