topic Re: Wiring issue in Nest Thermostats

Wiring issue

vanvreede — Sat, 01 Jan 2022 16:55:03 GMT

My old thermostat has a R wire (jumped to RC), Blue, White, Yellow and Green Wire. I tried installing the Nest using the Blue wire as suggested for C (Common). I get a e79 error code indicating no power to the common. A volt meter between B and G shows 27 volts. Thoughts?

Re: Wiring issue

sporfrog — Sat, 01 Jan 2022 21:02:26 GMT

While the color of the wire often indicates the letter, not always. How were things connected before? What letters? For example is/was Blue connected to B, or C, or something else? A photo of the old thermostat connected may help.

Re: Wiring issue

johnCNA — Mon, 03 Jan 2022 04:02:45 GMT

You should be measuring between R-red and C-blue for your 24v, not B&G. If all else fails, go look at the furnace connection to be sure that Red=R, White=W, Yellow=Y, Green=G and Blue=C. You might have to run the setup again on the Nest just to be sure it remembers the correct wire settings.