Nest Thermostat will not ignite gas furnace that ignites properly with old thermostat.

[JohnPaul]

I have a Trane single stage gas furnace and air conditioner.  Everything was working properly before installing a Nest Learning Thermostat.  It is a 4 wire system with no C connection.  A fifth unused blue wire is present.  After the Nest was installed the inductor fan comes on but the furnace does not ignite.  The inductor fan cycles repeatedly on and off.  When I reinstall the old thermostat everything works fine.  The old thermostat has a jumper between Rc and Rh.  Any suggestions?

[Jenelyn_O]

Hi JohnPaul,

Thanks for reaching out and for being a step ahead of us. I'd like to know more about it. A few questions: in what country are you located? What is the make and model of your HVAC system? Do you have access to your system's control board? I'd like to check if we can use your spare wire as your common (C) wire. Please take a photo of the wires connected to your system's control board. Ensure that the labels of the wires are visible.

Best,

Jenelyn

[JohnPaul]

Jenelyn,

  I am in the USA.  The system is a Trane XE80. I cannot see how to attach a photo.  However, the photo does show that no wire is attached to the C terminal on the control board.  The other end of the blue wire in the thermostat cable that is unattached at the thermostat end is also unattached at the control panel location.

John

[Jenelyn_O]

Hi there JohnPaul,

Thanks for the information you've provided. Could you please try clicking this link then scroll down to your response and hit reply? Check for the camera icon or click the three horizontal dots to expand the toolbar. From there, let's see if you'll be able to reply and share a photo.

Regards,

Jenelyn

[JohnPaul]

I was not able to find a camera icon or 3 dots on  the link you provided.  Is there another way, or should I just try hooking up the C wire?

[zoeuvre]

Hi folks,

@JohnPaul, you can either hook a C wire on your Nest Thermostat base or have a Nest Power Connector installed on your HVAC system itself. Also, if you're unable to send a photo here in this thread, kindly upload the relevant photos instead in your Google Drive and send us the link here so we can check.

I appreciate your help, Jenelyn. 

Thanks,

Zoe