Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (2024)

This page includes instructions on how to wire a furnace thermostat for homeowners that want to install a new or replacement thermostat for their HVAC system.

It includes a furnace thermostat wiring diagram that is color coded to make your job simple at home.

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Furnace Wiring Diagram

This gas furnace wiring diagram is a guide to wiring most thermostats that will control a gas furnace system whether or not it includes an AC unit.

You might not need all the wires. Various system configurations and thermostat wiring including 4 wire, 5 wire and 7 wire thermostats are discussed on this page.

Note on Electric furnace thermostats: This diagram is correct for most electric furnace units. However, it is always best to consult the thermostat wiring guide provided by your furnace manufacturer to ensure it is done correctly.

Furnace Wiring Color Code

Here are the wiring colors used in most furnace thermostats and the functions they serve.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (1)

Starting on the upper right side of the diagram above:

RC – Red: This wire provides 24 volt power from the furnace to the thermostat so that the thermostat can control air conditioning.

R – Red: This wire serves the same purpose as the RC wire but for heating.

Y – Yellow: Cooling Mode. This wire runs to the furnace control board which in turn controls the activation of the compressor and related components required for AC.

G – Green: This wire controls the blower motor in the furnace.

Y2 – Light Blue: This wire is necessary if you have a two-stage AC compressor. If the AC is single stage, this terminal will be empty.

W2 – Light Brown: This wire functions like the Light Blue wire but for furnaces with low and high stages of heating.

W – White: Heating Mode. For a gas furnace, it controls the sequence of furnace functions needed to fire the furnace. For an electric furnace, it controls energizing the heating coils to produce heat.

C – Dark Blue: The Common wire or C-wire completes a circuit between the thermostat and the furnace to provide continuous 24 volt power. This keeps the thermostat in constant communication with the furnace.

C-Wire Note: If your thermostat uses batteries, then the C-wire isn’t crucial. However, it makes sense to keep it connected in case your thermostat batteries fail.

What do thermostat wire colors mean? Nothing, technically. The wires inside the housing are the same. The industry has adopted these standardized colors to make thermostat installation as uniform and easy as possible for technicians and homeowners too.

The key is to connect the same wire to the proper thermostat terminal and the proper location on the furnace control board.

Do you have only one red wire?

For some thermostats used for a system with both heating and air conditioning, there is only one red wire. In this case, the red wire will likely be attached to the RC terminal. And a jumper wire will connect the RC and R terminals. Though this could be reversed – the wire attached to the R terminal and jumped to the RC terminal.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (2)

How to Wire a Furnace Thermostat

Knowing how to wire a furnace thermostat depends on whether it is a replacement thermostat or a new thermostat.

New thermostat wiring: If you’ve installed a new furnace in a new home or addition, for example, use the wiring diagram above for the new thermostat. Separate the bundle of wires coming out of the wall. Strip the housing off the last quarter-inch of the wire, and connect the wires to the terminals as shown.

Thermostat replacement: Take a picture of the thermostat wiring before you start disconnecting wires. Yes, most thermostats are wired using the wiring colors and terminal connections shown above. But in rare cases, it is different. Having a picture will ensure you wire the replacement thermostat just like the old thermostat.

How Do You Wire a Single Stage Furnace Thermostat?

How to wire a single stage thermostat is the same as the diagram given above with a few exceptions.

Single stage heating with no AC: This is 4 wire thermostat wiring. Use the R, G, W and C wires. If the thermostat is designed for a single stage system, it won’t have Y2 and W2 terminals. You can use a 2 stage thermostat with a single stage system, but the Y2 and W2 terminals won’t be used.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (4)

Single stage heating and single stage AC: Use the R, RC, Y, G, W and C terminals. This is 6 wire thermostat wiring.

If the thermostat only uses 4 or 6 wires and you have wires left over that’s OK. Not all wires have to be connected to a terminal.

How Do You Wire a Two Stage Furnace Thermostat?

See the Furnace Thermostat Wiring diagram above for reference as you go through this section. It shows the standard wire colors and connections for wiring a 2 stage thermostat for a system with 2 stage heating and air conditioning.

But not all systems with a 2 stage furnace have AC. Therefore, two stage furnace thermostats require 5, 7 or 8 wires depending on the components in the system and their performance.

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2 stage heating with no AC: This system requires 5 wires. You won’t use RC, Y or Y2.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (6)

2 stage heating with single stage AC: This is a 7 wire configuration. The only wire not used is the Y2 wire for a second stage of cooling.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (7)

2 stage heating and 2 stage cooling: This installation uses 8 wires. Most thermostat wiring bundles have 8 wires, though 10-wire bundles are also used on some systems. Not all wires are used on many systems.


How do you wire a Trane gas furnace thermostat?

Wire a thermostat for a Trane furnace using these wire colors and terminals:

  • Red to the R terminal for heating.
  • White to the W terminal for heating – the thermostat will close the connections between the R and W terminals and contacts in the furnace to create a circuit and energize the furnace to go through the heating sequence.
  • Green to the G terminal for the blower fan.
  • Dark blue (sometimes black) to the C terminal for continuous power between the furnace and the thermostat.

Those are the required connections for single stage gas furnace heating.

Additional connections you might need for a system with AC or staged heating/cooling are:

  • Red 2 to the RC terminal for cooling – or use a small piece of wire to jump or connect the R to the RC terminals.
  • Brown to the W2 terminal for second stage heating.
  • Yellow to the Y terminal for cooling.
  • Light blue to the Y2 terminal for second stage cooling.

How do you wire a 4 wire furnace thermostat for a Goodman furnace?

For a Goodman furnace and most other brands, wire the thermostat this way: Red wire to the R terminal. White wire to the W terminal. Green wire to the G terminal. Blue wire to the C terminal.

What voltage is a furnace thermostat?

A furnace thermostat uses 24 volt power. The furnace is powered by 110-120 volts. A transformer in the furnace converts it to 24V to power the thermostat.

What’s the best thermostat wire for a furnace?

18-8 wire is the best furnace thermostat wire. The 18 refers to the gauge or thickness of the wire. 8 refers to the number of individual wires in the bundle.

If you have extras on your system like a humidifier or air cleaner, then you might need 18-10 wire with ten wires in the bundle.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (8)

Written by Rene Langer

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

Gas / Electric Furnace Thermostat Wiring (Diagrams & Color Code) (2024)


What color wires go where on a thermostat? ›

Thermostat Wiring Tips

If your thermostat controls your heat, you will have a white wire. The Y wire is yellow and connects to your air conditioning compressor. The G wire is green and connects to the fan. Rc and Rh: The red wire(s) are the power source for your thermostat.

What is the wire code for the furnace thermostat? ›

Blue or Black (C): The Unsung Hero
Wire ColorTerminalFunction
Red (Rc)Power24-volt power supply for cooling
Red (Rh)Power24-volt power supply for heating
White (W)HeatHeating system activation
Yellow (Y)CoolCooling system activation
3 more rows
Sep 6, 2023

What are the 5 thermostat wires? ›

This configuration includes a 24-volt red wire for power, a white wire for the heating elements, a yellow wire for the AC unit, a green wire for the fan component, and a common wire which is most likely blue but could be a different color.

Where does the orange wire go on a thermostat? ›

While thermostat wire colors can vary, you can also sometimes use color as a guide if a wire is in your thermostat's O/B - W terminal. If the wire in the O/B-W connector is orange: you probably have a heat pump.

What happens if you wire a thermostat wrong? ›

A mismatched thermostat can damage the HVAC system and the electrical system. The clash between the instructions from the thermostat's computer algorithms with the HVAC system can harm components.

What is the basic HVAC thermostat wiring? ›

For most battery-powered thermostats, you'll typically see a system with four wires. These consist of red, white, green, and blue wire. While the red and white wires control power and heating, the green wire controls fans, and the blue “C” or “B” are either your common wire or for your heat pump.

What is the blue wire from the thermostat to the furnace? ›

Blue wires ('B') are also called C wires because they are common wires. C wires are necessary for any smart thermostat that requires a 24/7 power source, regardless of type of heat pump you have. (Make sure to choose a thermostat that is compatible with your specific heat pump.)

How do you wire a 4 wire thermostat to a furnace? ›

Attach the red wire to the R terminal (Call for heating and/or cooling). Attach the green wire to the G terminal (Fan). Attach the white wire to the W terminal (Heat). Attach the yellow wire to the Y terminal (AC).

Where do I connect the C-wire on my furnace? ›

On the C-Wire Adapter's "Equipment" side, connect the wires to the corresponding terminals on the control board. Note: Be sure to remove the plastic tip on each wire before connecting to a terminal. Use the included mount and adhesive pad to attach the C-Wire Adapter inside your furnace or heating system.

Where does the brown wire go on a thermostat? ›

W2 - This is the thermostat 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. Brown for W2 Terminal.

What does RC and RH mean on a thermostat? ›

RC: The RC terminal is the 24-volt cooling power supply. RH: The RH terminal is the 24-volt heating power supply. (Note: The RC and RH terminals are jumpered together in a four-wire heat/cool system and a single-stage heat pump system but not in a five-wire heating/cooling system.)

What are the color codes for thermostat wires? ›

Common Thermostat Wire Colors and Their Functions

Yellow wires: They connect to your compressor and control your air conditioning system. The yellow wires connect to the Y terminals on your thermostat. Green wires: The green wire connects to the fan of your furnace or air handler.

What color does the orange wire go to? ›

Orange wires are often used in the same capacity as red wires, to serve as a secondary power supply line. Although red is probably used more frequently, electricians sometimes pair orange wires with black primary power supply wires.

Which wire is positive green or orange? ›

AC has no positive or negative. The NEC does not contain a required color code for voltage marking, the industry standard is 208/120V (A) black, (B) red, (C) blue and grounded/neutral white; 480/277V (A) brown, (B) orange, (C) yellow and grounded/neutral gray. Green or green/yellow is ground.

What is W1 and W2 on a thermostat? ›

W/W1 and W2. In a conventional system, W/W1 controls your heating system and, where applicable, W2 controls your heating system's second stage, which helps warm the home faster. If you have a heat pump system with auxiliary heating, you'll normally connect the AUX/AUX1 or W2 wire to the ecobee's W1 terminal.

How to hook up a thermostat with 4 wires? ›

Attach the red wire to the R terminal (Call for heating and/or cooling). Attach the green wire to the G terminal (Fan). Attach the white wire to the W terminal (Heat). Attach the yellow wire to the Y terminal (AC).

What are the wires for thermostat heat and a C? ›

Typically, the "hot" wires, or the wires that provide power to the unit, are the heating and cooling wires, labeled Rc and Rh respectively. They provide the source of 24V power that comes into the main HVAC control board itself.


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