N72 error

[dabovskiy]

Hello all, need a little help with this one. I’ve got an “older” Trane XV80 system (2015) The existing tstat only ran 4 wires (Y, W, G, R) and it was working fine.
I installed a Nest with an external 24v adapted but it's giving me an error "N72" - no power to Rh detected.
Checked the FAU, everything seems to be okay, manually jumped it and all works but Nest is still giving me the error. I removed the nest and installed the old tstat back on and everything came back to life - what am I missing here?

background on me, used to be an HVAC tech years back, so not completely clueless when it comes to things.  

[Patrick_Caezza]

Post a picture of your original thermostat's wiring and another one of how you wired your Nest

[dabovskiy]

I didn’t take a pic when I had the Nest installed, but the current terminals in use are: 

W, Y, G, R

on the nest I had:

W1, Y1, G, Rh - plus from the external 24v power source: C, Rc

[Patrick_Caezza]

Try swapping the Rc and Rh wires so the thermostat has the external 24VAC on the Rh terminal and the red wire is going to the Rc terminal.

[MelbaDT]

Hey folks, 

Thanks for your answer and suggestion, Patrick_Caezza. 

dabovskiy, I'm chiming in to check if you've got the chance to try Patrick's suggestion. How was it?

Best, 

Melba

[dabovskiy · Accepted solution]

That didn’t work, but I found the actual issue. 
So I just said F-it and rewired it with a new tstat line thinking that will solve the problem, but it gave me the same error. Called tech support and was told to switch from Rh to Rc, did that and the error code went away. Was advised that I had a faulty thermostat but could still run it off of Rc. They were wrong, the compressor kept cutting in and out so I just took everything down went back to Best Buy and exchanged it. Came home, reconnected and all is working perfectly. 

[MelbaDT]

Hey dabovskiy, 

Understood and thanks for sharing what you've done to have it resolved and sorry that you had to go through this. We'll definitely pass this to our team. 

Feel free to let us know if you have more questions. 

Best, 

Melba

[MelbaDT]

Hi, 

I haven't heard from you in a while so I'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