Common Wire Power Not Recognized (E79): Losing Voltage When Connecting Wires to Nest Backplate

[djroze]

Hi there,

   I am trying to install the latest generation Nest Learning Thermostat to replace my old thermostat. Originally I did not have a C wire, however I did have two extra wires going from my furnace to my thermostat and so I connected one of them as the C wire on both the furnace and thermostat end. The voltage difference shown when nothing is connected on the thermostat end is >24V between RH and C, which suggests to me that the C wire is connected properly and able to provide power in conjunction with RH. I have tested the voltage difference on the furnace circuit board multiple times and it consistently shows >24V between RH and C when the system is on (safety latch depressed).

   When I plug in *both* of the C and RH wires to the Nest backplate, however, I then lose the voltage difference between them and it's something like 1V ("ghost voltage"?). With either one connected, but not both, there remains a 24V difference. This is without the faceplate connected or anything, just putting both wires into their respective holders on the backplate of the thermostat. From my limited electrical knowledge, this seems like there might be a short circuit in the backplate, as I don't know why else the voltage difference should go away when these wires are attached to their connectors. When the faceplate is attached, I get an error E79 that the C wire is connected but not providing power.

   Should I return the thermostat for a replacement? Is there another measurement I can take to help troubleshoot this? Are there any user-replaceable fuses or the like inside the Nest backplate?

   Thanks in advance,

Daniel

[djroze · Accepted solution]

I ended up solving this when I discovered that there were actually two different bundles of thermostat wires: one leading out of the boiler's control panel enclosure, and one entering the main building to continue through to the hallway where the thermostat resides.

3 of the 7-8 colors on the "furnace wire bundle" were joined with their counterparts on the "thermostat wire bundle" after exiting the furnace, and thus the bundle of the wires coming from the furnace actually contained a couple other unused colors like brown and black which were not found (used or unused) on the thermostat's wire bundle. So, the circuit with my "added" C-wire appeared to never have been completed... I guess something in the Nest backplate wiring decreases the Rh-C voltage when those two spots are occupied?

Anyway, I ended up joining the "new C-wire" (blue in my case, a slightly different color blue in the furnace bundle than in the thermostat bundle) where the two thermostat wire bundles meet, and now my Nest recognizes the C-wire and everything works swimmingly.

- Daniel