Using Nest Learning Thermostat & Heat Link V3 and 2-circle OpenTherm Boiler (Geminox)

[P_K]

Hi,

I have been using the Nest Learning Thermostat & Heat Link V3 for 5 years in my apartment, and I am happy with it. I would like to use it in my house as well, where I have installed the Geminox 17THRi DC boiler controlled by Siemens QAA73 (I need remote access, which is why I am considering Nest...). However, I found that I cannot use it with this type of boiler because the Nest V3 can only control hot water and one heating circle via OpenTherm. I require two heating circles. Currently, I am using the same schedules for both heating circles, so I can use only one schedule to switch the heat on and off for both circles. I am considering an OpenTherm bridge that I can insert between the boiler and Nest, and modify the commands to control both circles. Alternatively, perhaps someone here has already figured out a solution and can point me in the right direction. Or, maybe there is a slim chance that Google will modify the firmware in the future (just kidding...). Thank you.

[LarisaL]

Hi there, thanks for reaching out! As you mentioned, the Nest can control only 1 heating zone since it is connected to the boiler. You can check more about the OpenTherm and zoned system in these 2 articles:

• OpenTherm and what Nest thermostats can do with it
• Learn about zoned system compatibility with Nest thermostats

[P_K]

Hi, thank you for your reply and links, but this doesn't help. The valves that control both circuits are built into the boiler and are controlled by OpenTherm commands sent from the original Siemens QAA73. (The boiler itself has two circuits.). I theoretically could use two tehrmostats with two HeatLinks, but unfortunately, I don't see there an option in the Nest to select which circuits it will control and I am also not sure if I could connect two HeatLinks to the boiler (so the first controls Circuit 1 and the second Circuit 2...). Typically, the 2nd circuit is employed for floor heating, while the first is used for radiators. In my case, the first circuit is dedicated to ground floor radiators, and the 2nd circuit serves 1st floor radiators. If it were possible to address two circuits (+ hot water), I would be able to utilize two Thermostats. I believe achieving this would require software modification or hacking the commands sent to the boiler...

[AlexD]

We can't advise on any modification in relation to the boiler. The thermostat is designed to work with compatible OpenTherm systems, the ones that have the particular logo, but these are generally only Combi boilers. The thermostat can also work with zoned systems but these are on/off valves usually. 

If your valves do use the compatible OpenTherm protocol it might be possible to connect a thermostat to each but it would be best to check in with an installer or a Nest Pro as well as the manufacturer of the boiler as they may have further information on this. 

[AlexD]

Hi, I wanted to check in. Is there any further assistance I can try and provide? 

[P_K]

Hi, that's all. Thank you. I discussed with two companies doing heating systems and they said better to forget OpenTherm - no perspective.

[Vladut]

That's great. Let me know if you need anything else.

[LarisaL]

Hi there, just checking in! Is there anything else we can help you with?