4 Wire Thermostat Color Code and Wiring

The 4 wire thermostat wiring color code is simple, one of the easiest thermostat wiring diagrams of any type.

Using 4 wire thermostat wiring is common for systems with basic heating and cooling, usually a system with a gas furnace indoors and a condensing unit/AC outdoors.

Nest thermostats also use 4 wires along with ecobee, some Honeywell models and other smart thermostats.

4 Wire Thermostat Wire Color Code

The most common 4-wire color code is:

Wire ColorStandard Color Wire CodePurpose

Here are the connections to be made when wiring a 4 wire thermostat:

  • White wire to the W terminal for heating.
  • Red wire to the R terminal for 24V power from the transformer in the furnace or air handler – or RC on some thermostats.
  • Yellow wire to the Y terminal for cooling.
  • Green wire to the G terminal for indoor fan or fans.
4 wire thermostat wiring

How can I check a 4 wire color code?

There are two ways to ensure that this color code works for your thermostat.

The easiest way to check if the wires and terminals are connected properly is to try the system. When you adjust the thermostat setting or mode, does the system respond properly? Does both the heating and AC mode work? Does the fan come on? If these functions are working, then the thermostat is properly wired.

The second way to check is to take the cover off the furnace and view the furnace terminals. If the wire colors and terminal are the same as listed above, the wiring is correct.

What HVAC Systems Use 4 Wire Thermostats?

This simple wiring configuration is suitable for use with a furnace and AC systems and a few other HVAC systems with basic heating and air conditioning.

What is the blue wire for?

Some 4 wire thermostat wire bundles might have an extra blue wire – or sometimes it is black – that isn’t used. This is most common in older thermostats made by major brands like Honeywell and White-Rodgers.  

If you replace that thermostat, the blue wire can be used as the common wire, or C-wire, to continuously power the digital display instead of it only being powered when there is a demand for heater or cooling. This is the most common difference between 4-wire thermostat color code and 5-wire thermostat wiring.

It also helps to use the C terminal and a C-wire for a nest thermostat when it is controlling a heat pump. This allows the thermostat and system to remain in constant communication in order to more precisely control the heating and air conditioning modes of the system.

Switching a 2 Wire Thermostat to 4 Wire Thermostat

If your thermostat has 2 wires connected, it is likely quite old and should probably be replaced with a 4 wire thermostat. If your HVAC system has more than one stage of heating or air conditioning, you might need 5-8 wires.

But here is how to change a 2 wire thermostat to 4 wire connections.

1). Choose your 4 wire thermostat. Nest, ecobee and a few Honeywell thermostats use a minimum of 4 wires.

Tip: Before you purchase a thermostat, read up on it online to make sure it will operate the system you have. Once you install the thermostat, you might not be able to return it if it doesn’t work with your system.

2). Once you have your new thermostat, read the installation manual thoroughly and completely.

3). Connect these wires to these terminals:

  • Red to R for 24 volt power.
  • White to W for Heat Mode.
  • Yellow to Y for AC/cooling Mode.
  • Green to G for powering the indoor blower fan.


Can you hook a 4 wire thermostat to 2 wire?

Yes, there are some system types that allow you to connect a 4 wire thermostat to 2 wire heating systems. They typically have 1 white wire and 1 red wire. The 2 wire systems usually include just a furnace. The simplest method is to connect the red wire to the R terminal for power and the white wire to the W terminal for heating.

If you have additional wires in the bundle, connect the green wire to the G terminal in the thermostat. This will control the fan independently.

Some systems that run on 2 wires are 240 volt electric baseboard systems.

If that’s the type of system you have, then without seeing how the system is wired, it is impossible to give safe guidance on how to wire a 4 wire thermostat to a 2 wire heating system. The right decision is to contact a local HVAC technician or an electrician to ensure that the thermostat is properly and safely wired.

What is the difference between a 2 wire and 4 wire thermostat?

In high voltage systems, 2 wire thermostats are single pole thermostats. 4 wire thermostats are dual pole models.

For low voltage systems, 2 wire thermostats are usually old and for a furnace-only system. They should be replaced with a 4 wire thermostat on systems with basic heating and AC. If your system has 2-stage heating and/or cooling, you’ll need a thermostat with up to 8 wires depending on system functionality. 

Written by

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.
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