Opentherm Monitor finished

This post will be the last one about the Opentherm Monitor. OTOH, when is something really completely finished…

Arduino Serial Monitor

I could spend some more hours on the Opentherm (OT) Monitor and in particular the sketch, but for now it’s good enough. I should add some extra code to validate the OT frame but that would also mean I won’t be able to analyze ‘strange’ frames with unknown Data ID etcetera on my PC. So I’ll leave it as-is for  now. The Opentherm Gateway is waiting 😉

From what I’ve seen during the last 24 hours, the ‘quality’ of the frames I receive is quite good; somehow there seems to be an invalid frame on the wires every minute or so, and I can’t find out what it is. This same thing happens with the Opentherm Gateway Monitor, so I think both are having the same problem. The Data ID tells me it’s probably an OEM frame…?

OpenTherm Decoder

The Opentherm Decoder running on my PC receives the 4 OT bytes from a serial port and decodes those bytes to something human readable: whether the frame came from the Thermostat or the Boiler, Message type and the meaning of the Data ID. The 16-bit data value (there where you can find the temperatures, pressure and status bits) is not decoded yet; well, it’s all in the Opentherm Protocol documentation, so that should be no problem.

Now I can use this Opentherm Monitor as an additional display near the boiler! The Remeha Calenta already has a rather large display showing stuff like status, water pressure, whether the pump is running, but it doesn’t display flow- and return temperature, control setpoint and I’m sure I can think of some more interesting stuff I wanna see – that’s what the Opentherm Monitor is going to do for me. I already have a 16×4 LCD, so all I have left to do is finding a suitable enclosure, build everything in there and I’m done!

I really liked getting this Opentherm Monitor to work without errors; in fact, getting it to work was more exciting than building it. Learning on the job about ATMega timers, Manchester decoding and programming the whole thing in C from scratch was one big adventure.

The most important references I used were:

And here‘s the sketch- no additional libraries needed, free to use and no guarantees that it will work for you just as well as it does for me. Have fun!


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5 Responses to Opentherm Monitor finished

  1. Freddy Martens says:


    I guess the sketch will work with my shield as well. I need to find some time to test but I will certainly do. Thanks for the nice posts on this topic.


    PS: The synology software is progressing.

  2. Freddy,

    IIRC your shield uses digital 7 as input pin, so it’s just a matter of changing the RXpin in the sketch and off you go 😉

    Good luck with your Synology project!

  3. Pingback: Domotica using a NAS, a CA42, a Remeha heater and HTML5 | ATS TechLab

  4. Martin says:

    Hi Robert,

    i also want to monitor my remeha quinta.
    Therefor i need this recom interface cable (GMI), i wanted to ask you, where you have initially purchased your interface cable?

    Remehe doesn´t sell directly to customers and the sales partner in austria has no glue about this interface.
    Thank you.

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