E79 C Wire Issue

[Atarykris]

I had a Nest Thermostat E and it worked fine with my Bosch ac/heat pump.  But when it came time for winter to use my gas heat it didn't work so I had to rewire just the heat in and disconnect everything else.  That worked just fine.  Apparently the issue was that the Thermostat E couldn't handle 2 different systems.  So, I got the Learning Thermostat because it can.  I wired it back the way it was wired when the ac worked and wired the gas heat in the way the gas heat worked.  It's throwing a power C wire E79 error.  I can get the gas heat to fire but not the heat pump or ac because of it.  I don't know what to do.  There is an extra green wire but I've tried swapping the 2 and it doesn't even register an attempt at a c wire at all.  My HVAC guy has no clue and I'm kinda frustrated because technically it means he never really finished the job because both systems have never been able to work together not to mention he sold me on getting a $1200 rebate for the system that the system didn't end up qualifying for so I don't want to be out even more $.  It just hit the 80's and I need to get this resolved.  I've attached the pic of it wired in the Thermostat E when the ac only worked and how it's wired now with just the gas heat working.  The solid wires go to the gas heat and braided go to the heat pump/ac system. Yes I know braided aren't recommended but that ship has already sailed.  Help is greatly appreciated.

[Patrick_Caezza]

Do you have a picture of how the original thermostat was wired?

It looks like you have separate heating and a separate cooling system. This is called a Dual transformer system and needs to be wired a certain way or you'll have problems. What type of heating system do you have? boiler, furnace, electric wall?

One problem I see is you have several stranded wires, stranded wires are not compatible with the type of terminal blocks that Nest uses. You need to use some short pieces of solid wire and wire-nut them to the ends of the stranded wires

Another issue is the heating side should only be connected to the W and Rh terminals and all of the cooling wires need to be connected to the Y, G, OB, C, and Rc terminals only. I'm going to guess you have three wires coming from the heater (wht, and red) and five wires from the AC (wht, brn, blu, red, and black).

[Ryan_G]

Hey Atarykris,

Thanks for being here in the Google Nest Community.

We appreciate you including the wiring configuration from the thermostat base. We would like to know how these wires are connected to the main board of your HVAC. We'll need to verify the green wire connection to your system.

Thanks for your help, Patrick_Caezza.

Best regards,

Ryan

[aatienza]

Hey Atarykris,

Appreciate the help, Ryan. I wanted to follow up and see if you are still in need of any help? Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

Thanks,

Archie

[Atarykris]

I'm not Ryan but Bosch advised Nest isn't compatible with their systems.  So, I returned and bought an Ecobee which is.

Thanks

[MelbaDT]

Hey there, 

Understood — thanks for getting to us and for sharing the information from Bosch. Feel free to reach out again should there be anything else. 

Best, 

Melba

[MelbaDT]

Hey folks, 

We haven't heard from you in a while so we'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the Community.

Best, 

Melba

[Atarykris]

I included photos in my post & mentioned I know stranded wires are not ideal but obviously I'm not going to rewire and they do work as they worked just fine when I had the Nest Thermostat E but that system couldn't handle both so in the winter my heat didn't work and all the ac wires had to be unplugged to get it to.  

Yes, I have forced hot water for my main heat & then the ac/heat pump.  The solid wires are my forced hot water & the only wires plugged in are the white & red for that.  I selected dual system when I installed.

So if you don't see a problem then my assumption that he probably didn't connect the C wire in the attic is probably correct.