AC Thermostat Wiring (Diagram Explained For HVAC)

HVAC systems need control points and those come in the form of thermostats. That control can be digital or analog, but it all ends up going through the thermostat wires. When you need to wire a new thermostat or install a new thermostat from scratch, it can get confusing.

Whether you have a simple 2-wire system or a more complex 8-wire system, thermostat wiring doesn’t have to be difficult. This guide will take you through all the steps to have you wiring your new thermostat like a pro.

Common AC & Furnace Thermostat Wiring – How To Wire Them

There are a lot of different home climate control systems, from a basic furnace to a complex system with humidifiers, ventilation fans, and other components. Each of these needs to be controlled by a thermostat and its wires.

Air Source Heat Pump Vaillant Aromtherm Plus

2 Wire Thermostat Wiring

2 wire systems are used for every thermostat and HVAC component out there. While many options use more than two wires, all systems will use at least 2. The only system that uses exactly 2 wires is a furnace. When it is the only component in the system, you only need the red wire and white wire.

The red wire is for powering the system, which is connected to the 24v power terminal. This is marked with an R or RC on the thermostat side. The white wire is for heating and is connected to the W terminal on the thermostat.

3 Wire Thermostat Wiring

A 3 wire system is just like a 2 wire system but adds the complexity of a common wire or C-wire. This is found on more complex heating systems like central heaters and boilers. The C-wire is typically blue or black; however, any non-used wire can be repurposed for C-wire use. Make sure you look at your current thermostat terminals to see what color is used as the common wire.

To connect a 3 wire system, the red wire will still (and always) go to the R or RC terminal. This also continues for the heating wire (white). Make sure this is connected to the W terminal on the thermostat.

Once you have identified which wire is used for the C wire, connect this to the C terminal on the thermostat. When originally installing a system, many techs will use the green wire instead of blue because 16/4 and 18/4 wire bundles don’t always include a blue wire, but will have a green one.

4 Wire Thermostat Wiring

4 Wire systems are becoming more common due to the popularity of heat pumps, which use a fan, heat, and cooling to operate and keep your home at the right temperature.

Once again, the red and white wires are connected to the R (or RC) and W terminals, respectively. This ensures your system has power and heat. The green wire is now used for the fan (as designed) and is connected to the G terminal on the thermostat.

The new wire in use is the yellow wire. This is designed for cooling or AC control and is connected to the Y terminal on the thermostat. It is fairly obvious by now what the markings on the thermostat terminals mean (letters equal wire colors).

5 Wire Thermostat Wiring

The 5 wire system is the most popular setup. It is used for central heat and air systems and newer heat pump systems that use aux heat or two-stage heat pumps. The 5 wire system is essentially a 4 wire set up with the addition of the C-wire once again. It is important to note that from here on out, the C-wire is required.

A C-wire, as covered earlier, can be either blue or black. Blue is the most common, but you need to double-check your current setup.

For a 5 wire connection, the red, white, yellow, and green are still connected to their namesake terminals on the thermostat. The C-wire is now also used and connected to the C terminal.

6 Wire Thermostat Wiring

6 wire setups include using the C-wire and will be used on advanced heating systems with air conditioning. This will be found on two-stage heat pumps, an aux heat system, or one that uses reversing valves that require power.

The first 5 wires are connected the same. Red to R or RC, white wire to the W terminal, black to the C, and yellow and green for the Y and G terminals, respectively. The 6th wire will depend on your system and setup.

For example, if you use a single-stage heat pump with aux heat, the light blue wire will connect to the X (or Aux) labeled terminal. However, if you have two-stage cooling, the light blue wire connects to the Y2 terminal. Finally, the two-stage heating system uses the brown wire to the Y2 terminal.

7 Wire Thermostat Wiring

Using 7 wires is pretty uncommon, and it details a system with a lot of components. Central heating and air with dehumidifiers or vent fans, for example, or heat pumps using reversing valves and two-stage heating will use this wiring type.

The first 5 wires are the standard, as we have discussed above. Red, white, yellow, and green go to their respective terminals (R/RC, W, Y, G). You also need a spare wire for the (typically black) C/Common terminal.

The 6th wire is the aux heat wire, and you should connect light blue to the X/Aux terminal. This brings in the 7th wire. If using a system with reverse valve heating, attach the dark blue wire to the B terminal. On the other hand, if you have a reverse valve cooling system, attach the orange wire to the O terminal.

8 Wire Thermostat Wiring

Finally, we come to the systems that use all 8 wires in the bundle. Almost without fail, the only systems that will use these are HVAC heat pump systems with auxiliary heating. There is essentially no difference in the connection between an 8 wire system and a 7 wire system.

The distinguishing factor is that the dark blue wire (B terminal connection) and the orange wire (O terminal connection) are both used instead of one or the other.

One thing to make a note of, though, is that if you are replacing an older thermostat, the wire coloring can be faded. Sometimes it can be difficult to distinguish between the white and yellow or dark blue and black wires. You do not want to mix these up.

If this happens to you, simply pull the wire bundle from the wall and peel back the sheathing to expose fresh wire. You should be able to untwist and find the right colors and follow them to the ends.

Thermostat Wiring Diagrams (Color Code Guide)

When you buy a new thermostat wiring bundle, it will come with a slashed number. You will typically find 18/5, 18/6, or 18/8 on the label. The first number (18 in this case) is the wire gauge. Thermostat wire should be 18 gauge to meet the electrical code.

Stranded 6 Wired Electric Line

The number after the slash is how many wires are in the bundle. For example, an 18/6 label means the bundle has 6 wires twisting in a sheathing that are 18 gauge each. If your system requires 7 or 8 wires, then 18/6 will not work for you.

It is always advised to buy 18/8 wire, though. This will allow for HVAC system expansion in the future without needing to run new wiring. You can twist and bend the unused wires out of the way.

All thermostat wiring is color-coded to help you out when installing your thermostat or running wire to the components. A typical setup will use red, white, yellow, green, and blue. Less common will use dark blue and brown or even black. While technically the colors don’t matter, it is important to stay with convention to avoid confusion later.

How to Install a Thermostat Step

The red wire is the 24v power supply line. It is always connected to the R terminal unless the RC terminal is used. Yellow, connecting to the Y terminal, is the wire for AC or heat pump cooling.

Like the yellow wire, the white wire is used with the W terminal and controls the signal for turning on and off the heating. Green wire, connected to the G terminal, is for fan control. Finally, the black wire is used for the C-wire or common terminal connection. These are the standard wires used in almost all modern systems.

Beyond that, you have the brown wire, orange wire, and light blue wire. The brown wire connects to the Y2 terminal for two-stage heating systems. The orange and blue connect to the O and B terminals for reverse valve cooling (orange) and heating (blue).

Honeywell Thermostat Wiring Diagram

A typical smart thermostat will use a common wire and is required to have one. You can get kits to create a c-wire without running a new wire bundle if needed, and Honeywell makes them available with every thermostat they sell.

Honeywell is one of the biggest names in 3rd party thermostat systems and is compatible with all major HVAC brands. They typically use a 5-wire connection system, as seen here.

Honeywell Thermostat Wiring
The red wire is connected to the R terminal (with an RC jumper). This supplies the power to the system to let each component turn on or off when the signal is sent to that respective wire. The green wire on the far left is for fan control and is connected to the G terminal.

Left to right, then you have the white wire in the W terminal for heat control, blue in the C/Common terminal for connectivity and WiFi control, and the yellow wire in the Y terminal for cooling control.

Trane Thermostat Wiring Diagram

Trane has specialized thermostats for their systems and typically uses a 5 to 7 wire setup depending on the features you have in your HVAC system. For example, dehumidifiers require the use of the X or Aux line.

In this diagram, Trane covers the use of all 8 wires in a typical 18/8 bundle.

Trane Thermostat Wiring Diagram

Starting on the right side from top to bottom, we have the common connections. Red wire to the R terminal with an R – RC jumper installed. Orange goes to the O terminal for reverse valve cooling, yellow to the Y terminal for AC cooling, and green to the G terminal for fan control. The light blue wire is connected to the B terminal for reverse fan heating control (and in Trane systems doubles as the Common wire).

On the left side, you find the less common applications. The brown wire connects to the Y2 terminal for the two-stage heating system, while the white wire goes into the W terminal for heating control. The black wire is used in the X terminal for auxiliary use for aux heating with the heat pump system.

Goodman Thermostat Wiring Diagram

For Goodman systems, it is a more common 6-wire setup. In this diagram, you will notice two extra wires (black) that connect separately to external sensors. The control wires, though, are standard for all 6-wire applications.

Goodman Thermostat Wiring Diagram
In this instance, we have a two-stage furnace system that uses the red power, yellow heat, and green fan control for the primary connections. Orange and blue are used for the O and C terminals that control reverse valve cooling (orange) and C-wire (blue).

The brown wire connects to the Aux terminal for auxiliary heating, while the white wire (pink in this diagram) is connected to the E terminal (W2 terminal), which will control emergency heating through the thermostat controls.

Differences Between AC, Furnace, and Heat Pump Wiring

When wiring a heat pump, AC system, or furnace, the wire colors and set terminals are essentially the same. There are a few differences to make a note of, though.

For starters, a furnace system doesn’t use supplemental heat and offers zero cooling options. It is the most basic type of thermostat-controlled system available. Furnaces are either on or off, and as such, they only use two wires. Red for power and white to turn the system on or off.

With an air conditioner, you don’t have the heat control, so you will use a yellow wire to control the cooling and green for the fan control. Of course, you still need the red wire for power. In the central heating and air system (the most common type), you will use all five wires for power, heating, cooling, and fan, with an optional common wire for smart controls.

A heat pump can go many ways, and it is essential to know what type you have for the correct wiring. For example, some heat pumps require supplemental heat or have reverse valve control for heating, cooling, or both. Multi-zone systems and single-stage systems will also have different wiring.

How to Replace Thermostat Wire (Step By Step Guide)

If you are replacing an existing thermostat, it can seem like a daunting task. However, there is a simple method to ensure you get it right every time, without fail.

  1. Remove the thermostat from the mount to expose the wires and terminals. In most cases, this area will be covered in dust and needs to be cleaned off. Make sure you can see the terminals and the wire colors easily.
  2. Take a photo of the thermostat with your phone or tablet so you can easily identify the wires and where they are connected. This will be used later.
  3. Remove the wires from the terminals, being careful not to cut the wire or break off the ends. If you do accidentally cut or damage the wire, you will need to pull the wiring bundle from the wall and strip new ends on the wires, so they are all the same length. Don’t worry, almost every install has at least an extra foot of wire tucked in the wall.
  4. Tape the wire bundle to the wall. You don’t want it to fall back into the wall while you are removing the mount.
  5. Remove the screws holding the thermostat mount and remove them from the wall. You will need to remove the tape and wire bundle to slide the mount over them. Reattach the wires to the wall with tape.
  6. If you need to patch the wall or paint behind the old thermostat, now is the time. Otherwise, move to the next step.
  7. Open the new thermostat and mount the bracket to the wall. You will need to pull the wires through the center hole in the mount and make sure they don’t fall back into the wall.
  8. Using the photo you took earlier, attach the wires to the correct terminals as they were before on the new thermostat.
  9. Once secure, push any excess wiring back into the wall and secure the thermostat to the mounting plate.
  10. Test each mode and function to ensure proper operation.

Related Article How to Reset an AC Thermostat

Heat Pump Behind a Wooden Paneled House

People Also Ask (FAQ)

And here are frequently asked AC & furnace thermostat wiring questions.

Can you add a thermostat C wire?

You can add a new wire to a thermostat. You will either need to run a new bundle, a single wire, or use an adapter kit. If you are rewiring an entire system running a new wire bundle is the best option. If you don’t have an adapter, you can run a single wire, though it isn’t recommended. The best option, though, is to add an adapter that will connect inside the furnace circuit box.

What happens if you wire a thermostat wrong?

If you wire a thermostat incorrectly, the best thing that can happen is nothing. In certain bad instances, though, you can damage the thermostat beyond repair, cause damage to the HVAC system itself, or even char and corrode the wires.

What is the common wire on a thermostat?

The C-wire, or common wire, is a wire that connects your thermostat to a furnace to allow for additional controls using a smart thermostat. Since furnaces only use two wires for power and operation, smart controls cannot function properly without the extra wire.

What color is the common wire?

In almost all cases, the common wire is either blue or black. However, because running new wire is difficult on a finished home, a c-wire can be any color of unused wires at the thermostat. For example, if a 5-wire system is already wired, the orange wire may be used for the common wire.


Air conditioner and furnace thermostat wiring shouldn’t be a daunting task. It can be difficult depending on the type of system you have and how complex it is. However, you will use either 2, 3, 4, 5, 6, 7, or 8 wires, and they won’t change duties or terminal positions when you replace a thermostat.

As long as you take your time and use photos to help you remember which wire went where, you shouldn’t have any problems. Hopefully this guide has helped you learn which wires to use, when, and how.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.