Thermostat Wire Color Codes For 3-8 Wire Thermostats (Color-By-Color)

The thermostat wiring is a bit complicated. The 1st thing everybody has to learn is what each thermostat wire color means. To help you get started, we have prepared a full thermostat wiring color code (we’ll go wire-by-wire color-by-color for blue wire or black wire (C wires), orange wire, yellow wire, red wire, white wire, green wire, light blue HVAC wire, and so on).

We have also included wiring codes for thermostats with a different number of wires, from the simple 3-wire, 4-wire, 5-wire thermostat wiring color codes, and up to the most complex 8-wire thermostat wiring color codes.

5 wire honeywell thermostat wiring color codes
5-wire Honeywell thermostat. We see a black C-wire (blue wire in most cases), white heating wire (W), red power wire (connected to Rc, jumper to Rh), green fan wire (G), and yellow cooling wire (Y).

Many homeowners seem most confused about 4-wire and 5-wire wiring color codes, what each wire color is, and into which terminal it connects to. You will find the full thermostat wiring article for 2-wire to 5-wire thermostats here; here is the short summary:

  • Typical 3-wire thermostats will use red (R), white (W), and blue (C) wires; used for water heating, boilers, ACs.
  • Typical 4-wire thermostats will use red (R or Rc), white (W),  yellow (Y), and green (G) wires; used for heat pumps, HVAC systems. Very common for Nest thermostats.
  • Typical 5-wire thermostats will use red (R or Rc or Rh), white (W, W1),  yellow (Y, Y1), green (G), and blue C-wire; used for furnaces, heat pumps, modern air conditioners (mini splits, central air). Mostly for new digital thermostats that require continuous power via C-wire.
  • Typical 6-wire thermostats will use red (R or Rc or Rh), white (W, W1),  yellow (Y, Y1), green (G), black or blue C-wire, and orange (O/B) wire for reversing valve switch; used for complex heat pumps.

Thermostat wire color meanings are standardized. That means that you will be able to decipher what each wire color means for all thermostats; red wire (R) is always the power wire (providing power from a 24VAC transformer). We have to make an important distinction, however, between older and newer thermostats (the addition of heat pumps meant extra thermostat wires/colors):

  • Older thermostats have the basic wire colors (black, red, green, yellow, white). Most older Honeywell and White Rogers thermostat wiring color codes are based on these classic wire colors.
  • Newer thermostats (modern digital programmable ones) like Nest thermostats or programmable Honeywell thermostats will have extra wires (C-wire and wires for heat pumps). These include orange, dark blue, light blue, brown wires, and so on.
what do thermostat wire colors means red orange yellow green blue wire
Newer thermostats are digital are require continuous 24V power. That’s why we see blue (5) or black wires; there are the common C-wires.

In short, heat pump wiring is a bit different; that’s why we will look at both traditional thermostat wire colors (for older Honeywell and White Rogers thermostats) and modern thermostat wire colors (for heat pumps connected to smart digital thermostats like Nest and new Honeywell thermostats).

Thermostat wire colors are an extensive topic; that’s why we will take as simple as possible structured approach (we don’t want to get lost in all these colors, wires, and thermostats). Here is how we will fully learn what each wire color means (function, within a thermostat, for certain appliances like furnaces, AC units):

  1. We will start with a general summary of thermostat wire colors (2 charts). You will find thermostat wire color code charts (traditional and modern) that list all relevant wire colors (traditional and modern/digital), their functions (furnace wire colors, for example), and a general explanation of what each does. This is just to get a grip on the variety of wire colors and their functions.
    thermostat wire colors and terminals
    We will also see to which terminal differently colored wires connect to.
  2. Then we will go wire-by-wire color-by-color. We will look at each color wire in turn, what terminal it connects to, what is its primary function, and in which types of thermostat you will use it. Example: Blue wire in Nest thermostats is a common 24-volt wire. It provides continuous 24/7 power to a Nest thermostat.
  3. Finally, we will look at wire colors used in 3-wire, 4-wire, 5-wire, 6-wire, 7-wire, and 8-wire thermostats. We will look at each of these thermostat wiring color codes one by one. Example: 4-wire furnace thermostat (mostly used for heat pumps) will usually have a red wire connected to Rh (hot wire), a blue wire connected to C (C wire), a white wire connected to W1 or W2 (1-stage vs 2-stage furnace), and a green wire connected to G (furnace blower or fan). In a 5-wire thermostat, you will usually see (standard configuration) yellow wire connected to Y1 or Y2 for cooling (1-stage vs 2-stage AC unit).

Equipped with this knowledge, you will have the requisite knowledge about all thermostat wire colors, what they do, which terminal they should connect to, and, probably most importantly, what is their standard configuration in 3-wire to 8-wire thermostats.

Note: Despite thermostat wires being low-voltage 18 AWG wires (below 14 amp current), you should follow the safety measures when wiring any and all thermostats. Also, be aware that every thermostat is specific and comes with a wiring manual; you should follow that manual alongside this guide.

Alright, let’s have a look at a thermostat wiring color code chart:

Thermostat Wire Color Charts (General Summary)

We have to differentiate between older and newer thermostats. Here is the 1st chart for traditional older thermostats (5 basic wire colors you will find in old Honeywell thermostats, for example):

Standard Thermostat Wire Color: Terminal Letter/Label: Function Or Meaning: Explanation:
Black Or Blue Wire (C Wire) C Terminal Common Wire 24-Volt Common C-wire completing 24V electric circuit; linked to the transformer.
Red Wire R or Rc Terminal Power Wire Every thermostat has a red wire. It provides the voltage that turns on heating systems like furnaces (Rh or Red hot) or cooling systems like AC (Rc or Red cold)
White Wire W, W1 Terminal Heating or Furnace Wire Connects to heating systems, primarily furnaces.
Yellow Wire Y, Y1 Terminal Cooling  or AC Wire Connects to the air conditioner (Stage 1) or heat pump (cooling mode). Y2 connection is used on modern thermostats for 2-stage ACs
Green Wire G Terminal Fan Wire Connects to HVAC fans; these can be furnace blower motor, indoor air handler fan motor, etc.

With the invention of heat pumps, thermostats became a bit more complex. Heat pumps have a dual function (heating, cooling), reverse switch between these two modes, and so on; all these functions are run from the thermostat. Here are the wire colors found in modern thermostats:

Modern Thermostat Wire Color: Terminal Letter/Label: Function Or Meaning: Explanation:
Black Or Blue Wire (C Wire) C Terminal Common Wire 24-Volt Common C-wire completing 24V electric circuit; linked to the transformer (L2)
Red Wire R, Rh, Rc Terminal Switching To 24VAC Red wire provides the voltage that turns on heating system like furnaces (Rh or Red hot) or cooling systems like AC (Rc or Red cold)
White Wire W1, W2 Terminal Switch To Heating White wire is for heating; furnaces, heat pumps. You connect them to W1 (White 1-stage) or, if you have a two-stage furnace or heat pump, to W2 (White 2-stage)
Yellow Wire Y Terminal Cooling Wire Wires in the air conditioner (Stage 1) or heat pump (cooling mode). It’s connected either to R or Rc.
Green Wire G Terminal Fan Wire Connects to HVAC fans; these can be furnace blower motor, indoor air handler fan motor, etc.
Orange Or Dark Blue Wire O or B Terminal Heat Pump Switch Wire For heat pumps; switching from cooling to heating, and heating to cooling.
Light Blue Wire Y2 Terminal 2-Stage Cooling Switch Cooling switch for 2-stage heat pumps
Brown Wire W2 2-Stage Heating Switch Heating switch for 2-stage furnaces

Alright, this is the general wire color codes overview.

Let’s get into specifics wire-by-wire and color-by-color:

Blue Wire (Same As Black Wire; C-Wire)

The blue thermostat wire is a bit of a mess. We have different shades of blue wires (dark blue, light blue), a Nest thermostat blue wire, and a regular blue C-wire that is interchangeable with a black C-wire.

Let’s start with the basic blue wire you will find in all new thermostats (Honeywell thermostat blue wire, Nest thermostat blue wire). The thermostat C wire color is blue or black (connects to the C terminal). New digital thermostats require continuous power (24V). The blue wire (sometimes this is a black wire) is connected directly to the transformer and provides continuous 24V power to a smart digital thermostat.

In many older thermostats, you will see that the thermostat blue wire is missing or is not connected. That’s because older thermostats work on-demand (when the thermostat needs power, the power is supplied, no blue wire needed for continuous power). You will see that the thermostat blue wire is not connected in some older thermostats because there is no need for a C-wire power supply.

Newer thermostats can have many different blue wires. We have to be careful not to mix them up. Namely, here are the blue wires that the new Honeywell thermostat could have:

  • Basic blue wire. This is the all-important common C-wire for smart thermostats (completes 24V circuit).
  • Dark blue wire. This wire is used when wiring heat pumps (it can also be orange). It connects to the B terminal and serves as a heat pump switch. Heat pump has to switch from cooling to heating, and heating to cooling; the dark blue wire is used to send that signal to the reverse valve located between outdoor and indoor heat pump unit.
  • Light blue wire (sometimes referred to as HVAC blue wire). We rarely see this wire (connects to Y2). It is only used in 2-stage heat pumps. When the 2nd stage in a heat pump has to switch between cooling/heating, the light blue thermostat wire sends the signal.

Here is the general rule about thermostat blue wire:

In most cases, this is the C-wire. In rare cases, this is the heat pump switch wire (it’s usually orange when the C-wire is blue in order to avoid confusion). In the rarest cases, the blue wire will be lightly colored; you only have to use it if you are wiring a 2-stage heat pump to the thermostat.

Red Wire (Power Wire)

You will find red wire in all thermostats. This is a power wire that turns on HVAC systems; heating systems like furnaces and cooling systems like air conditioners.

In general, it is connected to the R terminal. In modern thermostats, you can see it connected to an R terminal with a jumper to Rc (cooling) or Rh (heating), or both. The red wire can also be connected directly to the R terminal.

When you see the red wire connected to the R terminal and jumped to Rc (or directly to Rc), you know this is the wire for cooling (Rc = Red cooling). Similarly, when you see the red wire connected to the R terminal and jumped to Rh (or directly to Rh), you know this is the wire for heating (Rh = Red heating). If the red wire in the R terminal is jumped to both Rc and Rh, you know this is the wire that turns on both your furnace or your air conditioner.

If there is no power to the thermostat red wire, the furnace or air conditioner won’t turn on. This is a common thermostat-originated issue when we see the furnace not coming on or the air conditioner not turning on.

White Wire (Heating Wire)

White thermostat wire is always used for heating. It connects to the W, W1, or W2 terminals and turns on the furnaces or heat pumps. You may also see it connected to R or Rh (heating) for on-demand heating.

In heat pumps, the white wire can be jumped to the Y terminal (for switching from heating to cooling). Jumper to W2 is usually used when you are connecting a 2-stage furnace (2-stage gas furnace, for example) or a 2-stage heat pump. If you don’t have a 2-stage HVAC unit, the W2 terminal is left unused.

Yellow Wire (Cooling Wire)

As the white wire is used for heating, the yellow wire is used for cooling systems (air conditioners, cooling mode for heat pumps). It connects to Y or Y1 (but not to Y2) terminals that turn on the air conditioner.  In heat pumps, it can also connect to R or Rc (cooling) for on-demand cooling.

You will notice that the yellow line in the thermostat never connects to the Y2. That’s for 2-stage heat pumps (cooling mode) and we have a special wire for that; light blue HVAC wire.

Specifically, the yellow wire will turn on the compressor in the outdoor unit (for AC units, heat pumps). When you heat the compressor kick-on or when the AC starts a new cycle, that’s the yellow wire sending a signal from the thermostat to the compressor control panel to power on.

Green Wire (Fan Wire)

You will find the green wire in almost every thermostat. That’s because in most HVAC units (furnaces, ACs) the thermostat controls require a fan to function adequately.

The green wire (G terminal) sends a signal from the thermostat to the fan motor control circuit (furnace blower motor, indoor air handler fan motor) and directs it to power on.

This is pretty much the simplest thermostat wire to install; just connect the green wire to the G terminal and you’re done.

Now, let’s have a look at more colorful wires that you will find mostly in new thermostats controlling heat pumps:

Orange Wire (Same As Dark Blue Wire; Reverse Valve Switch)

With heat pumps, you will have to switch between heating and cooling. To perform this action, every heat pump has a reverse valve; when switched, it reverses the flow of refrigerant (be it R410A, R22, or any other). This will switch the heat pump to the mode you want.

From the thermostat, the orange wire connects directly to the reverse valve and facilitates this switch. You can find an orange thermostat wire or a dark blue thermostat wire; both of these perform the same valve reversal switch.

The orange wire always connects to the O terminal. You will find it in most heat pumps; Carrier, Goodman, Lennox, Amana heat pumps.

With Rheem and Ruud heat pumps, the dark blue wire is used (connects to B terminal).

If you see that your heat pump doesn’t adequately switch from cooling to heating, or heating to cooling, you either have a problem with the reverse valve itself, or the thermostat orange wire (or dark blue wire in Rheem, Ruud heat pumps) is not correctly wired.

Light Blue Wire (HVAC Blue Wire)

If you have a simple heat pump (1-stage), you won’t find a light blue wire in the thermostat. However, if you have a 2-stage heat pump, you will always see the light blue wire in the thermostat. Be careful not to switch this light blue wire with the blue C-wire.

This light blue wire has only one job; power on the 2nd stage of a heat pump. It connects to the Y2 terminal.

Brown Wire (Furnace Wire)

The brown thermostat wire (sometimes referred to as furnace wire) is similar to the light blue wire for heating. That means that you won’t find it in 1-stage furnaces, but you will find it in 2-stage furnaces.

The job of the brown wire (connected to W2) is to power on the 2nd stage of a furnace. You won’t find it often; in many cases, the white wire is connected to the W terminal, and you will see a jumper to W2. It is also important to note that brown wire is standard for W2, but the wire color is not always brown; it can be colored differently (check the manual that comes with the thermostats/HVAC unit).

Alright, these are the thermostat wiring color codes. Now let’s look at how these differently colored wires fit into thermostats. We will start with the simple 3-wire thermostat wiring color codes and progress (one by one) to the most complex 8-wire thermostat wiring color codes:

3-Wire Thermostat Wiring Color Codes

Most 3-wire thermostats are used for heating only. This can include simple furnaces, boilers, and water heaters.

The typical 3-wire thermostat wire color codes are these:

  • Red wire. This is the 24V hot wire that provides power.
  • Blue or black wire. This is the common C-wire.
  • White wire. This is the heating wire.

In this basic 3-wire thermostat setup, the blue (or black) C-wire provides continuous power, red wire is the power wire for a heater, and the white wire turns on the heating appliance (furnace, water heater, boiler).

4-Wire Thermostat Wiring Color Codes

The 4-wire thermostat is one of the most common thermostats (made by Honeywell, primarily). This wiring setup can be used to power heat pumps, for example. That means we will need wires for both heating (W terminal, white wire) and cooling (Y terminal, yellow wire).

Here are the usual 4-wire thermostat wire color codes for a heat pump:

  • Red wire. This is the 24V hot wire that provides power.
  • White wire. This is the heating wire, primarily used to turn on the heating mode of a heat pump.
  • Yellow wire. This is the cooling wire, primarily used to turn on the cooling mode of a heat pump.
  • Green wire. This is a fan wire that powers the indoor air handler fan motor.

This is the minimum wire setup for wiring a heat pump to a thermostat. You may notice the lack of the common C-wire. That means that these basic 4-wire thermostat wire colors will be found in older Honeywell thermostats. Newer Honeywell and especially Nest thermostats will have that common C-wire (blue wire).

5-Wire Thermostat Wiring Color Codes

Many of the new thermostats being installed today have 5 wires. If you compare 4 wire vs 5 wire thermostat wire color, you see that the 5-wire thermostat will have an addition of a common C-wire (blue wire). This is just a requirement for new contemporary thermostats that require continuous power supply via the C-wire.

This thermostat wiring can be used for heat pumps, as well as air conditioners and furnaces.

Here are the common 5-wire thermostat wire color codes:

  • Blue wire (sometimes black wire). This is the common C-wire providing power to smart programmable thermostats like Nest thermostats or new Honeywell, White Rodgers thermostats.
  • Red wire. This is the 24V hot wire that provides power.
  • White wire. This is the heating wire, primarily used to turn on the heating mode of a heat pump. It can also be used to turn on the furnace.
  • Yellow wire. This is the cooling wire, primarily used to turn on the cooling mode of a heat pump. It can also be used to turn on the air conditioner.
  • Green wire. This is a fan wire that powers the indoor air handler fan motor and/or furnace blower motor.

Here is an example of a 5-wire circular thermostat with a black C-wire:

nest thermostat wiring color codes

Let’s look at some of the more complex multiwire thermostats as well:

6-Wire Thermostat Wiring Color Codes

The 6-wire thermostat is similar to the 5-wire thermostat, with the addition of the orange or dark blue wire. This is for heat pumps that require a special thermostat wire to switch the reversing valve (for switching from heating to cooling or cooling to heating).

It is commonly used for wiring heat pumps via the new digital thermostats.

Here are the typical 6-wire thermostat wire color codes:

  • Blue wire (sometimes black wire). This is the common C-wire providing power to smart programmable thermostats like Nest thermostats or new Honeywell, White Rodgers thermostats.
  • Red wire. This is the 24V hot wire that provides power.
  • White wire. This is the heating wire, primarily used to turn on the heating mode of a heat pump. It can also be used to turn on the furnace.
  • Yellow wire. This is the cooling wire, primarily used to turn on the cooling mode of a heat pump. It can also be used to turn on the air conditioner.
  • Green wire. This is a fan wire that powers the indoor air handler fan motor and/or furnace blower motor.
  • Orange wire or dark blue wire. This is a reversing switch wire. It switches a heat pump from heating mode to cooling mode, or visa versa.

7-Wire Thermostat Wiring Color Codes

7-wire thermostat is reserved for either 2-stage heat pumps or 2-stage furnaces. The extra light blue wire or extra brown wire are used to turn on the 2nd stage heat pump cooling or 2nd stage heating (heat pump, furnace). If you only have 1-stage HVAC units, you won’t need a 7-wire or an 8-wire thermostat.

Here are the typical 7-wire thermostat wire color codes:

  • Blue wire (sometimes black wire). This is the common C-wire providing power to smart programmable thermostats like Nest thermostats or new Honeywell, White Rodgers thermostats.
  • Red wire. This is the 24V hot wire that provides power.
  • White wire. This is the heating wire, primarily used to turn on the heating mode of a heat pump. It can also be used to turn on the furnace.
  • Yellow wire. This is the cooling wire, primarily used to turn on the cooling mode of a heat pump. It can also be used to turn on the air conditioner.
  • Green wire. This is a fan wire that powers the indoor air handler fan motor and/or furnace blower motor.
  • Orange wire or dark blue wire. This is a reversing switch wire. It switches a heat pump from heating mode to cooling mode, or visa versa.
  • ONLY ONE: Light blue wire or brown wire. This is the 2nd stage wire for either a heat pump or furnace. In a 7-wire thermostat, you will either find a light blue wire (for the 2nd stage heat pump) or a brown wire (for the 2nd stage furnace); you will never see both wires in a 7-wire thermostat.

8-Wire Thermostat Wiring Color Codes

An 8-wire thermostat is the most complex home thermostat setup. It is used when you have both a 2-stage heat pump and a 2-stage furnace (usually for a heat pump with a furnace backup setup for colder days).

Here are the typical 8-wire thermostat wire color codes:

  • Blue wire (sometimes black wire). This is the common C-wire providing power to smart programmable thermostats like Nest thermostats or new Honeywell, White Rodgers thermostats.
  • Red wire. This is the 24V hot wire that provides power.
  • White wire. This is the heating wire, primarily used to turn on the heating mode of a heat pump. It can also be used to turn on the furnace.
  • Yellow wire. This is the cooling wire, primarily used to turn on the cooling mode of a heat pump. It can also be used to turn on the air conditioner.
  • Green wire. This is a fan wire that powers the indoor air handler fan motor and/or furnace blower motor.
  • Orange wire or dark blue wire. This is a reversing switch wire. It switches a heat pump from heating mode to cooling mode, or visa versa.
  • Light blue wire. This is the 2nd stage heat pump wire.
  • Brown wire.  This is the 2nd stage furnace wire.

All in all, you can see that we are dealing with up to 8 different thermostat wire colors. Other terminals like BK, RS1, RS2, ODT1, ODT2, AUX NO, AUX C, AUX NC will be connected to by wires of varying colors. There is no specific code for those; here the manual that comes with the thermostat and HVAC units is priceless.

We hope that quite a complex topic of thermostat wiring color codes is a bit more understandable now. For a detailed way how to wire a thermostat, you can check this wiring post.

Thank you.

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