What Does Brown Wire Mean on Nest Thermostat [Color Meaning]

Have you just noticed the brown wire on your Nest thermostat, and now you’re unsure about its function? The wires on the Nest thermostat are color coded differently to identify them concerning their functions.

So, what does brown wire mean?

The brown wire of your Nest thermostat connects it to an emergency heating system. This emergency heating system is a backup if your heat pump system ever fails. If you don’t have a brown wire, it may indicate that your thermostat’s HVAC has no backup system for emergencies.

Read on to learn all about the brown wire on your Nest thermostat. We shall also talk about the other colored wires on your system.

What does brown wire mean

Why do I have a brown wire on my Nest thermostat?

If you, by any chance, open your Nest thermostat, you’ll see a brown wire on thermostat either placed as a spare wire or connected with the rest of the colored wires. In any case, this wire is of great use and is essential for your thermostat.

The Nest thermostat has multiple colored wires, each with a particular use. The brown one connects the Nest thermostat to the emergency heating system. This emergency heating system activates when the heat pump fails to function and breaks down.

Therefore, this brown wire will enable your emergency heating system to work in any emergency. Thus, you may utilize its function until you call a technician to repair your thermostat.

In addition, the proper connection of the brown wire at the proper place is very important. Otherwise, the data from the HVAC system would not be sent to the Nest thermostat, and preventing it from working properly would create problems for the system.

The brown wire is left inside without any connection often. This is to prevent the activation of the emergency heat system. Thus, you may connect it whenever needed, such as failure of your heating system.

In some cases, you may not find any brown wire in your thermostat. This means that your Nest system doesn’t have any emergency heating pump.

What do thermostat wire colors mean?

As discussed earlier, each colored wire on your Nest thermostat has a specific function and identity. This color coding is for your benefit, such that it would help you to differentiate each wire easily and thus utilize them efficiently when needed.

Furthermore, color coding may help you effectively when you’re repairing your old thermostat or when you’re installing a new one. This may prevent any damage or wrong connection of the wires.

Below is the list of Nest thermostat’s colored wires along with their function:

  • White:The white wire works as a primary wire that relays heat.
  • Orange:The orange wire is a cooling relay in the Nest thermostat.
  • Red: The red wire transmits 24-volt power to the heating and cooling mechanisms.
  • Brown: The brown wire works in emergencies to relay heat.
  • Black: The black wire regulates the secondary heating systems by providing additional heat, thus controlling their temperature.
  • Green: The green wire regulates the heating fan and the cooling mechanism.
  • Blue:The blue wire increases the cold temperature by completing the first cooling stage to the second one.
  • Yellow: The yellow wire works for the cooling mechanism by activating the process of releasing cool air. Thus, it’s used in the first stage of the cooling apparatus.

What happens when you wire the thermostat incorrectly?

An incorrect connection to your Nest thermostat is quite dangerous. It may affect your system to a great extent and may also cause trouble for you.

The wrong wiring may cause an electric shock, which is dangerous for you and your surroundings. Moreover, the circuit may even blow up or can damage permanently. In any case, calling the technician for immediate help is crucial; otherwise, you will lose your thermostat if no urgent fixation has been done.

Frequently Ask Questions

Can a thermostat wire be brown?

Yes, the brown wire on the thermostat is most likely the heat pump wire. It is connected to the O/B connector of your thermostat. However, if you have a blue wire as the B wire, it should e connected to the C connector.


The color coding of your Nest wire is crucial as it may help you identify its function efficiently when needed. Moreover, it may also aid you while fixing the old thermostat or activating the new one.

The brown wire is the heat pump wire which activates the emergency heat pump when the actual one fails to function or burns down. Dig into this blog to quickly review the functions of all the important wires on the Nest thermostat, including the brown one.

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