What Is The Brown Wire On Thermostat For?

Each connected wire on a thermostat has a specific purpose and has to be connected correctly to let the thermostat function effectively. To make the installation of a thermostat easier and to ensure that the right wires are connected to the right thermostat terminals, the wires can be identified according to a commonly accepted color coding. When you open your thermostat and look at the wiring, you often won’t see a connected brown wire, although you can see it is tucked away in the wall behind the thermostat. Now you might wonder what the brown wire for when it is connected to a thermostat and why it is not connected to your thermostat is.

The brown wire connects a thermostat to a heat pump, the dual system switch on the furnace, or the emergency heating system. If your HVAC system is not equipped with any of these, there will not be a brown wire connected to your thermostat.

If your home’s wiring includes a brown wire and your thermostat doesn’t need it, it will be tucked away in the wall.

brown wire on thermostat
brown wire on thermostat (source)

In this article, I’ll provide you with more general information about the different wires being used for wiring thermostats and then specifically more detail regarding the brown wire. When you perhaps have to inspect the wire connections on your thermostat in the future, you can quickly refer to this article. 

What is the brown wire used for?

If there is a brown wire connected to your thermostat, it can be the connection to any of the following:

  • A heat pump, if your HVAC system uses heat pumps.  
  • The dual-system switch on your furnace or boiler, if your home is equipped with a two-stage heating system. The W2-terminal will be connected to the brown wire.
  • A heat pump used for additional (auxiliary) or emergency heating. The W2-terminal will be connected to the brown wire.

If your current thermostat doesn’t need a brown wire connection, the wire is tucked into the wall for possible use when you install another thermostat that needs the wire.

Different thermostat wires 

Although you have only been wondering about the brown wire in your thermostat, it is always good to understand the color coding of all the thermostat wires and which wires are usually found in the different types of thermostats. You will then also better understand what is meant by a “2-wire thermostat,” a “4-wire thermostat,” etc. 

To explain this, let’s look at two aspects:  

  1. The number of wires usually needed for thermostats with specific functions.
  2. The identification by the color of the thermostat wires.

1. The number of thermostat wires for different functions

Thermostats come with different numbers of wires in them. Some thermostats will have only two wires, whereas some have eight wires. Given below is information on what the wires are used for on different types of thermostats;

  • 2-Wire Thermostat – Typical when the thermostat is controlling furnaces.
  • 3-Wire Thermostat – Used if the HVAC system utilizes boilers and water heaters.
  • 4-Wire Thermostat – Used in HVAC systems with heat pumps. Many contemporary smart thermostats also use 4 wires.
  • 5-Wire Thermostat – Nowadays, this is probably the most common thermostat for most HVAC systems. The 5 wires connect all kinds of things, such as furnaces, heat pumps, and air conditioners. Newer smart thermostats that require a C-wire also feature 5 wires.
  • 6-Wire Thermostats – Thermostats with 6 wires are usually used for complex two-stage heating systems and heat pumps with complicated heating and cooling systems.
  • 7-Wire Thermostats – These thermostats are usually part of air-conditioning systems, Two-stage heating systems, and control alarm systems.
  • 8-Wire Thermostats – They are used for very complex heat pumps.

2.  Color coding of thermostat wires 

In the following discussion of the color coding of thermostat wires, it will become clear in which thermostats you can expect connected brown wires. If the original wiring of your thermostat and HVAC system has been done correctly according to the commonly accepted color coding, you’ll most probably only find a connected brown wire in 7-wire and 8-wire thermostats and sometimes the orange wire in a 6-wire thermostat might be brown. 

The list of wires I’m providing here is constructed in such a way that the already mentioned wires are always used in the next thermostat and the new colored wire is added. For instance, wires 1 and 2 are used in all thermostats. Wires 1, 2 and 3 are used in a 3-wired thermostat, etc. 

Wire 1 – Red wire 

The red wire forms part of every type of thermostat. It is a power wire and it connects the thermostat’s R-terminal to 24V power.  Nowadays this wire is designated by the letter “R,” while In older thermostats, it is designated by the letter combination “RC.” 

Wire 2 – White wire

The white wire is also part of every type of thermostat and is used for heating. It can be connected to either “W” or ”W1.” A 2-wire thermostat only has a red and white wire. 

Wire 3 – Green wire 

A 3-wire thermostat usually uses red, white, and green wires.  The green wire is used to connect an indoor fan and it connects to the G-terminal. 

Wire 4 – Yellow wire 

A 4-wire thermostat usually uses red, white, green, and yellow wires. The yellow wire is for cooling and is connected to the Y-terminal.  

Wire 5 – Black (or blue) wire 

A 5-wire thermostat usually uses red, white, green, yellow, and black/blue wires.  The black/blue wire is the 24V common wire (C-wire) and is linked to the transformer. The main purpose of the black/blue wire is to complete the 24V electrical circuit. 

Wire 6 – Orange (or dark blue) wire – in some instances, it is a brown wire

A 6-wire thermostat usually uses red, white, green, yellow, black/blue, and orange /dark blue/brown wires. The orange wire is designated for reverse valve cooling or reverse valve heating. The wire is connected to the condenser of the heat pump outdoor part.

Wires 7 and more – Light blue, brown, and other colors 

7-wire and 8-wire thermostats use all the previously mentioned wires and the light blue “Y2” wire is used in 7-wire thermostats while 8-wire thermostats use the brown “Y2” wire for second-stage cooling systems. A brown wire with a “W2” designation is used for second-stage heating systems. And a brown wire with the letter “E” is used for emergency heat. 


Q: Can I use an unconnected brown wire to connect extra fans?

A: In principle, you can use any wire to connect extra fans or other devices and if the brown wire is not already utilized in your HVAC system, you can use it. Just remember, this is now “outside” the normal color code and you have to inform any HVAC technician who might work on the system in the future about the connection you’ve made.  


The brown thermostat wire is usually used in 6-wire, 7-wire, and 8-wire thermostats for advanced smart device functions and emergency heat connections. You don’t have to worry if there is not a brown wire connected to your thermostat – it simply means that your HVAC system is not equipped with the devices and functions where brown wire connections are needed. 

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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