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Thermostat Wire Colors Explained

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

Understanding the wiring of your thermostat can be difficult, the wires can be red, green, white, blue, black, green, orange, or sometimes something more obscure.

Most of these wires have different functions and roles in controlling this important part of your house, but sometimes the manufacturers make it difficult to know which wire does what and how to work it out. 

An important thing to consider before reading this guide, however, none of these answers to wire color meanings are guaranteed to be correct and there is very little standard protocol on a certain wire having to have a certain meaning. 

So while this guide covers what it is likely these wire’s colors mean, it is always preferable to have a guide from the manufacturer to have the proper definitions available.

You can get a definite answer by checking the wiring in an HVAC system if you have access to yours as it will give you the exact answer you are looking for.

While this guide may give you the answers you are looking for do not attempt to work on or change any of the wiring in your thermostat if you are even partially uncomfortable with the work or feel qualified or competent to do so.

If you go in not confident in your abilities it is likely that you can cause more damage than good and may even require the help of a professional to fix your mess. 

And when working on your thermostat you should isolate your power supply to the HVAC system and the wiring through your circuit breaker.

The Different Types Of Wiring In Your Thermostat

The wiring that is present in your thermostat will rely on variables like the style of HVAC system you have, how many devices or appliances are connected to it, as well as the era in which your thermostat was wired. 

In the modern era, most thermostats will have a normal 4 or maybe 5 wire templates with the main stand-out being whether a C-wire is there.

There are older thermostat setups that may have just 2 or 3 wires in the system which are usually just connected to a single simple boiler, this is compared to modern systems that are usually connected to devices like air conditioners and heat pumps.

So what is it likely that the color of your wires means?

Red Wires In Your Thermostat

Terminal Name – R, Rh, and RC

What Is It Usually Used For? – Power

The red wire in a thermostat will usually provide power (usually 24V) from your transformer.

Green Wires In Your Thermostat

Terminal Name – G

What Is It Usually Used For? – Controlling the fan

The green wire will usually control your blower fan that is in the air ducts by turning it either on or off.

Yellow And Light Blue Wires In Your Thermostat

Terminal Name – Y and Y2

What Is It Usually Used For? – Cooling

These wires are usually used for AC relay to control your air conditioner.

Black Wires In Your Thermostat

What Is It Usually Used For? – No standard function

A black wire in your thermostat does not have a standard function and to know what it means it is best to refer to your HVAC system.

White And Brown Wires In Your Thermostat

Terminal Name – W and W2

What Is It Usually Used For? – Heating

White and brown wires in your thermostat are usually connected to your heater or your heat pumps that control it.

Orange And Dark Blue Wires In Your Thermostat

Terminal Name – O and B

Orange or dark blue wires are usually used to control the heat pump reversing valve.

The Occasional Use Of Blue and/or Black Wires

Terminal Name – C

What Is It Usually Used For? – Complete Power Circuit

This use which is usually used on a black or blue wire is not always easily available but has control over the complete power circuit.


Now you have a general overview of what each wire type does, let’s go into more detail, so we can get a more in-depth understanding of what each wire type achieves within your thermostat.

Details On What The Wire Color Of Your Thermostats Mean

Red Thermostat Wire (R, Rh, Rc)

This wire is almost universal for all thermostats and is what is used to power an HVAC system.

Depending on the country you are in this wire will connect a 220V/110V system directly to your heating device connecting it to your power source.

The device is usually used to connect to a furnace and will usually be connected by a transformer at one end. 

Thermostat Wire Colors Explained (1)

If you have a modern thermostat, you will likely have 2 red wires that are marked as Rh (used for heating) and Rc (used for cooling).

These wires work separately to power different systems meant for heating and cooling respectively using a different transformer.

If you are using a system that has a transformer shared by heating and cooling, a jumper is used to separate them.

Yellow And Light Blue Thermostat Wires (Y, Y2)

If your thermostat uses yellow wires they will come from the Y terminal into the compressor relay that controls your air conditioning.

The yellow wire color is usually dedicated to a cooling system that comprises a condenser, compressor, as well as an air handler.

The compressor as well as the condenser will be left outdoors, in contrast to this the air-handler will be placed indoors.

Your yellow wire will go from your thermostat into a splitter that is placed near (sometimes inside) your air handler.

There will be a single wire going from your splitter to your condenser then onto your air-handler.

Some people have a dual-stage type cooling system which is when the Y2 terminal is in use as well.

Most systems tend to have a single compressor but with these systems there are 2; therefore the presence of a Y2 terminal.

A Y2 terminal is usually connected with a light blue wire into the second compressor.

So generally speaking, there is a yellow wire usually intended for cooling and if you have two compressors there will be a light blue wire present as well.

White And Brown Thermostat Wires (W, W2)

If you have a white wire, it will usually be located behind the thermostat, and it is generally used to control your heating section on your HVAC.

This wire will be connected from your W terminal as well as your heat source, a boiler, or a furnace.

A lot of houses use a dual-stage type heating system that is much more efficient than not using one and is now common to have as part of a more modern HVAC.

If this is what you have, the thermostat will control both a low fire and a high fire setting.

If you use this type of two-stage type heater, you will tend to use a brown wire that would connect to a W2 terminal located on the dual-stage switch which is on the boiler or the furnace.

Heat pumps will usually utilize staging to have more heating (often called auxiliary heating).

If this is what you have, your heat pump will be connected to your W2 terminal using a brown wire.

Green Thermostat Wires (G)

If your thermostat uses a green wire this will usually be connected to a blower fan relay, this is used to control your airflow through your ducts.

Sometimes there will be a split type system, in this case, the fan will be indoor in your air handler, while inside a package unit, your fan will be in an outdoor unit.

These fans will blow air back against where it is naturally tending to go, a good example of this is the warmth inside which is achieved during winter as naturally the airflow should be going from warm temperatures to cold.

Your fan will be using electrical power to go against this to keep warm air flowing inside your house.

The reverse of this is true for cooling during warmer weather. Over time, your heating or your cooling appliances will work less making a fan have to work harder.

Common Thermostat Wires (Usually Black) (C)

This wire has become a more common topic in the past 5 years and is labelled the C wire and is usually black.

Most modern systems run on 24V and have a C wire that is utilized to complete your power circuit between for your thermostat if it uses electricity.

Most of these modern thermostats will need electricity to remain operational, meaning that a C wire is definitely a necessity.

There are some thermostats powered by batteries that will not necessitate a black C wire.

However, if you need a C wire, you can also install a power kit for more control over it.

If you do not have a C wire but need it for operation you will need to buy an extension kit to connect it to your power supply.

Orange And Dark Blue Thermostat Wires (O, B)

Orange and dark blue wires are most commonly used for controlling your heat pump, more specifically your reversing valve used on the heat pump.

The color used for this will depend entirely on the manufacturer and only one or the other will be used.

E Terminal Thermostat Wires

The E terminal will need wires to power its function of powering back up emergency heating through the heat pump.

These wires tend to not have a standard wire color. These are used in case your primary heating system fails with a lot of heat pumps featuring a lesser emergency heating setting for this occasion.

This wire will come from your heat pump.

What Type Of Wire Is Best To Use For A Thermostat?

For this purpose both 18 and 20 gauge wires are used, and the name labels the thickness for the wire.

Generally speaking both will work fine but try to use a thicker wire for shorter distances and the opposite for longer.


Make sure to always check your HVAC system to make sure the wire definitions match, but these wire coloring systems are the most commonly used.

Always shut off power before working on wires and make sure to not leave an open job and not leave wire work half-finished.

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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