Thermostat Wiring

There is no official standard for wiring colors, or wiring terminals. To be safe it’s always best to refer to your equipment manufactures wiring diagram. Don’t trust the wire colors. The installer can use any color to complete any circuit. Verify each wire first.

In the following table is a note for each terminal and its common function. The normal color wire is listed. DO NOT ASSUME these are what you have.

Terminal          Purpose                             Normal Color          Notes

R                       24 V Power                              Red

Rh                    24 Vac Heating Power             Red

Rc                    24 V Cooling Power                   Red

C                      24 V Common                             Black

Y                   Cooling /Compressor                Yellow          Cool or 1st stage ht heat pump

Y2              Second stage Cooling               Blue or Orange

W               Heat                                                  White                           First stage heat could require jumper to Y for heat pump. Could be second stage heat

W2            Second Stage Heat                      Various                       First stage Aux heat for heat pump

G               Fan                                                    Green                      Fan switch or call for cooling or heat pump

E               Emergency Heat Relay             Various                   Disable the heat pump or make Aux heat first stage

O             Reversing Valve for Cooling         Various

X                 See notes

X2              Second stage heat, indicator light for some thermostats Various            See notes

T                 Outdoor anticipator                        Various

L                  Service Light                                      Various             Used to notify you of a problem with the system


Wires that cause greatest confusion are B and X

B terminal NEMA standard use B terminal for heating changeover valve.

Trane and  older Honeywell common side of transformer.

“X” standard for common.  some use “X” or “X2” for emergency heat relay.