Thermostat Terminals

The sites below gathered these from manufacturers like

and others to show usual terminal designations:

From
http://highperformancehvac.com/thermostat-wiring-colors.html :

" R – The R terminal is the power for the thermostat. This 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, it is a good idea to kill the power at the condenser and the air handler before changing or working on the wiring at the thermostat. If you have a package unit then the transformer is in the package unit.

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. 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.

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 for a heating and cooling system equipped with a single transformer.

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 and then this wire is spliced for the separate wire pull which is made to the condenser. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed.

Y2 – This is the thermostat terminal for cooling second stage if your system is so equipped. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling.

W – This is the thermostat terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler, electric heat, auxiliary heat on a heat pump w defrost output from outdoor unit to activate electric heat and turn on the AUX. heat lamp

W2 – This is the thermostat 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.

G – This is the thermostat terminal used for the fan relay to energize the indoor blower fan. On a split system the blower fan is in the air handler while with a package unit the blower fan is in the outdoor package unit.

C – This is the thermostat 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. Many digital thermostats require 24 volts for power so the common wire is necessary.

O or B – These thermostat terminals are for heat pumps and the B thermostat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Most other manufacturers of heat pumps will utilize the reversing valve for cooling and the O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located. Energize to heat some systems, Rheem Ruud.

B or X – common side of transformer. some electronic thermostats indicator lamps. confused with B Reversing valve energize to heat above. York and Trane use B common.

E – This thermostat terminal is for heat pumps and stands for Emergency Heating. 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. Basically this simply utilizes the back-up heat source many heat pumps have to heat the home without sending a signal to the condenser to run for heat."

More on this can be found from jsmeenan site as he gives colors others might choose on wiring harnesses if available:
" E] emergency heat relay on a heat pump. . X2] on Trane, outdoor anticipator reset.
Used on GE Trane American Standard and some Carrier.
L] service indicator lamp.
Numbers, see diagrams, re stages of electric heat especially on York Borg Warner Coleman-Evcon, Frasier Johnson Air-pro.
X2], 2nd stage Electric Heat on GE Trane American Standard. indicator lamp or misc. contacts on other systems"

Compiled from Honeywell thermostat manuals:

"Old Label_________New Label__Description
R, V-VR or VR-R ______ R_____ 24 VAC, Return
Y,Y1 or M____________ Y_____ Stage 1 Cooling Heating Circuit
O or R _______________ 0______Reversing Valve, (Cooling Mode]
B ___________________ B_____ Reversing Valve (Heating Mode]
F or G _______________ G_____ Fan control relay
Y2 __________________ Y2____ 2nd Stage Cooling Circuit
W1 or W2 or W-U _____ W2____ 2nd Stage Heating Control
L or X _______________ L _____ System Monitor LED
E or X2 ______________ E_____ Emergency Heating Circuit
C,X or B _____________ C_____ 24 VAC, Transformer Common

How to set a thermostat anticipator.