Nest 3rd gen Thermostat low battery issue after buying the second one

[apollonin]

Hi, I have two zone HVAC system (one for the main floor and one for the 2nd floor). 

I have bought nest 3rd gen thermostat few months ago for the main floor and everything was fine. Today I bought the second one an right after the installation my first one started to indicate Low battery. Also approximately after 4-5 hours, the second one showed Low battery as well. 

Note. I don't have a c-wire for either of them, although the main floor was working perfectly for the last couple of months. Is it possible that my HVAC system can not handle two thermostats simultaneously? What can I check to confirm? 

Readings are as follows right now:

main floor: 

  battery: 3.738 V

  Voc: 37.74 V

  Vin: 31.85 V

  Lin: 20 mA (i)

second floor: 

  battery: 3.680 V

  Voc: 7.06 V

  Vin: 7.06 V

  Lin: 20 mA (i)

Thank you

[Jhonleanmel]

Hi apollonin,

Thank you for your posting.
 

Give these steps a whirl:

1. Ensure that the wires are connected properly
2. Remove the thermostat display from its base and check if any connector pins are bent.
3. If the issue persists, contact a pro installer to discuss your options.

Keep us posted.

Best,
Mel

[Juni]

Hi there,

I just wanted to follow up to see if you still need our help. Please let us know as we would be happy to answer any questions you may have.
 

Thanks,

Juni

[Markjosephp]

Hello apollonin,

I wanted to stop by and see how I could help.

Do you have a zoning system? If you do, some zone relay panels don't provide the Nest Thermostat with enough power to charge its internal battery. In this case, the Nest Thermostat needs the Nest Power Connector or a common or "C" wire connected to provide power.

Please let us know if you have more questions. Otherwise, this thread will be locked in 24 hours. 

Regards,

Mark

[apollonin]

Hi, everyone. Thank you a lot for the answers and suggestions I appreciate that. What I found is more likely to be a coincidence of few things. 

First of all I have bought a new thermostat and temporarily turned off power to install it. 

Secondly, we have had pretty rough and cold 2 weeks (-29 to -32 C) and this could have let to furnace working extra hours. And as far as I understand, without a C wire, thermostat only charges during period of inactivity. But we had two weeks of really active heating days and nights, so it could just mean the batter was drained slowly over a time. and my 20 minutes power off led to low battery indicator. 

It has been another week with two thermostats and I see no issues with them now. So could be a false alarm here, but I will be monitoring and posting here in case of any new information

[Markjosephp]

Hi there,

Thanks for the update and for letting us know. It's good to hear that your thermostats are behaving well now. From this point on, please keep observing their behaviors and keep us posted if the same issue persists.

Regards,

Mark

[Markjosephp]

Hi apollonin,

I wanted to see if you still needed help. Please let us know if you have any other questions or concerns.

Thanks,

Mark

[apollonin]

hi, Mark. 

While there is no more issue with the low battery (seems like), I experience really weird results after adding the second thermostat. It is not related to the topic, however, as I mentioned I had one thermostat on the main floor and bought this new one for the second floor. 

After around 1 week since installation I checked the usage history in Google Home app and discovered that the old main floor thermostat started to turn on furnace more frequently (1.5-2hours per day instead of 30-40 minutes as before). However, there is no big difference in outdoor temperature since then...

Also it is considerably colder on the second floor now after the installation of the new thermostat, despite usage is around 3.5 hours per day. I don't know the previous usage of the old thermostat, but second floor was much more warmer. 

I have not changed anything except a new thermostat addition... 

Do you have any ideas how this could be related? Is it possible that internal temperature sensor broke in thermostats? Main floor shows 21 C however, when I put two other sensors I have nearby they both show 20 C (19.9 and 20.1 respectively).

[Markjosephp]

Hey there,

I'm sorry for the late reply. I know it has been a while, but I wanted to ensure that everything is covered here.

How's your thermostats now? Are you still experiencing any ambient temperature issues? Let us know so we can assist you right away.

Regards,
Mark

[apollonin]

hi, thank you for followup. Tbh, yes, it has not changed either to positive nor negative direction. 

Upper floor still colder than it used to be with an old thermostat set to the same temperature (21C). So I don't know what to do and currently don't do anything here...