What is the influence of the "supply temperature" setting when OpenTherm is in use?

[fortean]

Hardware: Nest V3 (sw 5.9.5-2) + heat link (sw 2.1.2-1), connected to an OpenTherm capable heating system (combi heater with radiators). OpenTherm active and apparently working (my room gets warm and both the heating and warm water icons appear in my app).

My question: what is the influence of the "supply temperature" setting when OpenTherm is in use?

To me the "supply temperature" setting the Nest offers makes no sense, unless it LIMITS the maximum temperature the Nest can tell  the heater to use.  In which case I figure it should be set as high as possible, as the Nest should be allowed to figure out the proper temperatures to set by experience.  Anybody here who can enlighten me?

[AlexD]

Thank you for your questions and your interest in our product @fortean . I would be happy to try and assist.

You are correct. When using OpenTherm you can set the water temperature for the heating system and for the hot water. Setting it to a maximum higher temperature will mean that the location can reach the requested temperature faster. Depending on the system and location you may want a lower temperature and have the location heat slower or you may want it to heat and reach a set point faster. You can read more about OpenTherm control and scheduling here. 

[fortean]

Hi @AlexD , thanks for your reply.

Given your answer I initially thought I was correct in assuming that the "supply" setting reflects the upper limit that the Nest will use to set the flow temperature.  In other words: the Nest can freely choose to set any temperature its algorithms feel it should, but never above the value set in the "supply" setting in the Pro-configuration menu.  In which case the setting is superfluous and meaningless when one has a simple on/off setup.

However, upon further reading (and some conversations with the Google Support folks) I'm still in doubt, perhaps you can re-assure me?

So, reduced to a simple yes or no answer, can you tell me if indeed the "supply" setting in the Pro-configuration menu serves to set the upper limit used by the Nest to set the CH flow temperature? In other words: say I set this to 70 degrees (C), it may well be that the Nest orders the heater to use a flow temperature of say 50 degrees, or 60, or 59.2.. whatever, but NEVER higher than 70 degrees?

BTW: the link you sent does not work (page can't be found).

[AlexD]

Sorry to hear the link is now working. Clicking on it opens up the support page for me. Here is the full link: https://goo.gle/3jUOLvg . 

The set temperature will be the maximum the boiler will run at. While it is not as important for some setups, something like in-floor hydronic heating where you can't have more than a certain temperature or the wood floors can crack it can be helpful. Also, as I mentioned, in some locations you may not need such a high temperature or to heat the location as fast. 

A modulating boiler will vary the temperature between a minimum and the set maximum on the thermostat. So, yes, that setting will be the upper limit the modulating boiler will reach for the water temperature.

[fortean]

@AlexDNot sure why, but I still get this:

Howevever, I'm happy to read that indeed the "supply" setting demarcates the upper limit of the temparature the heater will use for the flow temperature (supply). That totally makes sense to me. 

I assume, just to make sure, that in case of a simple on/off setup, the setting is meaningless?

[AlexD · Accepted solution]

That is strange. I got the link again, https://goo.gle/3bGxNYq , try and see if perhaps it works now. 

Correct, in an on/off it is not as important. In an on/off system the heating will either be on, and go to the set water temperature on the boiler or be off. And if not set up as a OpenTherm system, you will not have the ability to set it on the thermostat. Though the maximum temperature may still be important if you have something like an in-floor system under wood floors, as I mentioned, as there you can't pass a certain temperature. 

[fortean]

Ah, success - the link now works, thank you 😀

Thanks for helping me out here! All clear now 😎

[AlexD]

No worries. I am happy to try and help. I will go ahead and close this thread but if you have any other questions, make sure to open up a new one.