How do I know if this blue wire is a C-wire?

[LVSunsFAN]

My old thermostat had 4 wires attached to it: W (white), Y (Yellow), G (Green) and Rc (Red). It also has a blue wire that wasn't attached to the old thermostat (it's along the white wire in the picture).

I bought the "Google Nest Thermostat - Smart Thermostat for Home - Programmable Wifi Thermostat" and connected those four wires. After getting error messages for low power I bought a Google Nest Power Connector but it appears my furnace is on the roof, too high to reach even with my 22 ft ladder.

So I can't use the Google Nest Thermostat without getting regular error messages unless that blue wire is actually a C-wire. How do I go about checking if it is? And if I do find out it is a C-wire, do I just connect it to the C slot between the W and Y slots on my Google Nest Thermostat?

Thanks

[sporfrog]

Ideally a photo of the thermostat will include the letter indicators for each wiring terminal.  Based on the wiring colors it probably clear, but not always.

If you don't have access to the HVAC unit, you can test the blue wire with a multimeter.  Test R (usually red wire) to your unused blue. If it is a "Common" it should be about 24V AC.  Without a multimeter I don't know.  Hopefully someone does!

If it is a common (C) wire you would connect it to the C terminal on the thermostat base plate.

Good Luck.

[Dutch00]

Even with the C wire plugged in, the Nest is still been finicky with the May 22 update and my furnace. I gave up and bought an Ecobee to replace it, have not regretted it.