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Lets open the furnace and look at the control board. Tell me what the five wires each connect to on the furnace board.We will go from here.
The C (Common) wire is optional on many thermostats but mandatory with the NEST.
Some thermostats using AA batteries do not need a common as the batteries are replaceable and provide power source,. However the NEST requires a 24 volt AC power supply. That is obtained through the R wire and the C wire.