Nest Thermostat for split system

[8475nester]

Hello,

I got a new Nest Thermostat to replace an existing Trane (TCONT802AS32DAA) thermostat. 

My HVAC is Trane XR heat pump system which has wiring as shown below. I have both O and B wire and when I tried the nest compatibility test, it said my HVAC is incompatible. I have watched a few videos and read some forums and others were able to replace the thermostat. 

Has anyone else replaced a similar thermostat and suggestions on what wire should go in O/B in the Nest thermostat?

 

 

[CoolingWizard]

@8475nester , the blue wire attached to B is actually your Common wire.  So, use these wire settings:

Red Wire R: 24Vac Power In. Assign to Rc

White Wire O: Heat Pump Mode Switch Relay, your is an O model.  Assign to *OB
Yellow Wire: Compressor Relay, Assign to Y1

Green Wire: Fan Relay, assign to G

Blue Wire B: 24Vac Power Common, Assign to C

Orange Wire S1: Outdoor Temperature Sensor Send, Unused by Nest 

Black Wire S2: Outdoor Temperatures Sensor Return, Unused by Nest

AC Cooling Wizard

[8475nester]

Thank you, Cooling Wizard. I'll try this weekend and will provide an update. 

[DaveLister]

The O wire on your current thermostat goes into O/B on the Nest. Trane likes to label the common wire as B, so that one would go into C on the Nest. If you want to be sure, check the wiring on the air handler side and if I'm right it'll be labeled "B/C".

[8475nester]

Yes, air handler is super dirty so hard to see but I did look at the wiring diagram and was labeled B/C. Thanks!

[Markjosephp]

Hello there,

I wanted to ensure that everything was covered here.

Thanks for the help here, @CoolingWizard and @DaveLister.

@8475nester, I appreciate you confirming the purpose of the blue wire by checking it in your system. As they mentioned above, I can also confirm that it was a 24V common, and you should connect it to the C terminal for the Nest Thermostat. Please keep us posted on how the installation goes, as I would be happy to know what the result will be.

Regards,
Mark

[8475nester]

Follow-up update - I have Nest 1st generation device. I connected the wires and its not cooling at all. I set the temperature to 78 degrees and after running for 2 hours, it shows 84 degrees. When I installed it, it was 75 degrees in the house.  Do I need a different Nest thermostat?

[CoolingWizard]

@8475nester , can you post a picture of your Nest Thermostat backplate show the wires attached please, thank you.  You might need to change the Equipment Heat Pump settings from O to B type.  

AC Cooling Wizard

[8475nester]

@CoolingWizard - Here is the photo of the wiring. There was no W wire to attach. Let me know if I missed anything. 

[CoolingWizard]

@8475nester , The wiring all looks good. You say that you did not get any cooling instead you got heating? We need to double check your settings for your heat pump, equipment and verify where the checkmark is for the mode type.  Goto SETTINGS > EQUIPMENT > HEAT PUMP.  There is a check mark in front of either O or B.  You need to change the current selected mode. If O is selected , change to B and vice versa. 

AC Cooling Wizard

[8475nester]

@CoolingWizard - Attached are the photos of the change I made. It was at O and moved it to B.  Now its cooling or at least keeping the temperature same. Do I have to change this in winter? 

Other thing I noticed that it says Primary Hear Source is Electric and that's not true. My primary heat source is gas since its a split system and I can't change the heat source, its grayed out. 

 

  

 

[CoolingWizard]

@8475nester , no sir. All Heat Pumps are either O models or B models.  This all has to do with the heat pump reversing valve.  O models default to Cooling and needs the OB wire energized to switch to heating mode.  The Heat Pump B models are Heating by default and need the OB wire energized to make the reversing valve to switch to cooling.  Your Nest thermostat knows this and now that the correct mode is set, it knows to not energize the reversing valve ( the OB wire) for winter heating.  The original concept was B models would sold in northern states and O models would be sold in the southern states.  The thinking was if the reversing valve failed, it would fail in the default position.  That way you still had Heat, or Cooling while you wait for the repairman to visit.  

AC Cooling Wizard 

[8475nester]

@CoolingWizard - thank you for patiently answering my questions. What about the Heating Source? Even though it says Electric, which is incorrect, will it heat the house? I haven't tested heating yet since its 100+ degrees outside where I am at. 

[CoolingWizard]

@8475nester , what you have is a Heat Pump, and that is your primary source of heat. Heat Pumps are electric.  That is why Nest reports it as Electric. 

AC Cooling Wizard

[Jenelyn_O]

Hi there,

I hope you got the answer you're looking for. Please let us know if you still have any questions or concerns, as we'll be willing to assist you further.

I appreciate your help, @CoolingWizard and Mark.

Best,

Jenelyn

[Jenelyn_O]

Hi 8475nester,

Checking if you still need help. Please let me know, as I'll be willing to assist you further. 

Thanks,

Jenelyn

[8475nester]

Thank you asking. I am good for now and if it doesn't work, I'll ask here. 

[Jenelyn_O]

Hey 8475nester,

Thanks for the update. Please feel free to create a new thread if anything comes up or if you have any questions or concerns, as we'll be willing to assist you from there. Otherwise, I'll be locking this thread after 24 hours. 

Cheers,

Jenelyn