Need to figure out what to do with S1 and S2. Can I use one of that to be C wire?

[eva1028]

I am trying to convert my old thermostat to Nest Learning Thermostat.  After connecting the wires based on a Youtube video, Nest will not power on.  I used an external wire to power it on and Nest gave an E79 error code.  Currently use the Blue wire (which used to be on S2) but seems like not having enough power.  I still have the S1 (brown color) wire that is for external temperature sensors.  I knew Nest doesn't need this, but can I repurpose it to use as a C wire?  

[CoolingWizard · Accepted solution]

The Blue wire on your old thermostat is connected to the S1 sensor and not a Common. You had no common wire on your old thermostat. The good news is you don’t need the sensor wire so it can be re-purpose by re-wiring it to the common wire. What you will need to do is go to your air handler open up the panel where the wire goes in, find the blue wire and disconnect it from the sensor, S1, and connected to the common of the transformer.

AC Cooling Wizard

[Jenelyn_O]

Hi there,

Thanks for posting in the Google Nest Community forum. I'm sorry for the delay. I hope you get the answer you're looking for. Please let us know if you still have questions or concerns, as we'll be willing to assist you further.

I appreciate your help, @CoolingWizard.

Best,

Jenelyn

[eva1028]

Thank you for reaching out.  We tried the method from @CoolingWizard.  Our blue line is already in the B/C spot.  We tried to switch the blue line to under C but still not enough power to turn on Nest.  We looked at all the wires that tied together and found there are green and blue lines tied together.  Our next thought is to tie same color together, like all three greens wires together and all three blue wires together.  Not sure if that is ligit.  by the way, we are an electric heat pump and gas furnace.  Hope this make sense.    

[zoeuvre]

Hi eva1028, 

Thanks for posting here in the Community forum, and I'm sorry to hear about the situation. Let's sort this out. 

Can you help me with your Nest Learning Thermostat's power reading?  

1. From your Nest Learning Thermostat screen, select Technical Info, then press the ring to select it.
2. Select Power. 
   • Battery
   • VOC
   • VIN 
   • IIN

I'll look forward to your response.

Thanks for your help, @CoolingWizard and Jenelyn.

Best,

Zoe

[eva1028]

Hi Zoe:

As I mentioned in the previous thread, we tried to tie all blue lines together and all green lines together.  Current setting provides:

* Battery: 3.908

• VOC: 31.59
• VIN : 31.59
• IIN: 200mA(c)

[zoeuvre]

Hi there, 

Awesome! It's great to hear that the suggested steps work on your device. Let me know if you have other questions or concerns, or I'll consider this post complete and lock the thread in 24 hours.

Regards,

Zoe