Gen 3 Learning Nest- no power through any wires

[ColoradoYeti]

I tried to install my nest on my own, but I received an E74 code- no power to Rh line. I have an Rh and a W1 line going into my thermostat, and I believe they are called COM1 and H1 on the furnace side. I followed all the troubleshooting steps I could find with google, and it resulted in telling me the issue was on my furnace side. I  worked with Google chat to confirm that everything was compatible on paper. (case number:  8-2394000032432) 

My next approach was to have OnTech come out to diagnose and fix the issue. They arrived this morning and spent a good bit of time in my crawlspace looking at my boiler.  Their diagnosis was that there were no lines sending power and that it was an incompatibility issue. I asked if the google C-adapter would work, but since there is no power to any of the lines, this would not solve my problem. I also asked about providing a tertiary C wire from a plugin adapter (common for a doorbell) and both he and his friend (an HVAC specialist) did not think that this would solve my issue either.

What are my options here? I would really like to convert to my nest thermostat and not give up, as my current thermostat is just eating up batteries and requires reprogramming all the time.

[Patrick_Caezza]

So you have two wires coming from your boiler, a red and white one?

Lots of times a boiler is just looking for a switch closure and the Nest thermostats can't provide that by themselves, but they can if you add a simple relay and an external 24V transformer.

Here is a simple diagram of how you can probably make it work.

 

[ColoradoYeti]

Hey Patrick, I just received a 24V transformer and the relay that you referenced.

I am a little confused about your diagram... how do I connect the 1 wire to the nest and the relay? Do I need a long jumper line from the relay as line 3?

[ColoradoYeti · Accepted solution]

I added the external 24V power supply, and no further hardware or cables were required. The thermostat works as designed.

[aatienza]

Hey ColoradoYeti,

Thanks for letting us know! You can reply to this thread if you have other questions, we're here to help.

Thanks,

Archie

[aatienza]

Hey ColoradoYeti,

As we got our resolution here, I'm going to mark this one as resolved. Please feel free to open up a new thread if you need assistance. 

Thanks,

Archie

[Ryan_G]

Hey ColoradoYeti,

Thanks for posting here in the Google Nest Community Forum.

Have you tried the recommendation from Patrick? Let us know how it goes by replying to this thread.

Thanks for your help, Patrick.

Best regards,

Ryan