Thermostat signals and color code

Lack of standards makes this interesting.

The following information may have errors; It is not permissible to be used by anyone who has ever met a lawyer. Use should also be confined to Engineers with more than 370 course hours of electronic engineering and should only be used for theoretical studies.

While there isn't an official standard for thermostat circuit wiring colors, there is a general pattern. Your best bet is to see the manufacturer's documentation (both thermostat and HVAC).
schematic

Click to enlarge.

Older systems and base signals on newer ones

Terminal Probable
wire Color Signal Description

C Black 24 Vac Common From one side of the 24Vac transformer (Think of this as 24Vac neutral

R or V Red 24 Vac Power to be switched From other side of the 24Vac transformer (Think of this as 24Vac L1)

Rh or 4 Red 24 Vac Heat call switch power Same as R, but dedicated to the Heat call switch

Rc Red 24 Vac Cooling call switch power Same as R, but dedicated to the cooling call switch

G Green Fan Fan switch on thermostat - is connected to R when Fan /Auto switch is in the fan position

W White Heating call Is connected to R or Rh when themostat calls for heat. (Might be jumpered to Y on a heat pump. On others might be second stage heating

Y Yellow Cooling call Is connected to R or Rc when themostat calls for cooling. Also Cooling or 1^st stage heating on a heat pump
Most often connected to G when Fan switch is set to auto

New signals - for heat pumps and staged systems

Terminal Probable
wire Color Signal Description

Y2 Blue or Orange 2^nd stage Cooling

W2 Varies Second Stage Heating First stage Auxiliary heating on a heat Pump

E Varies,
blue, pink,
gray, tan Emergency heat relay on a heat pump. Active all the time when selected, usually not used. Disable the heat pump and turn on first stage Aux heating

O Varies,
Orange Reversing valve Energize to cool (Changes from heat to cool on heat pumps)

B Varies,
blue, black
brown, orange

Sometimes common side of transformer. Needed on some electronic thermostats or if you have indicator lamps
OR
Reversing valve (energize to heat) above. York and Trane sometimes use (B) as common. Might be heating changeover or on older stuff common of transformer.

X Varies SEE NOTES BELOW Might be common but sometimes emergency heat relay

X2 Varies Second stage heating or indicator lights on some thermostats Might be emergency heat relay

T Varies,
Tan or Gray Outdoor anticipator reset Used on GE/Trane/American Standard and some Carrier Products.

L Varies Service Light

Older systems and base signals on newer ones
Terminal	Probable wire Color	Signal	Description
C	Black	24 Vac Common	From one side of the 24Vac transformer (Think of this as 24Vac neutral
R or V	Red	24 Vac Power to be switched	From other side of the 24Vac transformer (Think of this as 24Vac L1)
Rh or 4	Red	24 Vac Heat call switch power	Same as R, but dedicated to the Heat call switch
Rc	Red	24 Vac Cooling call switch power	Same as R, but dedicated to the cooling call switch
G	Green	Fan	Fan switch on thermostat - is connected to R when Fan /Auto switch is in the fan position
W	White	Heating call	Is connected to R or Rh when themostat calls for heat. (Might be jumpered to Y on a heat pump. On others might be second stage heating
Y	Yellow	Cooling call	Is connected to R or Rc when themostat calls for cooling. Also Cooling or 1^st stage heating on a heat pump Most often connected to G when Fan switch is set to auto
New signals - for heat pumps and staged systems
Terminal	Probable wire Color	Signal	Description
Y2	Blue or Orange	2^nd stage Cooling
W2	Varies	Second Stage Heating	First stage Auxiliary heating on a heat Pump
E	Varies, blue, pink, gray, tan	Emergency heat relay on a heat pump. Active all the time when selected, usually not used.	Disable the heat pump and turn on first stage Aux heating
O	Varies, Orange	Reversing valve	Energize to cool (Changes from heat to cool on heat pumps)
B	Varies, blue, black brown, orange	Sometimes common side of transformer. Needed on some electronic thermostats or if you have indicator lamps OR Reversing valve (energize to heat) above. York and Trane sometimes use (B) as common.	Might be heating changeover or on older stuff common of transformer.
X	Varies	SEE NOTES BELOW	Might be common but sometimes emergency heat relay
X2	Varies	Second stage heating or indicator lights on some thermostats	Might be emergency heat relay
T	Varies, Tan or Gray	Outdoor anticipator reset	Used on GE/Trane/American Standard and some Carrier Products.
L	Varies	Service Light

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