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Temperature always too hot or too cold

Junior Homie

Hi, I have the following Bosch devices configured in my living room:


  • Smart Home Controller II
  • Raumthermostat II
  • 2 x Heizkörper-Thermostat II (I have two radiators in my living room)


They are all using the latest firmware, and all configured correctly with the Smart Home app on my phone.


I set the "warm phase" temperature to 23° and configured the schedule to begin the warm phase at 7:00 in the morning each day. Then I wake up at 8:00.


However, I have discovered that the thermostat never seems to stay at 23°. It is always either more than 1° above or 1° below the target temperature.


For example, when I wake up at 8:00, the room thermostat shows that the actual temperature is 24° or even higher. It seems that the radiator thermostats have opened their valves to 100% for too long, and this resulted in the temperature increasing beyond the target 23°.


I've attached a screenshot to show this. You can see from my screenshot that the target temperature is 23° but the actual temperature is 24.3° which is too hot.

Bosch Smart Home - temperature exceeded.png


Then the radiator thermostats close their valves to 0% and the room slowly cools down, but now the temperature drops down to 22° or lower - so now I feel it's not warm enough.


Then it repeats: the valves are opened again to 100%, and the temperature jumps up to 24° or more. So throughout the day, the temperature is jumping up and down beween 21.x and 24.x but never sitting stable at 23°. Therefore I feel either too hot or too cold in my apartment. (My body is very sensitive to the fluctuations in the temperature).


It would be nice if the radiator thermostats would start to close their valves *before* reaching the target temperature, so that the target temperature would not be overtaken, and then the thermostats should keep their valves open at a small amount (maybe 20%) to maintain a constant and stable temperature throughout the day.


When I used the original, mechanical thermostats that were attached to my radiators, I could set them to a certain position in the morning, and they would maintain a constant and stable temperature throughout the day. I would like the Bosch thermostats to function the same way.


I've seen some other posts in this forum from other users with the same problem, so it seems that this is a known issue. For the record, a couple of months ago I had the original Smart Home Controller (not the new model) and it had the same problem - in the end, I sent it back to Amazon for a refund. I hoped the new Smart Home Controller II would be better, but it seems not.


One other note: before I purchased the Bosch Smart Home devices, I tried the smart thermostats from Homematic IP. In the end, I sent them back to Amazon for a refund, because I didn't like the way they worked. However, one feature that I liked about the Homematic IP thermostats, is that they did maintain a stable and constant temperature throughout the day. And their app also has a feature called "Optimum start/stop function", which allows the thermostat to calculate the required lead time in advance, to slowly close the valve before passing the target temperature. I've attached a screenshot here of this feature. I think it would be helpful if Bosch can introduce something similar, to help prevent the thermostat from allowing the temperature to go above and below the target temperature throughout the day.


Homematic IP - Optimum start-stop function.png


In any case, if my mechanical thermostats can maintain a stable temperature, and if the Homematic IP thermostats can maintain a stable temperature, then I see no reason that the Bosch thermostats should have this problem. 


CEO of the Homies

Good summary of some of the known Radiator Thermostat II issues. Unfortunately, at this point your only options are to either return them or wait and hope that Bosch will eventually be able to fix them with a firmware update (which they have tried several times).


FYI, these issues are specific to the generation II thermostats and unrelated to the SHCs. The Radiator Thermostat I works just fine, which is no surprise since they were actually made by the same company which also makes the Homematic IP thermostats. So downgrading the thermostats instead of upgrading the SHC would have helped, but of course you couldn't have known that.

Privater Endanwender, kein Bosch-Mitarbeiter.

Thanks Maurice. Interesting to learn that the problem lies with the thermostat and not the SHC. I had previously owned the 1st-gen SHC - I bought it a couple of months ago - and had the same problems, and I assumed (wrongly) that the fault lay with the controller, so I sent it back to Amazon for a refund and waited to buy the new SHC II. Of course that didn't help after all - as you've explained, the fault is actually with the thermostat. 😞


It's quite funny that you say that the 1st-gen thermostats were made by the same company that makes the Homematic IP thermostats. That explains a lot! But so frustrating that Bosch have released this shiny new 2nd-gen thermostat, with its fancy display and everything, but it actually behaves worse than the 1st-gen ones.


I can only hope they'll eventually get it right with a firmware update. Otherwise, I'll be sending the lot back to Amazon again - the controller, all the radiator thermostats, the room thermostat, the smoke alarm - all will be returned to Amazon as "faulty".

New Homie

I have the same issue. Currently testing a few thermostats and the Bosch II seems to be "on and off" all the time and never uses a middle ground where it's gentle warmth. As much as I want to keep it for future proofing, I am considering selling it. 


Is there any word on when the next update is supposedly gonna drop?

Apprentice Homie

I wrote a small piece on my setup in this thread


Basically, imho the boiler needs to be integrated into the bosch smart home to close the loop. Else it works open loop with the known over en undershoot phenomenon. 


Tbh, I think Bosch not mentioning this has to do with marketing which I understand from a marketing perspective. However, it's a bit unfair to the people who are unable to figure is out themselves. A lot of money is spend before realising that either it's imposible to get the desired performance or they need to invest in another gateway to integrate their installation fully. 


@Bosch please treat your customers with respect. 

Enjoy your day, 

Best regards


Apprentice Homie

Btw, the first generation trv's work well because the ct200 is also an ems2 thermostat. It both integrates your boiler and your thermostatic valves. Actor and sensor are fully closing the boiler loop. The algorithm can only truly work as per how a stable system is designed. Close the loop!