I just installed a nest thermostat and need to hookup the c wire inside the furnace. I read where you need to connect the c wire to the secondary wire on the 24v side of the transformer. There are 2 yellow wires leaving the 24v side of the transformer. 1 wire goes up the panel and connects to a white wire (top of first pic) that is going out to the ac unit outside and the other yellow wire connects to a blue wire which is connected to the red thermostat cable (very bottom wire nut). From the looks of my wiring diagram, I believe I just need to connect the unused c wire to the wire nut with the yellow and white cable but I’m not 100% sure. Any help would be awesome. enter image description here
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Best Answer
In most but not all furnace setups the R and C are the 24V AC supply. Your thermostat already has the R as it is used to power all the relays etc when the thermostat requests that. The other side of these relays is already connected to C in the furnace. There for R is the 'hot wire' C is the 'common wire', normally not needed by a thermostat unless it requires power from the transformer to operate. You new thermostat needs this for power. You need to check the energy requirement of the Nest and be sure your transformer can support the additional load. Here is a link that will explain it much better than I can. https://thesmartcave.com/thermostat-c-wire-common/