Thermostats come with several wires on them each of which serves a different function. These wires are colored differently to help identify what the wire is for. Among them, there is a brown wire, and sometimes this particular wire is not connected. What is this brown wire for and why is it not connected sometimes?

The brown wire is what connects a thermostat to an emergency heating system, a backup heating system used if your heat pump ever fails. If the brown wire is not connected to the thermostat, it is because your thermostat’s HVAC system model doesn’t have an emergency system connected to it. 

If you ever open your thermostat you’ll be able to see the brown wire is either connected with the rest of the colored wires or just not connected to anything but that doesn’t mean it doesn’t have a use. Read on and you can learn more about the wires connected to your thermostat. 

Why Does my Thermostat Have a Brown Wire?

A thermostat has multiple strands of wires each one of them identifiable by color and they also serve different purposes in a thermostat. When the thermostat is being installed in your home each colored wire is placed in its right place which sends data from your HVAC system to your thermostat making it able to operate correctly. And if the colored wires are not correctly connected it might cause problems in your thermostat. 

Now that’s clear, we still have a question to answer. If all the wires are connected to the right places, why is the brown wire sometimes left out? And what is it used for? 

As mentioned earlier the brown wire is what connects a thermostat to an emergency heating system, this system is usually used when your heat pump breaks down. So when it does break down you have the luxury of enjoying your emergency heating system in your home before you call a technician to fix your broken pump.

 So how come you can find the brown wire not connected sometimes? This is mainly because your home doesn’t have an extra heating pump system connected to your HVAC model.

What Do the Colored Wires in a Thermostat Mean?

Knowing the colored wires in your thermostat and their purpose is quite helpful and really advisable especially when you decide to try and fix your old thermostat or maybe even want to install a new one. This is because knowing the thermostat’s wire color codes will help you prevent any DIY wiring problems.

Below are the thermostat’s colored wires and their meaning:

  • Red wire- This wire transmits 24-volt power to both the cooling and heating mechanisms.
  • White- This is the primary wire used to relay heat.
  • Orange- This is used as a cooling relay in the thermostat.
  • Brown- Used for emergencies to relay heat 
  • Yellow- this is part of the cooling apparatus and is used for cooling in the first stage by activating the process of releasing cool air. 
  • Green- this wire regulates the fan in the thermostat during heating or cooling. 
  • Blue- this completes the cooling stage from the first stage to the second stage increasing the cold temperature.
  • Black- this is where the secondary heating mechanisms are regulated by the black wire by providing additional heat as well as temperature control. 

What Happens if you Wire a Thermostat Wrong? 

If your thermostat’s wires have not been correctly connected then they would cause an electric shock, the circuit might even blow or the thermostat circuit might be damaged. If this happens call a technician to fix it. 


There are a variety of wires found in the thermostat each with its own functions including the brown wire even if it is sometimes not found connected like the rest of the wires. And the reason for that is because the brown wire can only be really effective in your home only when you have an updated HVAC system that has an emergency heating system in case something happens to your heating pump. 

And so when you find your brown wire not connected, that usually means that your HVAC system model doesn’t have a backup to it. 


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