TStatWiring


	Thermostat Wiring Guidelines

Thermostat Wiring Guidelines

We seem to be getting quite a few posts in our forums regarding thermostat wiring. Hopefully this document can aid you in the replacement of yours

First let me start off by saying, that there is no official standard for wiring colors, or even terminals for that matter. To be safe it’s always best to refer to your equipment manufactures wiring diagram to see what their terminals are meant for.

In the following section I will list each terminal and what it commonly does. As well I will give you guidance as to what colors are commonly used. DO NOT ASSUME these are what you have. Verify your wiring first by following it to each piece of equipment. Then you will be able to match the equipment with the terminal.

Terminal	Propose NEMA Standard	Normal Color	Notes
R or V	24 Vac Power	Red
Rh or 4	24 Vac Heating Power	Red
Rc	24 Vac Cooling Power	Red
C	24 Vac Common	Black
Y	Cooling /Compressor contactor	Yellow	Cooling or 1^st stage heating on a heat pump
Y2	2 stage Cooling	Blue or Orange
W	Heating	White	First stage heating may require a jumper to Y on a heat pump. On some systems this may be second stage heating
W2	Second Stage Heating	Various	First stage Aux heating on a heat Pump
G	Fan	Green	Fan switch on thermostat or on a call for cooling or heatpump
E	Emergency Heat Relay	Various	Used to disable the heat pump and make Aux heating first stage
O	Reversing Valve Cooling	Various
B	SEE NOTES BELOW	Various	SEE NOTES BELOW
X	SEE NOTES BELOW	Various	SEE NOTES BELOW
X2	Second stage heating or indicator lights on some thermostats	Various	SEE NOTES BELOW
T	Outdoor anticipator	Various
L	Service Light	Various	Used to notify you of a problem with the system

More information on the B and X terminals

The wires that appear to cause the greatest amount of confusion are the B and X wires.

Lets look at the B terminal first. The NEMA standard is to use the B terminal for the heating changeover valve however this is not always the case.

Trane and some other manufactures including older Honeywell thermostats use it for the common side of the transformer.

“X” on the other hand is the standard for common, but again like “B” some manufactures use “X” or “X2” as the emergency heat relay.

Should you have any other questions you can post them in our Forums