Thermostat Wire Color Code

All these colorful wires don’t mean a lot to people who don’t have any electrical knowledge. But the truth is that these colors are hiding quite a lot of information that might, at one point, come in handy.

The red wire is the one that powers the thermostat, the green wire connects to the fan, the white wire is connected to the heating system, and the yellow wire is responsible for the cooling system. Smart thermostats usually have a common wire that is either blue or black.

Thermostat Wire Color Code – Meaning and Purpose

Color-coded wires make it easier for technicians and regular homeowners to identify which wire is responsible for what. However, do bear in mind that it doesn’t always mean that the wire of one color has to be connected to the port of the same color.

Are Thermostat Wires Color Coded?

The absolute majority of thermostat wires are color-coded. This will help you fix the device if it ever goes bad or install a new thermostat.

Here are the most common thermostat wire colors and the ‘meaning’ behind them.

Color of the WireTerminalDetails
WhiteWConnects to the heating system and terminates at the furnace or air handler.Your thermostat can have more than one white wire if your HVAC system has a few heating stages.
YellowYConnects to the cooling system (your air conditioner). These wires go through the air handler and terminate at the compressor contactor.
GreenGConnects to the fan of the air handler or furnace (and terminates there).
RedR/V, Rc, RhConnects to 24VAC (Rh for heating, Rc for cooling). If you don’t have a dual transformer setup, then the Rh wire can be marked as R.
BlueBThis is a common wire that is connected to the heat pump. Your HVAC system is going to need a common wire if it has to be connected to a power source 24/7.

Read: HVAC Thermostat Problems

What Color Thermostat Wire Is Common?

The majority of smart thermostats require a C wire or the ‘common’ wire. It is a wire that powers the LED screen, internal processor, and Wi-Fi connection.

This common wire enables a continuous flow of 24VAC power to the device. Though the power is flowing from the R wire, the C wire is the part that makes the flow continuous by completing the circuit. 

If you used to have a regular thermostat and you are now considering switching to a smart device, then the chances are high that you’ll need to run a new cable from the HVAC unit that is going to be the C wire. 

The color of this wire is not standardized. Usually, it’s either blue or black; however, if the installer did his own rewiring, then the common wire can be of any color. 

Read: No Power To The Home Temperature Thermostat – Troubleshooting Guide

What Is the Blue Wire for On a Thermostat?

A blue wire is usually the common wire.

Homeowners that have a heat pump, in general, have blue common wires (if there is no heat pump, then the color might be black). 

The wire connects to the B terminal and allows the thermostat to constantly be connected to the power source. This helps control the device’s ‘smart’ aspects.

Nest Thermostat Wire Colors

Nest wire colors can vary, however:

  • Common wires are, in general, blue
  • If you have a heat pump and there is an orange wire in the connector labeled W and O/B, then put it in the thermostat’s O/B connector.
  • If you don’t have a heat pump and there is a white wire in the connector labeled W and O/B, then put it in the device’s W connector.

How Do You Identify Thermostat Wires?

Before beginning any electrical project, you should turn off the power to the device at the circuit breaker. Ideally, you would want to use insulated tools and high-voltage rubber gloves.

If you are not familiar with how electricity works and anything wire-related, then it would be better to call a professional to take care of the job.

To identify the thermostat wires, you should:

  • Turn the thermostat and the HVAC unit off.
  • Locate the HVAC control board – this is where the thermostat wires connect to the heating and cooling system.
  • The wires will most likely be color-coded. They should also be labeled. 

Read: Why Does The Room Temperature Not Match The Thermostat Setting?

What Are the 7 Thermostat Wires?

Different systems can have a different number of wires.

The chances are high that you’re going to have a 7-wire thermostat, in case you have a heat pump or a system that controls your home’s humidity or ventilation

Heat pumps need a wire to control the reversing valve and one to control auxiliary heat.

2 Wire Thermostat Wiring Diagram

If you can see only two wires, then it looks like you have a thermostat that can provide heat only.

Usually, the two colors are red and white. The red one powers the thermostat and the white wire is responsible for heating.

4 Wire Thermostat Wiring Color Code

This configuration is generally used with battery-operated thermostats or devices that regulate heat only.

The red wire powers the thermostat, the white wire connects to the heating unit, the blue wire is the common wire, and the green wire is connected to the fan.

5 Wire Thermostat Wiring Color Code

The most common configuration for HVAC systems that provide both heating and cooling.

You’ll have the red, green, white, and common wire (blue or another color), as well as a yellow wire that is responsible for cooling.

Read: HVAC UV Light: Everything You Need To Know

6-Wire Thermostat Wiring Color Code

You’ll have a 6-wire thermostat if your system has two stages of heat and one stage of air conditioning.

Two-stage heating systems have a low setting and a high setting.

Apart from the red, green, white, yellow, and black or blue common wire, a 6-wire thermostat will also have a brown wire that connects to second-stage heat and terminal W2.

8 Wire Thermostat Wiring Color Code

You might have an 8-wire thermostat, in case your HVAC system has auxiliary heat (can be brown, terminal E) and, perhaps, a two-stage cooling system (second-stage cooling wire might be light-blue and connected to terminal Y2).

Read: Why Does Home Thermostat Say HEAT ON But There Is No Heat?

What Do Thermostat Wiring Letters Mean?

Most thermostats follow the same standard lettering system. However, the instructions provided below are not universal.

In order to avoid any mistakes, it is always better to consult the diagram provided by the manufacturer.

Terminal LetterWhat It Means
GThe terminal controls the fan and is connected to the green wire
R/Rh/RcThe R terminal is the 24V power supply; Rh is for heating power supply; Rc – cooling power supply
Y/OThis terminal controls the cooling
W/BControls the heating
CThe common wire’s terminal

Usually, the letters stand for the color of the wire that is connected to that particular terminal (R – red, for example).

Does R Go to RC or RH?

The often-red R wire is the wire that powers the entire HVAC system. If you have both, an Rh and an Rc wire, then the R wire can be connected to any of those.

If you have just the R wire, then it has a single R connector.

In a single-stage heat pump system and a 4-wire heating and cooling system, the Rc and Rh terminals are jumpered together. However, that’s not the issue in a 5-wire system.

What Happens If You Wire a Thermostat Wrong?

If you don’t have basic electrical knowledge, then it’s better to leave the installation of a thermostat to a professional. 

What will happen if you wire the device incorrectly? If you are lucky, the thermostat will simply not function.

However, the other consequences of such a mistake can be a bit more serious. You might…

By the way, a professional will not only find the perfect wiring configuration for your device but will also suggest the right location for the thermostat.