The color of the wire used, for the most part is irrelevant. What matters is that the same wire at the thermostat is connected to the same function terminal in the air handler. For convenience sake, HVAC companies tend to put the red conductor on the R terminal. A green conductor on the G terminal. A white conductor on the W terminal and the yellow conductor the Y terminal. When a heat pump is used, a wire is put on O/B.
Did you note which colored wire was connected to which terminal in your original non-nest thermostat? You might need to to go look at your air handler and double check which colored wire is on witch terminal of the control board. So here is some help.
Rc is input from the 24V AC Cooling transformer.
Rh is input from the 24V AC Heating transformer. (Not needed unless separate heating system ie., Boiler system)
Y or Y1 is for stage 1 cooling.
Y2 is for stage 2 Cooling
W or W1 is for Stage 1 heating
W2 is stage 2 heating.
E is Emergency heat, typically used with heat pump systems.
G is blower Fan
O/B is the 4-way Heat Pump reversing valve.
C is out/return to to the common side of the transformer
Do you have access to a Volt Ohm Meter? If so, set the meter to measure alternating current. Then attach one lead to the red wire, and then the second lead connect to each of the other remaining wires until you reach a voltage that will be between 17 and 27 V AC. That wire will be your common wire. The red wire will go in Rc, and the common wire into C. I will need more information to guide you with the remaining wires.
Google Pro, Mechanical Engineer, Electrical Engineer and HVAC service company owner.
