5 Wire Thermostate Color Code and Wiring - PICKHVAC (2024)

This 5 wire thermostat wiring color code will assist you as you install a thermostat used to control a furnace and AC system or a heat pump system.

5 wire thermostat brands include Honeywell, White-Rodgers, Emerson thermostats and most other brands.

Let’s get started.

5 Wire Thermostat Wiring Color Code

This 5 wire thermostat wiring diagram shows the most common wiring color code and which thermostat terminal each wire attaches to. This color code is for most systems with a furnace and central air conditioner.

5 Wire Thermostate Color Code and Wiring - PICKHVAC (1)

While the diagram color code is clear and straightforward, a brief explanation of the colors and terminals could be useful for some readers. And the color code for a heat pump follows.

  • White wire to the W terminal is for heating. When only one W terminal is found, instead of W and W2, it indicates one-stage heating.
  • Red wire to the R terminal for 24 volt power. Your furnace is either 110-120 volt (gas and oil furnaces) or 220-240 volt (electric furnaces). A transformer in the furnace sends 24 volt power to the thermostat for its power as needed, not continuously.
  • Yellow wire to the Y terminal for 1 stage air conditioning. As with W, a 2-stage AC thermostat has Y and Y2. No AC? No problem. Don’t attach the Y wire to a terminal.
  • Green wire to the G terminal. This wire and terminal power the furnace fan, aka blower.
  • Blue wire to the C terminal. What is a common wire? It’s also called the C-wire. A common wire creates a loop with the red wire/R terminal. As a result, it provides continuous 24V power to the thermostat, so that the digital display is always on. Without a common wire, the thermostat only draws power from the furnace when the system needs to start a Heat or Cool cycle.

Is the common wire always blue?

No. In some thermostats, a black wire is used for the common wire. Color wire isn’t important, other than to have a uniform color code, as long as the wire is attached to the corresponding terminal in the thermostat and on the control board of the furnace.

Heat pump 5 wire thermostat color code: A heat pump and air handler system uses the following thermostat terminals and colors. Most are the same, with one exception:

Yellow to Y; Green to G; Red to R; Blue or Black to C; Orange to O.

The orange wire connects to the O terminal and operates the reversing valve in the heat pump, so that it delivers heat in winter and removes heat in summer.

Rheem and Ruud owners – a dark blue wire is connected to the O terminal instead of an orange wire.

4 Wire to 5 Wire Thermostat Options

You can upgrade a 4 wire to 5 wire thermostat by adding a C-wire.

Or you can use only 4 wires with a 5 wire thermostat.

1). Upgrading a 4 wire to 5 wire thermostat

Most thermostat wiring is a bundle of 6, 8 or 10 wires. This means that if you have a 4 wire thermostat, you very likely have extra wires. Usually, the extra wires are twisted together, not connected to anything, and “stuffed” into the thermostat opening in the wall.

The purpose of the upgrade is to add a C-wire.

Try this first: Pull the blue wire out of the bundle. Connect it to the C terminal. Reinstall the thermostat cover.

Now, is the digital display on the thermostat lit all the time? I

If so, you have effectively installed a C-wire. If not, remove the cover again.

Try this next: Remove the blue wire from the C terminal, and connect the black wire to it.

Put the cover back on, and see if the display is lit up. If it isn’t, and you want a C-wire for continuous power to the thermostat, then you probably need the service of an HVAC technician.

I only have 4 wires for a 5 wire thermostat!

If there are no extra wires, then you will have to run a wire from the terminal inside your furnace to the thermostat.

Remove the cover of the furnace, and find the terminals where colored wires are connected. Connect your extra wire to the C terminal in the furnace or air handler.

Run the thermostat wire through the wall to the thermostat, and connect it to the C terminal there.

You now have a common wire.

Tip: Choose a blue or black wire to fit the standard 5 wire thermostat wiring color code.

2). Use 4 wires only

Connect the red, white, yellow and green wires to the R, W, Y and G terminals.

This should power your furnace and AC system. You won’t have a c-wire, but it shouldn’t significantly affect your system’s performance.

Do you need a C-wire? If the answer is yes, here is an option for how to use 4 wires with a 5 wire thermostat. It is controversial, so you might want to consider consulting an HVAC technician for the best practice.

Remove the green wire from the G terminal both at the thermostat and inside the furnace or air handler.

Next, in both places, attach the green wire to the C terminals.

Finally, use a jumper wire to connect the Y and G terminals inside the furnace or air handler. This will eliminate the independent functioning of the fan. In other words, you will lose Fan Only mode. But when the system comes on, the blower will work. It’s like the thermostat being set to Auto instead of Fan-on or Manual.


Do thermostats use 4 or 5 wires?

Both are common numbers of wires. Most new digital thermostats require a C-wire for best performance. A 5 wire thermostat is essentially the same as a 4 wire thermostat with the addition of a C-wire or common wire.

What voltage is a 5 wire thermostat?

24 volts.

5 Wire Thermostate Color Code and Wiring - PICKHVAC (2)

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.

5 Wire Thermostate Color Code and Wiring - PICKHVAC (2024)
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