Need help wiring Nest Learning thermostat 3rd gen with heat pump, all electric.

Hi everyone, I need help hooking up my Nest Learning 3rd gen thermostat for my upstairs and downstairs. I currently have Trane XL824 t'stat for downstairs with 7 wires and Trane XR724 for upstairs with 6 wires. My system has a heat pump and ALL electric. Air handler make and model is Carrier FJ4DNXD60. Outdoor unit is Carrier 25HBC560A0030061. I posted photos of my current wiring as well. TIA!

Downstairs thermostat Trane XL824. 7 wires coming from the wall.


Upstairs thermostat Trane XR724. 6 wires.

 

Do you know what the black AUX1 wire is for on the downstairs unit? A humidifier or something else?
Otherwise straight-forward connects to Nest or any other thermostat. You can ignore the jumpers.

• Red -> Rc
• Blue -> C
• White -> W2
• Orange -> OB
• Green -> G
• Yellow -> Y1

Then tell Nest you have single fuel if asked.

 

Thank you. I don't have a clue what it is for. I don't have a humidifier in my unit. How can I find out? And how should I wire the upstairs stat?

 

Upstairs and downstairs use the same colors. (W2/AUX)
Heat pump with backup electric heat.

The only difference is the additional black wire.
To locate the black wire.... go to the first floor air handler and locate the thermostat wiring.
See where the black goes. You should see black spliced to something.
You can post a picture two if that wiring.

 

Here are the photos inside the air handler. Not sure which one so I took multiple shots. Hope this helps. TIA.

 

2nd to last picture.
In orange circle I see thermostat, condenser, red float wire and a mystery black wire.
In pink circle I see two transformers. Maybe one died and other is replacement ?

In the last picture.... it looks like the black and brown wires in the thermostat cable are wrapped around the cable as if they are spares and are not being used.

 

On the orange circle, the mentioned black and red wire, the red wire is connected to a device that’s attached to the drain pan and it’s for safety switch purposes if water fills up the pan. The black wire in there is connected to power a UV light that is also connected to one of the transformers you saw. It was installed by my AC guy before.


I also see a surge protector inside that was an add on before by my AC guy.

Any clue what the black wire is for on the thermostat?

 

Do you have one or two air handlers? If one that is shared between thermostats then there would be a zone controller mounted somewhere on or near the air handler and that is where to look for the wires.

 

Just one air handler. So do I just cap off the black wire in the Nest thermostat ?

 

FYI..................
You have one system with two thermostats. Both thermostats go to the zoning board.
The zoning board determines which damper is open.
Then one cable goes to air handler and one to the outside condenser.
Mention this next time when replacing thermostats.

In your latest picture is an orange wire nut connection..... the thermostat black wire goes to the white wire of a red-green-white cable. Green is not used and red goes to C. That three wire cable is currently connected. You'd need to trace where it goes. Probably some type of fresh air inlet/damper.

 

So for now do I keep the black wire capped?

 

Apparently the black wire is still connected/spliced to something.
You'd need to trace that three wire cable to see where it goes.