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Thermostat Wiring Colors & Terminals Explained
The wiring of your thermostat appears complicated, but when you understand the basic structure of it, including the terminals and thermostat wiring colors, it’s much less overwhelming of a task to install your own system.
The HVAC wiring is coded based on the action it performs while where the wire goes determines the terminal. Keep in mind, if you’re not sure how to wire a thermostat, your system could run improperly – or even worse.
Warnings and Special Considerations
Make sure you follow all safety instructions and heed all warnings when installing your HVAC wiring, whether you are installing heat pump thermostat wiring or any other HVAC equipment wiring.
First and most importantly, if you have any doubts about installing electrical equipment on your own, hire a qualified technician to install your thermostat. Improperly installing your equipment can cause serious complications. For instance, you could cause a system failure, damage to your devices or a blown transformer or a fire.
Important!
Before you begin connecting your thermostat wires, always shut off the power to your equipment. The voltage of the electric can harm you if the power is not completely off prior to the installation process.
Always double check to ensure all the power is off, and there isn’t any voltage. You may use a volt-ohm-milliammeter to do so.
Prior to getting involved in this project, you should always read the thermostat manufacturer’s instructions. You want to have a thorough understanding of the thermostat before you begin working.
No Standard Exists
You have to understand that wire colors are not standardized. No rules are implemented that require all HVAC equipment to follow the same color coding method. However, certain wiring methods are used most frequently.
Below is a guide to the most common method, but your system may not follow the same code.
Good To Know
Keep in mind, the person who installed your thermostat may not have followed the same procedure, so your system might not match what’s going to be explained in this article.
Tips for Changing a Thermostat
Before you uninstall the old thermostat, take a picture of its wiring to act like a thermostat wiring diagram. You’ll have a map of exactly where the wiring was connected to the base.
You want to take a photo of the current wiring because even when you have your old device removed, you’ll still have a map of how the wires were connected to the base. You may want to mark the terminal where each wire connected.
A majority of modern smart/wifi thermostats have stickers, so you’re able to label the old thermostat wires before you remove it.
It’s highly recommended that you use at least one of the tips mentioned above before you disconnect and replace the thermostat. It gives you a general idea of where the wires go in case your model doesn’t connect the way explained in this article.
HVAC Systems
More than likely, your heating, ventilation and air conditioning (HVAC) consists of an oil, gas or electric furnace and an air conditioning unit.
Although this is the setup in a majority of homes, some homes have a heat pump system. The heat pump system setup differs from a standard unit. Its wiring is slightly more complicated because it requires more circuits to work properly. This means you’ll have more wires to connect when you change your thermostat.
Good To Know
Keep in mind, not all thermostats are compatible with heat pump systems, so make sure the one you selected will work with your heat pump.
Thermostat Wiring Color Code
Typically, a furnace plus A/C HVAC system has four or five thermostat wires and a common wire. A heat pump system, on the other hand, has eight or more different thermostat wires.
Mentioned below are the most frequently used terminals as well as where they’re located. You may use this as a reference for the purpose of each wire.
Please be aware that the home thermostat wiring mentioned below might not be the same as yours since the person who installed yours may not have followed this particular color coding.
If you’re not sure which wire represents what, try tracing the wire to the source to determine its purpose.
Photo by gpshead 
R, RH & RC Terminals
The red wire is usually the 24 Vac power wire, which begins at the transformer. It runs to the R, RH and RC terminals, depending on how the thermostat is used.
In some cases, HVAC systems consist of a transformer for cooling and another one for heating. Additionally, a system may consist of a power wire for heating and another for cooling. The RC terminal is for cooling while the RH terminal is for heating.
Good To Know
You could install a jumper in between the RH and RC terminals if your system only has one transformer.
W Terminal
The W terminal the one that triggers the heating. It connects directly to the furnace, boiler or other heating source and is generally white.
Y Terminal
The wire that connects to the Y terminal is yellow and connects to the compressor in your cooling system. It enables the thermostat to control the air conditioning unit.
G Terminal
When you’re looking at your thermostat wire colors, you’ll notice a green wire connected from the fan. This wire is responsible for triggering the blower fan.
C Terminal
The common wire links to the C terminal. This particular wire extends from the transformer and is responsible for completing the 24 V power circuit in the thermostat.
The common wire doesn’t have a standard color. It’s usually black, but in some cases, it’s the thermostat blue wire. The installer could have used any color, though.
O & B Terminals
The reversing valve in a heat pump system connects to the O & B terminals. This portion of the heat pump changes the direction of the refrigerant flow. It alters the heat pump cycle from heating to cooling and vice versa.
When taking into consideration your thermostat wiring colors, the orange wire typically connects to the O terminal while the dark blue wire connects to the B terminal.
This depends greatly on who installed it as well as who manufactured your heating pump. A majority of brands use the orange wire for the reversing valve while a small percentage use the dark blue for it. You use this terminal for second stage heat.
Certain gas furnaces have low fire and high fire gas pressure adjustments, but certain ones have a two-stage heating thermostat that utilizes a W2 terminal. Heat pumps usually require a W2 terminal and use staging for auxiliary heat.
W2 Terminal
The W2 terminal is used for second stage heat. In some instances, furnaces have two-stage heating, which requires additional heating from a primary source. A heat pump requires a W2 terminal to operate.
You’ll usually find two-stage heating in locations where people experience extremely cold winters.
A majority of the time, the HVAC wiring color is brown for this particular terminal. However, an installer may have used any color when installing the wire to the W2 terminal.
Y2 Terminal
A cooling system with two compressors or a two-stage compressor need a Y2 terminal, so the thermostat is able to control the second stage of cooling. The color of the wire varies from installer to installer, but it appears light blue is usually the go-to color for this terminal.
E Terminal
The E terminal is for emergency heating, which occurs when the system runs completely on the second stage heat source.
When your thermostat triggers the emergency heat, it’s usually because it’s too cold for the heat pump to depend solely on the first stage heating.
The price to use second stage heat alone is high. Second stage heating uses more energy, so it should only be used as an additional heat source. No standard color exists for this wire.
About the Author David Waller
