OpenTherm Solution?

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Hi All,

I am about to start my new mini-project and I want to get some feedback from the community to drive in the right direction :slight_smile:

The goal: To be able to control the central HVAC system in my house via openHAB 2…

Hardware to be used:

  1. Hoval TopGas condensing gas boiler model TG-45
  2. Hoval TopTronic® T controller (installed on the boiler) Key Module 7 (2 mixing circuits, buffer storage unit and solar plant)
  3. 2 x Hoval RS-OT room station (installed in 1 room)
  4. 7 x Siemens RDF-110 Room thermostats (one in each of the 7 rooms in the house…but…not integrated with the Hoval System :frowning: )


The Hoval Boiler has an OpenTherm interface (2 wire bus system on connection pins #30 & #31) and I am planning to use this OT interface to be able to control the boiler directly from OH2.

I have found that there are many OT Gateways available from the manufacturer (Hoval), including a OT-KNX gateway as well as an Ethernet Gateway…

Additionally, I found another option from

I am planning to use this Gateway to interface OT to KNX and from there, use the KNX Binding in OH2 to control the Boiler.

Since there is no OpenTherm binding available (correct?)… I believe that I will need to use an intermediate translation protocol to achieve my goals…

More news soon as the project develops (of course…expect a full write-up at the end of it :))

Ps: I found some old info here:!topic/openhab/_hxrvt6D5t0
Ps2: Another way (instead of OT->KNX->OH2) would be: (a) + (b)… Opinions on this path?

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I’m using the the controller software puts message on an MQTT queue, which are processed by openHAB.

What my combination of Thermostat and Heating Unit I’m not able to override the setpoint so, I can only monitor in my setup.

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I am using the Atag One controller with a small Java program to set the temperature and read many parameters from the boiler. The openHAB exec binding is used to call the java jar.
more information about the controller:

about the java program:

The Atag One controller uses OpenTherm to communicate with the boiler, but this is all handled by the controller. You do not need any knowledge of the OpenTherm protocol.


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I had the first set back today… :frowning:

The Hoval TopTronic® T controller takes up the only possible OpenTherm Bus connection to the Boiler Controller (BIC) of the TG45 and it is not possible to connect in parallel (or in series) another external OpenTherm device.

I used a Nest Gen3 Thermostat with OT (EU version) and I couldn’t get it to work when the TopTronic controller was connected.

If I disconnect the TopTronic, the Nest works fine. This is not a working solution for me since the TopTronic controller performs many more functions (controls the pumps, thermostats, solar plant, etc) than the Nest…

This means that I cannot use OT to interface to the boiler system… :stuck_out_tongue: (so neither the will work)

Next method to try: ModBus with Hoval Part Number: 6014 389 (Gateway module ModBus TTT/ZM) and the Modbus Binding for OH2…