Monitoring the Remeha Calenta

Last week I attached some 1-Wire temperature sensors to the floor heating tubes to monitor flow- and return temperatures. While I was doing that, I looked at my Remeha Calenta again and thought what a pity it was that I failed to monitor the Calenta with the Remeha Gateway. I’ve known for a long time that there’s a 2nd solution to this: Remeha Recom. A “special cable” and the Recom software make it possible to monitor your Remeha Avanta, Quinta, Calenta (and more). However the price for this cable is rather high; you won’t be able to get it below 130 Euros with as only result a Recom window on your screen where you can see all kinds of things being monitored… all you can do is make a screen-dump or log the values to a file… 🙁

And if I would buy that “wonder cable”, would I be able to understand the protocol? I didn’t want to risk the chance of buying that expensive cable and not coming any further than using Recom, so i stopped thinking about it. Untill last week.

I started roaming the Internet for more information; I read about “it” being a null modem cable; I looked at the PCB and saw the RJ-11 connector for the Remeha cable was labeled RS-232; I took a long cable with a RJ-11 plug and stuck it into the connector and started measuring with a voltmeter; I saw the outer 2 were 5V and the other GND; that leaves the inner 2 for the serial part. I started thinking about what this super duper Remeha cable could hide inside to make serial communication work? There has to be something special to it… converting TTL to RS-232 perhaps? Why not give it a try…

I have a MAX3232CPE laying around for some time; so I planted it on a breadboard, added some 0.1 μF capacitors and built this schema I found (click it):

Here’s the result:

TTL <--> RS232

TTL <–> RS232

Guess what… it works!! My Remeha Calenta was recognized immediately and I watched the data arriving in Recom:

Remeha Recom v4.1.1

With no more than 8 Euros I created my own interface cable; too ridiculous… Special cable?  My A!@&%#%^!!!

OK, but this was just the beginning. With a serial port sniffer i saw the Calenta being queried and I started recognizing the flow temperature, return temperature etc. in the responses that came back from the Calenta. These packets look like this:

02 01 FE 06 48 02 01 F0 14 AA 14 00 80 00 80 86 F3 00 80 B7 10

00 80 40 1F 70 17 00 80 00 00 00 00 00 BC 02 00 00 00 00 64 00

00 00 00 C2 0B 10 00 FF FF 00 00 00 00 FF FF 17 00 BC 02 00 00

00 00 00 00 00 00 00 00 00 C5 9B 03

After an hour or so, i knew where to find some temperature values and stuff like that. The F0 14 marked in red for example, means 53.60 degrees Celsius. That was easy… not good enough for a quiz 🙂 But I also saw on the amount of values Recom could produce from the incoming bytes, some things had to be stored in bits; the on/off, yes/no, open/closed values appeared not to be stored in a byte, but ‘hidden’ somewhere in a bit maybe? Hmm, how do I know exactly when the Gas valve opens or closes, so how I can locate the bit for that? If it is stored in a bit? This could become a bit more challenging and time consuming than I would like.

Aha! There’s a config directory belonging to the Recom software package, containing XML files; one of them was named exactly like the model that Recom detected. Let’s have a look inside that file… Bingo! These XML files contain very detailed information; they tell me that I can find that Gas valve value in bit 0 of byte 38… Yeeha!

Now it’s just a matter of finding out some more about the leading and trailing bytes and I can start creating my own Remeha Calenta interface to my Domotica system.

Finally I’ll know what the Gas is being used for: DHW or heating. And when the water pressure drops below a value I find alarming, I can be warned before it’s too late. And I can see how much power is actually being produced; and …

Update: RJ-11 is incorrect; it’s a 4P4C connector!

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16 Responses to Monitoring the Remeha Calenta

  1. Rene Klootwijk says:

    This sounds very promising and even make me buy a Remeha kettle when my current Nefit (10 years old) needs to be replaced.

  2. Peter says:

    That is quite a development Robert!
    I’m really looking forward to your findings!
    Great work!

  3. WAUW! This is really cool information. I’ve got a Remeha Avanta and this information is invaluable 🙂


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  7. Robert says:

    Just got my new Calenta installed, and read about the possible readout of my Calenta. Could you please share with me your decoding scripts. So I can implement my Domoticz to read out the Calenta.

    Thanks much,

    • Hi Robert,

      In fact, I already shared my code with someone who’s going to build an interface for domoticz… this was about 2 months ago.
      So are you sure you also want to start doing this?


  8. Hi Robert and Robert
    I was just wondering if you’d made any progress with the interface for domoticz?

    I’m looking to build something, probably arduino based, to send the data from my remeha to my openhab server, probably using MQTT.

    Any code you have that might be useful to me would be really appreciated.


  9. HansN says:

    This looks very promising. At this moment I have a laptop running Recom hooked up at my Remeha Avanta. But I’m thinking of replacing it by some self-built project. I’m not sure yet what. The ESP8266 looks very nice, but then you need some voltage conversion.
    BTW, on the current version of Recom the XML doesn’t seem to contain the detailed information anymore.

  10. Peter says:


    Maybe it’s a bit a question for a n00b but how do you convert F0 14 (hex) to 53,60 (dec)?

    Kind regards,


    Ps. don’t you forget the information for the Homey?

  11. Shimager says:

    Actually the connector to connect with the Calenta is indeed 4P4C, indeed not RJ-11, but RJ-10 model.

    Then I am curious about experiences of others. I changed my thermostat to a Nest V.3. But the control pattern that can be read from the Calenta and that appears generated by the Nest looks like a hyper-nervous pulse width modulation (PWM) model, rather than a peaceful amplitude modulation (AM) that works towards an ideal PID control pattern. This way the Nest causes my system to switch on and off every 5-10 minutes! Very bad. I will get rid of the Nest device again, despite the pretty interface. But of course I am curious what other people see when monitoring the control pattern of a Nest with the Calenta. Anyone here experiences? Also hyper-nervous or peaceful like heaven?

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