What you will learn in Thermostat Wiring Colors Code article:

  1. What thermostat wire color is likely to go to which terminal on the thermostat.
  2. Basic electrical safety and standards for wiring color
  3. The function of each wire terminal on the average thermostat. Additionally highlighting some of the differences in those functions from manufacturer to manufacturer.
Sometimes the thermostat is not the problem. Therefore please check this article for a checklist of things to check before calling a heating and cooling repair service.

Thermostat Wiring Colors Code | HVAC Control – Safety and Standards

Thermostat Wiring Colors CodeYou will always want to follow the thermostat manufacturers instructions whenever changing the thermostat. Always turn the power off!You can do this at the air handler and condenser. Always make sure there is no voltage at the transformer before proceeding. The thermostat wire colors below are the most likely colors used for most installations. However, your thermostat wire colors may be different than what is indicated here. There is no official standard for thermostat wire colors. You can consider this the standard wiring colors for thermostat and HVAC equipment. Standards for thermostat wiring colors do not exist except for here and a few other references on the internet. However, these are the most common wiring methods used. This is based on my years of experience in the HVAC field where I successfully wired many thermostats.

Honeywell Thermostats Available Here
For instructions on how to wire a thermostat please see our how to wire a thermostat page.

Thermostat Wiring Colors Code – High Performance HVAC Thermostat Terminal Designations

Turn the power off before proceeding! Always follow safety instructions especially with electricity. This should be done at the circuit breaker for both the condenser and the air handler/furnace. The power for the condenser and air handler is done at the circuit breaker box or the equipment itself.

It is possible you could cause a dead short in the control circuit and blow the transformer. That could potentially cost you a service call for troubleshooting and repair.

Thermostat Wire Colors Code | HVAC Control

Thermostat Wiring and Wire Color Chart – Thermostat Wiring Colors Code

Tstat Terminal DesignationColor of Wire and Termination
R – The R terminal is the power. 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 tstat. If you have a package unit then the transformer is in the package unit.Red for the R terminal. *Although 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. 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 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. This is only for heating and cooling systems equipped with a single transformer.Red 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. Some manufacturers put a terminal board strip near the control board in the air handler. Therefore, a splice is not needed.Yellow 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. 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 completely up to the installer what color to use. For the thermostat wiring colors code for this terminal (if equipped) consult with the installer. 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 for W Terminal. *Although 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 brown but this varies and is completely up to the installer what color to use. 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. 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 for G Terminal. *Although 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. 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. 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 tstat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Other manufacturers of heat pumps utilize the reversing valve for cooling. The O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located.Orange for O and Dark Blue 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. Rheem and Ruud will usually utilize the blue wire for reversing valve.
E – This 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.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 strip board 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 but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.
S1 & S2 or (Outdoor 1 and Outdoor 2) – Some tstats have this terminal. It used for an outdoor temperature sensor. Special shielded wire is used for this run and completely separate form the other thermostat wires. Some manufacturers will show this the T terminals on their thermostat.Using shielded wire prevents electromagnetic forces generated from other wires from interfering with the signal inside the shielded wire. A remote temperature sensor is a solid state device. The signal needed to get an accurate temperature is sensitive to electromagnetic forces from other wiring inside the structure. This type of wire is different from the typical thermostat wire and a separate wire altogether.
Thermostat Wire Colors Code | HVAC Control - Table of wire colors along with explanations and exceptions.
Related Link: 4 Wire or 5 Wire Thermostat Wiring Problem – Question from a reader (opens in a new window)

Tracing a Wire to the Source – Thermostat Wire Color Codes

typical wire colors
Terminal Strip in an Air Conditioner Air HandlerIf you find the colors are not matching to the convention described here you can always trace to the source. Most of all, only if you are confident in your skills. Remember, the wires that run from the thermostat are terminated in the HVAC equipment. Most noteworthy, air handlers, boilers, or other equipment have high voltage. Some manufacturers provide terminal strip boards. While in other equipment you will simply find a bundle of wires with wire nuts holding them together. In either case, it is important to find the wiring diagram for the unit. In doing so, you can match up the appropriate wire color from the thermostat to the termination in the equipment.

The red wire should always come from the hot side of the 24-volt transformer. Consequently, the common wire (sometimes black in color) should come from the common side of the transformer. Most noteworthy, the wire colors on the transformer will be different than the thermostat wires. If you do not know what you are doing then please call a professional for a few reasons; as mentioned there is high voltage in the equipment and can be hazardous to your health. Therefore, if you wire it wrong or accidentally touch the wrong wires together you can blow the transformer.

Additional Resources

Thermostat Terminal Designations Explanations

You may also want to check our thermostat category for other detailed articles on thermostats. The category includes other detailed pages about thermostat wiring.

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