Thermostat wire color codes explained.
Please note. Not all installers use the proper color codes when they install or replace equipment. It may be necessary go go to the furnace or outdoor unit to verify what wires are for what purpose. Also be sure there are no splices in the wire that could change the wire colors between components.
• R (red) or Rh (Red heat) 24 volts from equipment.
• Rc (Red cooling) 24 volts from transformer in cooling equipment.
Note: If only furnace has a transformer and cooling equipment does not jumper Rc and Rh.
• W (White) sends 24 volts to furnace control to start the heating cycle.
• W2 (no standardized wire color, usually whatever wire color is available) controls second stage heat.
Note: W2 is most often used for heat pumps to control what is called emergency heat or Auxillary heat, and most often will use the white wire.
• Y (yellow) often (blue) is used, controls cooling unit (outdoor condensing unit) also is used for heat pump heat.
• Y2 (no standardized wire color, usually whatever wire color is available) controls second stage cooling.
• G (Green) controls the fan "on" operation of the furnace/air handler. Also often is used to start the blower for many electric furnaces.
• C (common) most often brown but can also be black or other color available to installer.
• O (orange) Energizes heat pump reversing valve for cooling (Trane and most other brands).
• B (Blue) Energizes heat pump reversing valve for heating (Rheem, Ruud and Weatherking).