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Thread: Raspberry Pi OpenTherm Weather compensation project

  1. #11
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    And that even has an input for an outside temperature sensor

  2. #12
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    Quote Originally Posted by bruce_miranda View Post
    And that even has an input for an outside temperature sensor
    If it's for a wired sensor it doesn't help me much - there is no good location to mount a wired sensor within reasonable cable reach of my boiler to get a proper outdoor measurement, (just sticking a sensor on the side of a brick wall is influenced massively by the heat from the house) and I don't particularly want to drill holes in the wall for it either.

    The wireless sensor on my weather station is perfect and that's one part of the project that's already in the bag so to speak. The Opentherm side of the design is by far the most challenging - and I'm not sure whether any of the opentherm projects I've seen are capable of being a full slave implementation by themselves rather than just being a pass-through bridge that alters communications slightly on the way to a real Opentherm slave. (boiler) I haven't had time since my previous posts to look into this further yet.
    Last edited by DBMandrake; 29th April 2019 at 12:04 PM.

  3. #13
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    You could reconfigure the wired temperature input to something else. Any way, the OT gateway can also run in stand alone mode. See http://otgw.tclcode.com/standalone.html#operation

  4. #14
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    I can see why it would be nice to use data from a weather station if you already have one but just to comment that I use the Open Weather Map as an online source for our local temperature etc.

    A Raspberry Pi running ebusd and pyowm controls the target water temperature of our Vaillant boiler (which is not Opentherm of course but Vaillant Ebus).

  5. #15
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    Quote Originally Posted by Jackthom View Post
    I can see why it would be nice to use data from a weather station if you already have one but just to comment that I use the Open Weather Map as an online source for our local temperature etc.

    A Raspberry Pi running ebusd and pyowm controls the target water temperature of our Vaillant boiler (which is not Opentherm of course but Vaillant Ebus).
    I could easily use an online weather service using pywapi - I already use this in my evohome-munin graphs to get the local forecast temperature, and it's literally about 5 lines of python, however there are a few reasons I'd prefer to use an actual outdoor measurement:

    1) The forecast temperature doesn't update very frequently and tends to lag behind so whenever the temperature is rapidly rising or falling it can be several degrees behind the true conditions. I've seen descrepancies approaching 6 degrees.

    2) There are times when the forecast temperature is a long way out even when when not rising or falling - not really sure whether it's due to a microclimate effect or whether the forecast is extrapolated from a far away weather station. I'm in Motherwell and many weather services even when told Motherwell just end up either giving me the same results as the Glasgow forecast or some extraplation of it. Glasgow might only be about 10 miles away but the weather and temperature is very frequently quite different!

    3) I wanted to design the system to not rely on an internet connection in any way. That even includes functioning correctly with an incorrectly set system time. (Since there is no realtime clock in a Pi, so no internet connection means booting in 1970....) Believe it or not its actually much more difficult in my house to get a reliable WiFi signal to the boiler cupboard (which is in the kitchen back porch out of Wifi range) than receive the outdoor temperature sensor in the boiler cupboard. And both require a USB device plugged into the Pi anyway.

    4) I just like the idea of using an accurate, fast responding weather station sensor that I already have to do this job. If I do end up getting an internet connection to the Pi somehow I can always write in the online weather report as an automatic fallback should something go wrong with the weather station, although it's not really necessary as I look at the weather station receiver every day and would soon notice if it had died. (And the code would just implement a manual fallback flow temperature if it lost comms with the sensor for more than 30 minutes)
    Last edited by DBMandrake; 25th May 2019 at 11:51 AM.

  6. #16
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    Iím 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.

    Iím 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?

  7. #17
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    Quote 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.

  8. #18
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    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.

    Iíll monitor it to see how it goes.

  9. #19
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    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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