3rd Gen Learning Thermostat does not heat when True Radiant is On

[MieszkoChmiel]

If True Radiant is On, then heating doesn't work. The room temperature is far away from the set point. If I set True Radiant to Off, then heating works fine. I use a Heatlink and a Viessmann Vitodens 111-W boiler. The configuration is in-floor heating and a combi boiler controlled by the OpenTherm protocol.

[virnab]

Hi MieszkoChmiel,

Thanks for posting in the community.

I understand that you've determined that your Nest Learning Thermostat 3rd gen, connected via Heat Link and OpenTherm to your Viessmann Vitodens 111-W boiler (for in-floor heating), fails to initiate a heating cycle when the True Radiant feature is enabled. I'm glad to help you with this concern.

With True Radiant, your Nest Thermostat learns how long your radiant system takes to heat up your home. It’ll turn the heat on early to reach the scheduled temperature, then turn it off early so your home won’t heat over the temperature you want.

True Radiant automatically adjusts when the system turns on and off. There’s no need to guess how early you need to turn on the heat to get the temperature you want when you want it.

For example, the Nest thermostat might learn that to reach 21.5ºC, it should turn the system off when the room hits 19ºC because the already hot radiators will take your home the rest of the way to 21.5ºC. The Nest thermostat will also maintain that temperature by turning your system back on before the temperature gets too low. To learn more you please check this Google Nest Help Center article: True Radian.

After considering the previous information I recommend you to check the following facts:

1. Confirm there are any exterior doors or windows opened.
2. Consider if you recently changed the control thermostat temperature on the boiler. If you do, the Nest Thermostat will need time to learn the new adjustments so it can correctly activate heating to reach a setpoint early or turn the heating off to prevent overshooting the target temperature.
3. Verify that the heating schedule is synchronized across the Nest Thermostat, Web page and mobile app.
4. Check the Nest thermostat temperature schedules are synchronized according to the True Radiant settings.
5. Check Eco Mode settings are not interfering with the True Radiant settings.
6. Consider that the Nest Learning Thermostat will prioritize the preferences set on the True Radiant feature rather than the temperature scheduled or Eco Mode settings. Example: If you have a scheduled setpoint of 20°C at 7:30 and True Radiant activates the boiler at 2:30 (pre-conditioning with max duration set for 5 hours).

If you have any other questions about Google Nest products or need more help with anything else, just let me know.

Best regards, 

Virna

[MieszkoChmiel]

Hi @virnab,

thank you for the feedback. I checked the facts as below but my problem is not solved.

1.  I confirm that windows and exterior doors are closed.
2. If Vitodens 111-W is set to the OpenTherm mode, then Vitodens 111-W does not allow to set any temperature manually on the boiler, all temperatures (heating and domestic hot water) are controlled by OpenTherm.
3.  The heating schedules are OFF on the Nest Thermostat, Web page and mobile app.
4. I'm not sure what you mean by "Check the Nest thermostat temperature schedules are synchronized according to the True Radiant settings". Can you elaborate?
5. Eco Temperatures are OFF and Eco is stopped, Eco/Heat to: OFF.
6. I don't use any temperature schedule, i.e. I want the same set temperature all the time. I don't use Eco.

Please also note that I use the following additional settings Auto-Schedule: No, Home/Away Assist: Stop Using. Please help me to solve the defect.

BR,

Mieszko