Understanding the thermostat wire color code is one of the first steps in any DIY wiring project. A typical thermostat may have a cable with multiple wires in different colors. Each of the wires does a specific function to run power through the circuit so you can change settings remotely.

Modern day thermostats need a continuous power supply to ensure that they can pick up Wi-Fi signals. That’s how they maintain constant communication with your household smart appliances. 

Thermostat Wire Color Code detailed guide

Safety Precautions to check Thermostat Wire Color Code

Before we talk in detail about the thermostat wire color code, know that this guide is an overview for beginners. The objective here is to give you a quick tutorial on the different thermostat wire colors and what they do.

Other manufacturers may have their own specifications, or the electrician wiring the house may get creative with the wires. If the thermostat wiring colors seem unfamiliar, step back from the DIY project and contact a professional electrician right away.

Since you’re dealing with wires here, you cannot risk a short circuit or a fire. Before you start working, make sure you have the necessary gear. Invest in shock-proof gloves and get a pair of safety glasses to protect your eyes. You’ll also need insulated tools for added protection. Ready to get started?

Thermostat Wire Colors – Let’s Start with the Basics

Here’s a quick rule of thumb to remember about your thermostat color code

Wire Color Terminal Function
Blue B The blue wire is the common wire that links to a power source and is critical for running the thermostat. Typically, the blue wire connects to the heat pump and can also be black.
White W The white wire connects to the primary heating unit and ends at the air handler or furnace. HVAC systems that have multiple heating stages may have multiple white wires.
Red R/Rh/Rc The red wire connects to the R terminal and relays a 24V power to the unit.
Rc RC Rc wires connect to the cooling power supply and are typically found in dual transformer systems.
Rh RHRh wires connect to the heating power supply.
Green G The green wire connects to the fan relay to run the fan in your furnace or air handler.
Yellow Y The yellow wire connects to the air conditioner and runs from the air handler, terminating at the compressor.
Orange OThe orange wire is typically present in homes that use an air source heat pump. This wire connects to the outdoor condenser. This wire may not be present if you have a geothermal heat pump.

When trying to understand how the thermostat wire color code works, know that the thermostat has two or more wires. The number of wires depends on the actual functionality you’re expecting from the gadget. 

2 Wire Thermostat Wiring Color Code

As their name suggests, 2-wire thermostats typically have just two wires, red and white. This device likely only controls your home’s heating system without managing the air conditioning.

The white wire connects to the heating unit, while the red wire powers the thermostat. You won’t need a common C wire for the system. It is also known as “heat only thermostat”.

2 Wire Thermostat Wiring colors
2 Wire Thermostat Wiring Color Code

3 Wire Thermostat Wiring Colors

Homeowners typically use the 3 wire digital thermostat to control their heating systems run by boilers and water heaters. The configuration in these gadgets includes:

  • Red wire or hot wire for 24V power connecting to the R terminal.
  • White wire connects to the heating unit, it links to the W or W1 terminal.
  • Green wire also for 24V power connecting to the G terminal. This wire is usually a repurposed C wire and completes the circuit. 
3 Wire Thermostat
3 Wire Thermostat Wiring Colors

4 Wire Thermostat Wiring Colors

If you’re looking for a battery operated, digital thermostat, you’ll go with the 4 wire thermostat that only helps you with heating. You’ll install this gadget to control the heating system in your home or workplace. The Nest and Ecobee brands typically need 4-wire thermostats. Here’s the thermostat wire color code to expect.

  • Red wire or hot wire for 24V power to keep the thermostat running linking to the R terminal. 
  • Blue wire or common wire for 24V to complete the circuit linking to the C terminal.
  • White wire connecting to the heating unit linking to the W terminal. 
  • Green wire connecting to the fan linking to the G terminal. 

5 Wire Thermostat Wiring Color code

HVAC systems that manage the heating, cooling, and climate control in the entire home or office complex use these thermostats. The gadget needs five wires to help you regulate heating and air conditioning to specifications.

You might also consider the 5-wire a 4-wire thermostat with an extra common wire configuration. This feature lends versatility to the gadget, so it can control heat pumps, air conditioners, furnaces, and any other heating systems you might have installed. Here’s what you’ll see below the face-plate.

  • Red wire or hot wire for 24V connecting to the power source linked to the R terminal. 
  • White wire connecting to the heating system linking to the W or W1 terminal.
  • Yellow wire connecting to the air conditioning or cooling system linking to the Y terminal.
  • Green wire for regulating the fan linking to the G terminal.
  • Blue wire or common wire, which can also be of a different color linking to the C terminal.
5 Wire Thermostat
5 Wire Thermostat Wiring Color code

6 Wire Thermostat Color Code

6 wire thermostats typically enable users to set their temperatures with two stages of heating and one stage of cooling. When trying to understand the thermostat wire color code, know that when you need to raise the room temperature, the thermostat turns on the heating.

At first, the device switches to a low-temperature setting. But, if you need a second stage heating with a higher temperature within a preset interval, the thermostat ramps up the furnace. 

Thus, the thermostat ensures the furnace works with a higher output of heat until the room reaches the desired temperature. Alternatively, the 6 wire thermostat may have a first-stage and second-stage cooling and one stage of heating. If you also have a heat-pump cooling system, that needs an 8-wire thermostat. Here’s how the thermostat wiring colors work:

  • Red wire or hot wire for 24V linking to the R terminal
  • White wire connecting to the W terminal
  • Yellow wire connecting to the Y terminal
  • Blue or Black wire connecting to the C terminal
  • Green wire connecting to the fan and G terminal
  • Brown wire connecting to the W2 terminal for second stage heating or cooling OR Orange wire connecting to O terminal Used on heat pumps to trigger the reversing valve
6 Wire Thermostat Color Code

The most critical factor to note about the thermostat wire color code is that the colors are not uniform. The brand you purchase may have a distinct code and you’ll want to study the accompanying manual.

Like, for instance, the color code of Honeywell thermostat wiring may be different from the code in say, Carrier or EcoBee. Read the information provided carefully before starting work on installing or rewiring the thermostat.

Frequently Asked Questions

Now that you understand what are the thermostat wiring colors and what they do, let’s answer some common FAQs.

What are S1 and S2 wires on thermostat?

Advanced thermostats provide you with a digital reading which indicates the outdoor temperature. But, for the device to get the reading, it needs the S1 and S2 wires that run outside. These wires are for an outdoor temperature sensor.

What do the letters on thermostat wires mean?

Each letter corresponds to the thermostat wire colors like Y for yellow and G for green. These letters help you identify the proper wires so you can connect them to your heating and air conditioning systems. The only exception to the rule is the common terminal marked with a “C.” You’ll connect the blue or black common wire to it. 

What happens if you wire thermostat wrong?

If you don’t understand your thermostat wire color code, it is advisable to let an expert handle the wiring. If you’re lucky, incorrect or faulty wiring may simply result in the device not functioning at all. Remember that this is electrical wiring and can be hazardous.

You risk electrocution or damaging the thermostat and HVAC systems irreparably, and spending thousands of dollars on replacing them. You could also blow the circuit breaker by wiring the thermostat wrong.

Where do I connect the C wire on my furnace?

You’ll connect the C wire to the C terminal in the furnace. If you don’t have a C wire, you may have to run a new C wire through the existing wiring. Alternatively, the electrician might choose to use the G wire in place of the common wire. The only downside is that you may not be able to control the fan independently. 

Following the thermostat wire color code makes it easy to connect the proper wires and have your thermostat working properly. But, if you see that the wires are different from what you expected, rely on professional services.  


  1. https://www.energy.gov/