Tstat Terminal Designation Color of Wire and Termination

R – The R terminal is the power. It is a red wire and comes from the transformer usually located in the air handler for split systems, but you may find the transformer in the condensing unit. For this reason, kill the power at the condenser and the air handler before changing or working on the wiring at the t-stat. Lastly, if you have a package unit, then the transformer is in the package unit. Red Wire for the R terminal - *You should be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

RC – The RC terminal is designated for the power for cooling. Some HVAC systems use two transformers. A transformer for cooling and a transformer for heating. In this case, the power from the transformer in the air conditioning system would go to the thermostat terminal. Furthermore, it should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. Red Wire for RC terminal. *Although be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

RH – The RH terminal is designated for the power for heating. See RC above for an explanation. It should be noted that a jumper can be installed between RC and RH. Finally, this is only for heating and cooling systems equipped with a single transformer. Red Wire for RH terminal. *Although be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

Y – This is the terminal for cooling or air conditioning and goes to the compressor relay. Typically a thermostat wire pull is made to the air handler on split systems. This wire is then spliced for the separate wire pull which is made to the condenser. Furthermore, some manufacturers put a terminal board strip near the control board in the air handler. Therefore, a splice is not needed. Yellow Wire for Y Terminal. *Although be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

Y2 – This is the terminal for cooling second stage if your system is so equipped. Finally, many systems only have a single compressor but if you have two compressors (or a two stage compressor) which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling. *The most common color I’ve seen used for this terminal and wire designation is light blue, but this varies and is entirely up to the installer what color to use. Furthermore, for the thermostat, wiring colors code for this terminal (if equipped) consult with the installer. Finally, if that is not possible, then trace the wire out to the source.

W – This is the terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler or auxiliary heating for a heat pump. White Wire for W Terminal. *You should be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

W2 – This is the terminal used for second stage heat. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal. Heat Pumps use staging for auxiliary heat and need a W2 terminal. *The most common color I’ve seen used for this terminal and wire designation is a brown wire, but this varies and is entirely up to the installer what color to use. Furthermore, for the thermostat, wiring colors code for this terminal (if equipped) consult with the installer or trace the wire out to the source.

G – This is the terminal used for the fan relay to energize the indoor blower fan. Furthermore, on a split system the blower fan is in the air handler. A package unit the blower fan is in the outdoor package unit. Green Wire for G Terminal. The fan wire. Furthermore, *be aware that this may have changed, especially if the person who wired the thermostat didn’t use conventional color coding.

C – This is the terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. Finally, many digital thermostats require 24 volts for power, so the common wire is necessary. C stands for common, and there is no universal color used for this terminal, although black is the most common color I’ve seen. For the thermostat, wiring colors code for this terminal (if equipped) consult with the installer. Finally, if that is not possible, then trace the wire out to the source.

O or B – These terminals are for heat pumps and the B t-stat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Furthermore, other manufacturers of heat pumps utilize the reversing valve for cooling. The O thermostat terminal is for this purpose. This wire goes to the outside heat pump condenser, for reversing valve control. Finally, this is for heat pump thermostats. Orange Wire for O and Dark Blue Wire for B, depending on the installer of the heat pump and the manufacturer. If you have a Trane, Carrier, Goodman, Lennox, Ducane, Heil, Fedders, Amana, Janitrol, or any other manufacturer other than Rheem or Ruud, you will be utilizing the orange wire for reversing valve. Lastly, Rheem and Ruud will usually use the blue wire for the reversing valve.

E – This terminal is for heat pumps and stands for Emergency Heating. Furthermore, if for whatever reason the heat pump condenser fails and it is necessary to run the heat there is an option on heat pump thermostats for emergency heating. The E terminal activates the back-up heat source. E – There is no universal color used for this terminal designation, but this should be wired directly to the heating relay or the E terminal on a terminal stripboard in the air handler or package unit if you have a heat pump package unit.

X or Aux – This terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler. X or Aux – There is no universal color used for this terminal designation. However, this should be wired directly to the heating relay or the Aux terminal on a terminal stripboard. It is terminated in the air handler or package unit if you have a heat pump package unit.