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connecting tado with opentherm vs relay

I recently changed my boiler/heater and decided to buy a Baxi Duo-Tec+ 33GA which has native OpenTherm support.

I bought a Tado Smart Thermostat and the Extension Kit, to connect wirelessly my thermostat to the heater/boiler which is located in a different room.

Following Tado tech-support advice, I decided to use low voltage connectivity with OpenTherm.

Baxi implementation of OpenTherm is quite rich, and Baxi sells an optional remote control device which is certified for this boiler. This remote control device speaks Baxi OpenTherm+ and just connects with two wires (polarity is not important) to contacts 1 and 2 available on M2 connector (the long one). Baxi remote control device gets power from the same two wires. Once remote control device is connected, the heater control panel is mostly disabled, and all controls are given to the remote control device, which bidirectionally talks to the heater control board.

I did not buy this remote control device. I connected Tado according to the specs, removed the bridge connection from the M1 conntector on the boiler (where the open relay / timer should go) and performed configuration thru the smart thermostat interface.

After this setup, I got no errors on tado, and tado mobile phone interface shows an icon in the main menu where I can control hot water temperature. I could see that the temperature set from the app went thru the opentherm link to the boiler, but hot water was not working.

I also realized that Tado does not allow any control on the hot water temperature going to the heaters. When you connect tado via OpenTherm, you lose control of the water temperature going to the heaters, because that setup is not anymore available from the Baxi panel too. Speaking with Tado support technician I understood that Tado can control the hot water temperature in the heating circuit, but this setup is not available to the customer, and only is accessible to Tado support, and defined in the cloud back-end application. Apparently, this temperature is independent from the boiler/heater model, and does not consider if the system is a condensing heater, which allows, with lower delta temperatures, the maximum energy efficiency.

Given these conditions, and having no real benefit in controlling the hot water temperature, I decided to switch my configuration to open relay mode, disconnecting the wires from M2 connector and moving to M1 relay, changing the connections in the extension kit to the relay side, and reprogramming the system from the thermostat panel according to the relay connection

With this setup I can easily control from the Baxi panel the heating water temperature, as well as the hot water temperature in the faucets. I also get all the benefits of Tado app with its connection with Weather forecast, away settings, preheating, etc...

I think that this setup is far more energy efficient, in my configuration, that the OpenTherm connection.

Best regards

Marco Guardigli

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  • During my experimentating with OpenTherm protocol connection to Baxi, i managed to have Baxi board stuck in a Error 83 situation, which was quite nasty, and the only way to reset the system was to temporarily connect the original remote control device. Luckily my plumber -who is a certified baxi specialist- was patient enough to allow these experiments.

  • Hello Marco, have you got on your boiler interface? In manual it is descrive6as interface which is needed before implementation of regulator (Siemens qaa73).
  • andyblacandyblac
    edited July 2

    I have a Baxi 830, and I have just link it to my Tado via OpenTherm and I can control the hot water temp in Tado App

    and also set up schedule to change temp at different times during the day, so I do not have to add cold for example when washing up dishes

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