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Thread: DIY OpenTherm Weather Compensator for Honeywell evohome

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  1. #1

    Default DIY OpenTherm Weather Compensator for Honeywell evohome

    Kevin Smart has created his own DIY Weather Compensator for evohomeÖ

    https://www.automatedhome.co.uk/tuto...l-evohome.html

    M.

  2. #2
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    Nice one!

    I ordered an OpenTherm Gateway, Soldering Service, Enclosure and FTDI cable from nodo-shop.nl out of curiosity, to monitor communication between the Evohome R8810A OpenTherm bridge and boiler. It turns out that the bridge does read the outside temperature from the boiler. Also, I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome.
    I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.
    Last edited by DBMandrake; 13th January 2020 at 04:27 PM.

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    On the topic of Opentherm. I have managed to decode all the OT messages between the Evohome Controller and the OT Bridge. If you are using an HGI80 or similar, in Domoticz, you will now see new devices appearing that allow you to track various OT parameters like the Control Setpoint, Boiler Flow and Return Temperatures, the Boiler's Relative Modulation etc. Until my PR gets merged in with the rest of Domoticz, you can download the code from https://github.com/brucemiranda/domoticz and let me know how you get on.
    This now means that boiler parameters can be monitored without needing a separate OT Gateway, eBUS monitor or any boiler software.

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    Quote Originally Posted by DBMandrake View Post
    Nice one!


    I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.

    This is the wrong conclusion as far as I'm concerned.

    As I've pointed out on a few occasions, connecting a more fully featured OpenTherm thermostat to a weather compensated Intergas boiler means that the boiler expects the thermostat to use its own compensation curve as well as its own maximum central heating temperature setting. The boiler just relays the outside temperature reading over to the thermostat so it can calculate a water temperature to send back to the boiler. It's then down to the boiler how it maintains that temperature.

    My OpenTherm/weather compensated Intergas boiler & Remeha iSense thermostat combo works perfectly as above, as does the inlaws Intergas/Nest v3 setup configured as an OpenTherm installation.

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    @kevinsmart given that you have studied the OTGW firmware in some detail, could you explain why the OTGW clamps the Control Setpoint to 60C even when it's in Monitor mode. Without the OTGW, my OT Bridge instructs my boiler to ramp up to 90C, ofcourse my boiler doesn't do that. However just the presence of the OTGW means that the Control Setpoint is rarely more than a few C over 60C, 66C is the max I saw. Why is the OTGW modifying messages even in Monitor mode? Is there some failsafe in the firmware that does that?

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    Quote Originally Posted by blowlamp View Post
    This is the wrong conclusion as far as I'm concerned.

    As I've pointed out on a few occasions, connecting a more fully featured OpenTherm thermostat to a weather compensated Intergas boiler means that the boiler expects the thermostat to use its own compensation curve as well as its own maximum central heating temperature setting. The boiler just relays the outside temperature reading over to the thermostat so it can calculate a water temperature to send back to the boiler. It's then down to the boiler how it maintains that temperature.

    My OpenTherm/weather compensated Intergas boiler & Remeha iSense thermostat combo works perfectly as above, as does the inlaws Intergas/Nest v3 setup configured as an OpenTherm installation.
    You seem to be reading and responding to something that I didn't actually say.

    I didn't even refer to weather compensation at all. I actually said "is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control". This was in response to Kevin's remark "I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome."

    So while Kevin was trying to implement weather compensation, along the way he discovered that the Intergas boiler in Opentherm mode does not send the CH max flow temperature setting from the front panel to the OpenTherm master, (Evohome) however when he intercepted and modified this value from the boiler Evohome did indeed respect this maximum flow temperature requested by the boiler in the flow temperature requests it sent back to the boiler.

    For a long time Intergas and Honeywell have pointed fingers at each other but in my opinion this proves that the blame lies with the boiler not sending the CH max temperature setting to the Evohome.

    That's all I was trying to say.

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    Quote Originally Posted by DBMandrake View Post
    You seem to be reading and responding to something that I didn't actually say.

    I didn't even refer to weather compensation at all. I actually said "is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control". This was in response to Kevin's remark "I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome."

    So while Kevin was trying to implement weather compensation, along the way he discovered that the Intergas boiler in Opentherm mode does not send the CH max flow temperature setting from the front panel to the OpenTherm master, (Evohome) however when he intercepted and modified this value from the boiler Evohome did indeed respect this maximum flow temperature requested by the boiler in the flow temperature requests it sent back to the boiler.

    For a long time Intergas and Honeywell have pointed fingers at each other but in my opinion this proves that the blame lies with the boiler not sending the CH max temperature setting to the Evohome.

    That's all I was trying to say.

    The article linked to at the start of this thread has some incorrect information in it that I have applied bold text to below.

    Evohome Article:

    I was frustrated that both the Intergas HRE SB boiler and Evohome controller ignore the outside temperature reported by a connected external sensor when using OpenTherm. The Intergas manual states…

    The outside temperature sensor can be used in combination with an on/off room thermostat or an OpenTherm thermostat.

    In fact, the boiler only performs weather-compensation adjustment when using an on/off room thermostat. With an OpenTherm thermostat such as Evohome, the thermostat is responsible for any weather compensation. Although the Evohome controller fetches the outside temperature from the Internet it does nothing with it other than displaying it.


    I was trying to point out that the Intergas boiler in OpenTherm configuration ignores its own internal max flow setting as it is expecting to get that from the thermostat - in this case Evohome. I can't see what purpose it would serve to send this figure to the thermostat knowing it will be ignored in favour of the one calculated by the thermostat itself, can you?

    In my experience, both iSense and Nest V3 OpenTherm thermostats require the installer to enter a max CH flow temperature to be used by the boiler, but I understand Evohome does not, it instead needs this to be set on the boiler itself, which is turning out to be problematic for many as Evohome seems to have no inbuilt limit to max CH flow temperature and many boilers besides Intergas expect an OpenTherm thermostat to decide what the max CH flow temperature should be.

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    Quote Originally Posted by blowlamp View Post
    The article linked to at the start of this thread has some incorrect information in it that I have applied bold text to below.

    I was trying to point out that the Intergas boiler in OpenTherm configuration ignores its own internal max flow setting as it is expecting to get that from the thermostat - in this case Evohome. I can't see what purpose it would serve to send this figure to the thermostat knowing it will be ignored in favour of the one calculated by the thermostat itself, can you?
    That's actually incorrect. If you refer to the "Weather - compensation adjustment" section in the Intergas manual you'll see "The maximum supply temperature (Tmax) is set via the temperature display".

    An OpenTherm thermostat can request a higher flow temperature than Tmax (as Evohome does). The boiler will of course limit it to Tmax as set via the temperature display.

    The issue is that the Intergas boiler does not report the max CH water setpoint (Msg ID=57) as Tmax, in fact the HRE SB does not support this message at all. Evohome then assumes the max CH water setpoint is 90C and proportionally reduces down from there.

    I think it would be useful for a thermostat to know the max CH water setpoint, I think that was DBMandrake's point.

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    Quote Originally Posted by DBMandrake View Post
    Nice one!


    I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.
    Thanks! Indeed as I found out, at least for me, the system works better with proportional temperatures calculated from a 90C maximum, rather than the 60C I have set at the boiler.

    So I ended up limiting requested flow temperatures to the weather compensation calculated maximum, rather than returning this temperature to Evohome.

    It may be that given time Evohome would adapt but I wasnít prepared to wait or risk this during peak heating season.


    I look forward to seeing how Resideo support weather compensation with their Q2 firmware update, but Iíll probably still use my solution to optimize it based on my local temperature sensor rather than weather station readings.

    I have my weather station registered with Wunderground but Evohome must be using a different service as temperatures vary.

  10. #10
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    Quote Originally Posted by bruce_miranda View Post
    @kevinsmart given that you have studied the OTGW firmware in some detail, could you explain why the OTGW clamps the Control Setpoint to 60C even when it's in Monitor mode. Without the OTGW, my OT Bridge instructs my boiler to ramp up to 90C, ofcourse my boiler doesn't do that. However just the presence of the OTGW means that the Control Setpoint is rarely more than a few C over 60C, 66C is the max I saw. Why is the OTGW modifying messages even in Monitor mode? Is there some failsafe in the firmware that does that?
    Bruce, when you have switched the OTGW to Monitor mode do you see any flashing LEDs?

    In Monitor mode you should see no LED activity and the OTGW should not be modifying data. If you are definitely in Monitor mode and seeing this behaviour Iíll try to work out why.

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