help with wiring a gen3 learning thermostat in a system with two zones (O and B wires involved)

[Valerio80]

Hi guys,

First time posting here. I'm upgrading the control of my home with smart devices and now it is time to work on thermostats.

My home system is set as the following:

- two separate units (Trans XR80)

- one unit controls the living area as a single zone, the other units control the bedroom areas in two zones

- the control plate at the unit is a Trol-A-Temp Mastertrol Mark II

- the old thermostats are battery powered Honeywell models

I successfully installed the learning thermostat to control the single zone unit on the first floor. Super straightforward process, with W, Y, G and R wires (no C wire) connected and system running within few minutes.

I have not tried anything yet on the rest of the house because the thermostat that seems the primary for the second unit (the two zone one) has W, Y, G, R, O and B wires. Consulting with the chat I was told that my system was compatible with the learning Nest and was suggested to hire a pro for the installation. I would like to know if it is something I could take care of myself.

Clearly, I cannot re-wire everything as with the old thermostat because of the O/B single port on the Nest. I opened the control board in the basement and saw that:

- O would control Cool

- G would control Fans

- B would control Heat

- the other 3 wires (W, Y and R) come from pins labeled with T or M, followed by numbers.

Unfortunately it seems I do not have permission to upload images, otherwise everything would be easier. Could anyone please advise if I can handle the installation by myself? Also, would I need to purchase a C-wire adapter?

Thanks in advance!

[Valerio80]

It seems that now I can upload photos! here we are. These are the pictures of the primary thermostat for the two-zone system and the control board at the actual unit.

[Valerio80]

Adding also this diagram as it might be useful. Any help is very appreciated! 🙂 

[Markjosephp]

Hi Valerio80,

Welcome to the Google Nest Community! It's great to hear about your progress with the Nest Thermostat on the first floor. Your detailed description of the system is helpful, and it sounds like you're on the right track.

About the thermostat for the second unit with the two zones, the additional wires (O and B) can be a bit confusing, but we can certainly figure this out. It's positive that you've consulted with the chat and were told your system is compatible with the Nest. For the O/B wire, Nest Thermostats typically handle both heating and cooling through a single O/B wire. The key is configuring the thermostat correctly during installation. 

As for the T and M labeled pins on the control board, it might be related to the zoning system. These might stand for Thermostat and Master, respectively. The numbers probably designate zones. You'll want to match the corresponding wires from the old thermostat to the correct terminals on the Nest. Regarding the C-wire, it's generally suggested for consistent power, but it's not always necessary unless you encounter power issues or the Nest Thermostat suggests a C-wire during setup. You can consider a Nest Power Connector.

In conclusion, Nest Thermostats don’t support stranded wires, they require solid-core copper wires. I'd suggest contacting a local pro to have your thermostat wiring replaced or adapted with solid copper wire so it can work with Nest Thermostats and to inspect its compatibility as well.

Let us know if you have more questions.

Warm regards,
Mark

[Valerio80]

So, as of now things have not worked out. The pro that was supposed to run the installation explained I would need to change my board, since both the O and B wires are in use. 

I'll have that done, but I have to be patient.

[Markjosephp]

Hi there,

Thank you for keeping us informed, and I appreciate your commitment to resolving this matter.

I understand that patience is now key in this situation, and I commend your patience and perseverance in tackling this. Please know that your efforts and dedication to ensuring a successful installation do not go unnoticed. If there's anything I can do to help you during this process, please don't hesitate to reach out—we're always here!

Best regards,
Mark