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