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The first thing to say is you shouldn't take the below thermostat wire color guide as gospel. While “generally speaking” each wire color has a purpose, there is nothing to say that the person who previously wired your thermostat did it correctly!

The only way to be sure is to check the wiring directly in your HVAC system. That way you'll know which color wire does exactly what in your specific circumstance.

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

While each color wire has a particular purpose, it is still recommended to check the thermostat wiring to your HVAC system to make sure it was wired correctly in the first place!

You should always isolate power to your HVAC system and thermostat wires before working on them, via the circuit breaker box.

Types of Thermostat Wiring

The type of wiring on your thermostat usually depends on factors such as the nature of your HVAC system, how many devices you have connected, and when your thermostat was wired. Most modern thermostats have the standard 4 or 5-wire template, with the only difference being whether a C-wire is present or not. However, older setups or simpler systems installed in places like cabins might use a 2 or 3 wire system. 2 or 3 wire systems usually have a single-stage boiler/heater installed. In contrast, newer systems might have air conditioners, heat pumps, etc.

RedR, Rh & RCPowerProvides power from the transformer (24V usually)
GreenGFan ControlTurns the blower fan on/off for air in the ducts
Yellow & Light BlueY & Y2CoolingAir Conditioner (AC relay specifically) 
BlackAnyDepends on your system.
White & BrownW & W2HeatingConnected to a heater or the heat pumps
Orange & Dark BlueO & BHeat pump reversing valve
Sometimes Blue or BlackCComplete Power CircuitCommon wire. Not always available

Details of Thermostat Wire Colors

Thermostat Red Wire (R, Rh, and Rc)

This wire is present in all thermostats and is used to power your HVAC system. A 220V/110V (depending on your country) system connects the power source directly to the heating device (a furnace usually). For all other systems, the wire connects to a transformer at the other end.

There will probably be two red wires in modern thermostats, marked by Rh (heating) and Rc (cooling). These wires power heating and cooling systems separately, usually through two separate transformers. However, if your system uses only one transformer to power both heating and cooling, a jumper can be installed between the two terminals.

Thermostat Yellow and Light Blue Wire (Y and Y2)

Yellow wires go from the Y-terminal to the compressor relay for your air conditioning. The yellow color is dedicated to the cooling system, which contains a compressor, condenser, and air handler. The compressor and condenser are placed outdoor, while the air-handler is placed indoors. The yellow wire goes from the thermostat to a splitter placed near or inside the air handler. Then, one wire from the splitter goes to the condenser and the other to the air handler.

If you have dual-stage cooling, then you’ll need the Y2 terminal as well. Most systems have one compressor, but in case there are two, the Y2 terminal is connected via a light blue wire to the second compressor. So to recap, yellow wire for cooling and an additional light blue wire if there are two compressors.

Thermostat White and Brown wires (W and W2)

The white wire behind your thermostat controls the heating section of your HVAC system. It is connected between the W terminal and the heat source, a furnace, or a boiler. Many houses use dual-stage heating,  which is more efficient and is quite common in modern HVAC systems.

In this case, your furnace will have low fire and high fire settings. If you have a two-stage heating system, a brown wire will be used to connect the W2 terminal to the dual-stage switch on your furnace or boiler. Heat pumps usually use staging for additional heating (auxiliary heating). In that case, the heat pump is also connected to the W2 terminal through the brown wire.

Thermostat Green wire (G)

The green wire is connected to the blower fan relay and controls the airflow through the ducts. If you have a split system, the blower fan is indoor in the air handler, while in a package unit, the blower fan is in the outdoor unit.

The fan is used to blow air against its natural tendency. For example, it is warm inside during winters, and the natural direction of airflow is from warm to cold. The fan takes up electrical energy to work against this and keeps on pumping warm air into your home. The same principle applies to cooling. So as time passes and your heating or cooling devices start reducing their workload, a blower fan actually has more work to do.

Thermostat Common wire (C) (no universal color, but usually black)

Ah, the infamous C wire! If you’ve replaced your thermostat in the past five years, there is a good chance you have heard about the common wire. There is no universal color for this wire terminal, but a black wire is often used. If your previous thermostat did not need this wire, the electrician would probably have rolled it around a wire or the wall terminal.

In most modern 24V systems, the C wire is used to complete the power circuit for the thermostat if it consumes electricity. The majority of modern thermostats require electricity to operate, so a C-wire is necessary for them.

However, there are battery-powered thermostats available that do not need a C-wire. If your thermostat needs a C-wire, there are options like installing an additional power kit. Take a look at our C-wire article for more information.

Thermostat Orange and Dark Blue Wire (O and B)

These wires are used for operating the heat pump, specifically the reversing valve on a heat pump. You’ll be using only one of these terminals, which will depend on the manufacturer of your heat pump. Some manufacturers such as Rheem and Ruud use the dark blue wire to connect B-terminal and the reversing valve. In this case, the reversing valve is used in heating mode.

Most of the other companies, such as Lennox, Ducane, Trane, Carrier, Goodman, Heil, Fedders, Amana, Janitrol, etc., use the reversing valve in cooling mode. An orange wire is used in this case to connect the O terminal with the reversing valve.

Thermostat E terminal (no color specified)

This terminal is used for the emergency heating back-up in your heat pump. In case the primary heating fails, heat pumps have a smaller emergency heating in them. There is no thermostat wire color specified, but it’ll be connected to the E terminal. It is better to check what wires originate from your heat pump to see what color the wire is.

Common Questions & Answers

What kind of wire should I use?

You’ll come across two types of wires for thermostats, 18-gauge and 20-gauge. These standards are based on the thickness of the wire. Regarding quality, the low-loss copper wire will do just fine and ensure it is not stranded. You can use the 20 gauge wire for a smaller distance (up to 200 feet). If the distance is larger, go for the 18-gauge!

If I don’t have a C-wire, can I leave it open?

If your thermostat requires electricity to operate, like most modern thermostats, you will need to connect a C-wire. If you leave it open, your system won't turn on, since the circuit won’t be complete. If you don’t have a C-wire system, there are remedies. Many manufacturers such as ecobee and Nest provide you with an extension kit that’ll let you power your thermostat externally. In addition, there are a few devices that are specifically made to bypass the C wire.

The Verdict

As discussed, it's best to check your wiring directly in your HVAC system before proceeding. Always turn the main power off before you fiddle with wires, and if you’re unsure about any aspect of the wiring, do not connect it randomly or leave it open. In this case, it is best to seek professional help.  


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