Help with wiring Nest Learning thermostat to 5-wire Trane xv80 with AC

[robmack]

Hi, recently replaced the aging "Direct Energy" thermostat with a Nest Learning thermostat.  Emulated the connections on the old thermostat onto the Nest. Heat & AC work but after a week or so, the Nest battery dies.  I understand this is because it is being charged only while the system is running (no "C" wire connection).  So I need to re-wire to correct this problem.  Picture below shows connections on the controller card in the furnace:

Picture below shows the original connections to the "Direct Energy" thermostat.  I wired the Nest according to the table below:

Furnace	wire	Nest
R	red	Rh
W	white	W1
G	green	G
C	black	Y1
Y/Ylo	orange	no connection

On the Nest, I wire it this way and it runs.  I tried to re-wire based on how the wires are connected to the controller card according to the table below but AC doesn't work:

Furnace	wire	Nest
R	red	Rh
W	white	W1
G	green	G
C	black	C
Y/Ylo	orange	Y1

I need assistance to find the correct wiring which will include a "C" so that the Nest battery remains charged.

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[Markjosephp]

Hello robmack,

Thanks for reaching out to the Google Nest Community Forum. Let's sort this out.

The Nest Learning Thermostat usually doesn't require you to have a Common wire, as it can charge its built-in battery using your heating and cooling wires. But there are situations where you still might need one to consistently deliver power to your thermostat.

To be sure, please share with us your Nest Thermostat's wiring setup as well and gather its power readouts in the Technical Info settings. A few questions: have you checked if there's a spare wire behind the wall intended for the cooling wire? I see a spare black wire on your old thermostat's photo. Could you also confirm its use?

I'll look forward to your response.

Regards,
Mark

[robmack]

Hi Mark,

Thank you for replying to my topic.  I did eventually get a professional in to do preventative maintenance on the furnace and during that service call, he looked at this problem.  Apparently a previous tech had miswired the thermostat connections, cutting wires and splicing others inside the furnace.  In any case, the tech corrected that error and wired up the Nest with the 5 available wires. That solved my problem. The Nest works as it should, now remains online and the internal battery gets charged through the "C" wire.  

[Alex_S]

Hello robmack,

We're glad to hear that this has been sorted out and thanks for sharing your resolution in the Community. Let us know if you have other questions and concerns, otherwise; we'll mark this as resolved and close the thread after 24 hours.

Best,
Alex