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Nest Power Connector to Weil-McLain CG (Series 12) boiler

Community Member

I'm trying to connect a Google Nest Thermostat to my Weil-McLain CG (Series 12) boiler for heat only.  I've noticed many online complaints and difficulties from pros about this product especially with boilers.  I'd like to get it working for the wifi communication (not for any learning or sensing benefit).  I only have two wires at the thermostat  (red and white), and Google Nest Tstat is requiring either a C-wire or Nest Power Connector.  I purchased a Power Connector, but still get an error that either I need a C wire or R isn't detected.  

The picture below shows the following connection:

Tstat R-  Transformer G

Tstat W - Transformer R

I kept the same type of connection where I connected the Tstat R and W to the Connector's R and W, and then:

Device C - directly to C lug on transformer

Device  R- Transformer G

Device W- Transformer R

Does anyone see a problem with this, or know why the Google Nest isn't recognizing power?



Weil-McLain CG Series 12 boiler


This has been a real pain, but I have a Google Nest on the 1st floor and would like the consistency to operate in the same app for my wife who has health issues and the app wifi is convenient for adjustments.

Thanks for any advice I can get to resolve this problem.

Thanks in advance!






Community Specialist
Community Specialist

Hi Buttermilk, 


Please help us with a photo of how the thermostat wires are connected to your old thermostat. In that way, it will allow us to identify its compatibility with our Nest thermostats and why the Nest thermostat requires a C wire. Also, kindly help us with the exact message or code that appears both on the thermostat and on the app that is asking you to connect a C wire.


Keep us posted. I'll look forward to your response. 





I received an e294 code prior to connecting the Power Connector.  I may have even gotten an e298 while trying different things.  After installing the Nest Power Connector I receive an message that some wires are not detected.  W and R are shown not connected.  The white/red wires are connected at the Nest tstat.  The Power Connector at the boiler transformer seemed a little different as described in my OP.   

 I attached a pic of the old tstat and wiring.  

I appreciate your help!  Thank you.



Community Specialist
Community Specialist

Hi there,


Thanks for getting back to us. After installing the Nest Power Connector, please gather the power readings of your thermostat by going to Settings > Equipment > Power Info (VIN, IIN, PS-*). Then, if the thermostat still shows wires not connected, kindly turn off the power to your system, then remove and reconnect each wire connected to your Nest Thermostat 2020's base, making sure that each wire is properly seated on its respective terminal. Turn the power back on and check if the same thing persists.




Community Member

Power readings (VIN, IIN, PS-*:  0.002 V, 10(W), 0

Power readings after reconnecting red/white on Nest Thermostat (and turning power back on): 0 V, 10(W), 0

Tstat panel toggles between "Error: E298" and "No Power" 

Is this a transformer / Power Connector wiring issue instead?  Does the Power Connector wiring makes sense that I supplied based upon how the pre-Power Connector wiring was completed?  Thanks!

Community Specialist
Community Specialist

Hey Buttermilk. 


Thanks for the information. This could be caused by your thermostat, the wires connected to the thermostat, or the power supply the thermostat gets from the Nest Power Connector. To isolate what causes the no power, please do the following: 


  1. Confirm that the power is back on and that you've closed the HVAC equipment cover. There's often a safety switch that turns off the system when the cover is open.
  2. Check the condition of the system’s fuses.
  3. Make sure the wires’ connections aren’t loose.
  4. Test the Nest Power Connector by going to the thermostat's Settings > Equipment > Power Connector > Power Test.
  5. Check for thermostat updates by going to Settings > Version Update.
  6. Update the thermostat to version 1.1 or later.


Keep us posted.


Kind regards,



Power was on, Power Test unsuccessful , version is 1.3 or something.  What should be the power reading voltage between the conductors on the Power Connector which go back to the Nest thermostat?  Is there any correction of the Power Connector wiring to the transformer that I described earlier?



I fixed the issue of the Google Nest Thermostat not being able to recognize the R and W white wires (only 2 wires are available) since they were not getting power even after installing the Nest Power Connector (PC).  The original transformer wiring to the thermostat within the Weil-McLain CG Series 12 boiler gave me the impression that PC-R should go to the transformer-G lug, but the PC-W wire should be connected to the trans-G.  After this was switched I was able to get a 25v reading on the multi-meter on the Tstat side of the Power Connector.

Final wiring:
Tstat-R to PC-R
Tstat-W to PC-W
PC-C to Trans-C
PC-R to Trans-R
PC-W to Trans G

This ended up taking a lot of time diagnosing, but was a very simple fix after getting the multi-meter out to check AC voltage at the Power Connector.  The Google support checks were circular checks and re-checks, when reasoning was "screaming" to get a multi-meter and check voltage at the Power Connector to isolate the problem.  The problem was then isolated as to the mis-wiring to the transformer.  

Community Specialist
Community Specialist

Hey Buttermilk, 


Great! I'm glad that everything has been sorted out, and I appreciate your sharing the steps you took to resolve this case. If you have questions or concerns in the future, feel free to start a new thread, and we'll be happy to help.




Weil-McLain CG (Series 12) boiler  with Nest Power ConnectorWeil-McLain CG (Series 12) boiler with Nest Power Connector