Nest Thermostat 3rd G Wiring

[Moman2]

As you can see, from the last picture I posted, one set of wires going to the left (labeled as W,RH,B,RC).   Another set going down (labeled as C,G,O,Y).  By looking at the wires of both OLD system and new Nest system,  I found out that the old system wiring has a seperate "O" and a seperate "B".   While the new NEST system has only one wire called "OB".  My question is which one from the old system goes to "OB"?

[GeremiG]

Hi @Moman2!

Thank you for posting in the community. I understand you're looking for assistance with wiring your new Nest Learning Thermostat 3rd Gen, specifically regarding the 'O' and 'B' wires from your old system and the 'OB' terminal on the new Nest Thermostat. I'm happy to help you!

Based on the configuration you've provided, the orange wire from your old system (labeled "O") should be connected to the O/B terminal on your Nest thermostat.

Here's a breakdown of the connections, incorporating the information you provided:

Yellow wire: Connect this to the Y1 terminal.
Green wire: Connect this to the G terminal.
Orange wire: Connect this to the O/B terminal.
Red wire: Connect this to either the R terminal. 
White wire: Connect this to the W1 terminal.
Brown wire: If you have one, connect this to the W2/AUX terminal.
Light blue wire: Connect this to the C (common) terminal.

If you have any questions or need additional help, please don't hesitate to let us know.

Best regards,
Geremi

[Moman2]

@GeremiG  Thank you @GeremiG for replying.  Based on your reply, I have couple of questions:

1 - Where does the "B" wire from the old system go?  must be left disconnected.?

2 - You said "Brown wire: If you have one, connect this to the W2/AUX terminal."  If you look at the picture I attached, you can see the Brown wire in the old here is the "B" wire.

3 - When I research about Nest comaptability, I find the following info below.  What do I answer during setup when Nest asks "if reversing valve is for cooling or heating (i.e., O or B)." (see below from research):

Nest Thermostat (3rd Gen or newer)

       -Has a single O/B terminal.

       -During setup, Nest asks if reversing valve is for cooling or heating (i.e., O or B).

       -Again, you cannot connect both O and B; one must be left disconnected.

[GeremiG]

Hi @Moman2!

Appreciate the information. Sorry for the delay in getting back to you.

To answer your questions:

The "B" wire from your old system should be connected to the W2/AUX terminal on your Nest Learning Thermostat (3rd gen). The cable can also be left disconnected by placing insulating tape on it.

Your system is compatible, but we'll need to do a quick test to confirm the wiring for both heating and cooling. During the setup of your Nest Learning Thermostat (3rd gen), you should select "O" (for cooling). Most heat pump systems use the "O" wire to energize the reversing valve for cooling. If you select "O" during setup and find that your system heats when it should cool (or vice versa), you can always go back into the Nest settings and change it to "B". 

Here's how you can do it:

1. On your thermostat, go to Settings.
2. Select Equipment.
3. Select Continue, and then Continue.
4. Select Heat Pump.
5. The Nest Thermostat defaults to an O setting for heat pumps.

• If the O option is highlighted, select B.
• If the B option is highlighted, select O.

To ensure everything is connected correctly, please test both the heating and cooling functions of your thermostat. If you test both heating and cooling, and in both cases, your system only produces heat, this indicates a specific wiring setup. It means your "O" wire and your "B" wire are currently operating separately. In this situation, the best course of action is to contact a local, qualified HVAC professional. They will be able to safely and correctly join the "O" and "B" wires so they function as a single unit, allowing your system to properly switch between heating and cooling modes.

Please keep me posted. I'll be waiting for your response.

Best regards,

Geremi

[Moman2]

@GeremiG can someone reply to my last post please