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The thermostat is the brain of your HVAC system, and if it's not wired correctly, your HVAC system will not be effective.
Thankfully, wiring your thermostat is much easier than brain surgery - you just need to have the thermostat wiring explained to you.
I have worked with every type of thermostat, from single-stage heating and cooling to advanced whole-home HVAC systems.
In this guide, I'll show you everything you need to know to get your thermostat wired in no time.
Key takeaways
How do thermostats work?
Thermostats are the component that makes your heating or cooling system work. They help control cooling appliances (ACs and fans), heating appliances, and HVAC systems.
Thermostats use a sensor to measure the temperature of a room.
If the temperature drops below or above the preset temperature, the thermostat processor determines if heating or cooling is required.
The thermostat uses the wiring to send an electrical signal to the appliance, activating the heating or cooling component.
Once the temperature adjusts and reaches the preset level again, the thermostat sends a signal telling the heating or cooling component to shut off.
By controlling the activation of your heating and cooling, the thermostat and the thermostat wiring maintain the set temperature in your home while lowering energy consumption.
Related article: How cold should the air from the AC be?
ACLAB Note:
If you are in the market for a new model, then check our recommendations for the best thermostats.
How does thermostat wiring work: Explained for beginners
Thermostats control heating and cooling systems through a set of electrical wires.
These carry low-voltage electrical power signaling for the heating and cooling system to turn on and off.
Typically, thermostats use small, low-voltage wires because they don’t need to carry a lot of power.
They’ll usually be 18-22 gauge, though they can vary slightly with different heating and cooling systems.
The wires are color-coded for their function (red carries the power, white carries the heating control signal, etc.).
A signal is sent through the corresponding wire when the thermostat determines that heating or cooling is needed.
Having separate wires allows the thermostat to control a fan, cooling, or heat pump system independently, allowing the HVAC system to function as needed.
To function properly, the thermostat wiring must be connected to the corresponding terminal (e.g., the red wire connects to the red terminal).
This allows the proper signal to be sent to the right place so the appliance takes the appropriate action.
Thermostat wiring isn’t particularly complicated or dangerous, and it’s one of the only electrical jobs I recommend as a DIY activity.
Just make sure you turn the power off and take your time.
Important Note
While thermostat wiring is often straightforward, if you are confused or if you do not have sufficient technical skills, its best to call an HVAC professional for proper and safe installation.
How many wires are used in a thermostat?
Thermostats typically have between 4 and 8 wires.
However, the exact thermostat wiring depends on the functionality of the HVAC system.
Essentially, the more advanced the functionality of your HVAC system, the more wires your thermostat will have.
Difference between AC, furnace, and heat pump
ACs, furnaces, and heat pumps have different functions and thermostat wiring configurations.
Furnaces are the simplest system, providing simple heat that's usually just turned on and off.
ACs operate on the same principle (but with cooling), though they often have a fan.
Heat pump systems are slightly more complicated because they offer heating and cooling, with more wires to control individual functions.
Important Note
You can find heat pump featured in other types of ACs too including portable ACs, Window ACs and also Mini Splits. Mini split heat pumps are particularly popular for single room heating/cooling.
The table below summarizes the differences:
System | Typical number of thermostat wires | Common wire options |
|---|---|---|
Furnace (heat only) | 2 | R, W |
2-stage Furnace (heating only with adjustable heat level) | 5-6 | R, W1, W2, G, C |
Heat and Air Conditioner (cool only) | 4 | R, Y, G, W |
2-stage Air Conditioner (cooling only with adjustable cooling level) | 5 | R, Y1, Y2, G, C |
Heat Pump (heating and cooling) | 5-6 | R, Y, G, O/B, C |
2-stage Heat Pump (heating and cooling with adjustable levels) | 6-8 | R, Y1, Y2, G, O/B, E, C |
Understanding thermostat wiring labels and guide to color codes
The key to thermostat wiring is understanding the thermostat wire color codes.
Here’s a breakdown of the most common wire options:
R, RC, or RH: Red
The red wire (R wire) is your power wire that connects to the transformer in your heat pump or AC to power the thermostat.
Every type of thermostat has a red wire.
Some heat pumps or HVAC systems have separate transformers powering the heating and cooling systems.
If that’s the case, the thermostat will have RH wires (heating power) and RC (cooling power) wires that connect to the appropriate power source.
G Wire: Green
The green wire (G wire) controls the fan, allowing the thermostat to signal the heating, cooling, or air circulation fan to switch on as needed.
This wire is part of a common configuration for heat pumps and HVAC systems.
Y or Y1: Yellow thermostat wire
The yellow wire (Y wire) controls the cooling system, allowing the thermostat to signal your refrigerant compressor system and start lowering the temperature in your home.
It usually works in conjunction with the green wire fan system.
Y2: Yellow / light blue
The Y2 wire controls the second-stage compressor and activates if you need extra cooling.
It is usually a light blue wire, but it can be yellow in some HVAC systems.
W, W1, AUX, or AUX1: White/other
The W or W1 wire controls the primary heating mechanism, and the AUX or AUX 1 controls the auxiliary or secondary heating system commonly used in zoned heating systems or dual fuel setups.
This is usually a white wire, but it can also be a red or brown wire.
W2, AUX2, E, E/AUX, E/EM: White/brown/other
The W2 thermostat wire controls the second stage heating system, and the AUX 2 controls the secondary stage of the auxiliary heating system (or emergency heat system).
Only multi-stage heating systems will have these two wires, and they are either blue wire or white wire.
O, B, or O/B wire: Orange/blue
The O/B wire connected to the thermostat is used in heat pumps to switch between heating and cooling modes.
They are used in HVAC systems with a reversing valve, allowing it to switch between heating and cooling.
The O wire is usually an orange wire for heating, and the B wire is usually a dark blue wire for cooling.
C-wire: Black/blue/other
The C wire completes the circuit and lets power flow back into the appliance transformer, providing continuous electricity to the thermostat so it can run continuously.
The C wire is typically black, blue, red, or white.
Not so common thermostat wires
Important Note
In most cases, you wouldn't have to worry about these wires as they are intended for industrial grade systems.
With this, you can attach a whole house dehumidifier or whole house humidifier to your HVAC system and control it via the thermostat.
Different types of thermostat wiring and their diagrams
The wiring configuration you use for your thermostat will depend on the type of HVAC system you’re using.
Generally, the more complex the system, the more wires you’ll have to connect to the corresponding terminals:
Useful Tips:
If you have an existing thermostat and are replacing all the wires, you can usually just put the new wires in the same place as the old wires. However, if you have a new thermostat, you will need to get the color coding right.
Below is a guide to the most common configurations with a wiring diagram for each, but I recommend following the manufacturer’s instructions to get the color coding right:
2 Wire
Two wires (red and white wires) are the minimum that every thermostat and heating/air conditioning system needs, but many use more.
Commonly used in:
Wires:
ACLAB Note:
These are the simplest thermostats out there. They offer the most basic temperature control for heating systems.
3 Wire
Commonly used in:
Wires:
The addition of the g-wire connection allows independent control of the fan system in this slightly more complicated configuration.
ACLAB Note:
These are also used for heating systems but have an additional wire for fan control.
4 Wire
Configurations with four wires offer a step up because they can activate heating or cooling components.
Commonly used in:
Wires:
The addition of the cooling wire allows it to activate the refrigeration equipment so we can now heat and cool a room.
ACLAB Note:
4 Wire thermostat is the least required for cooling systems. They can also be used for heating systems.
5 Wire
A configuration with five wires is the most commonly used in new thermostat models.
Commonly used in:
Wires:
ACLAB Note:
The additional C wire in a 5-wire thermostat is essential for a smart thermostat/Wi-Fi thermostat and lets the HVAC system run continuously.
6 Wire
A 6-wire thermostat is a more advanced configuration and allows smart thermostats to issue a wider variety of signals.
Commonly used in:
Wires:
OR
The 6th wire will depend on your HVAC setup and whether you have a reversing valve system:
ACLAB Note:
The 6-wire can control reversing valve to switch between cooling and heating.
7 Wire
7-wire systems usually have a reversing valve and can be more complicated. I recommend you thoroughly read the thermostat manual to get the correct wiring.
Commonly used in:
Wires:
ACLAB Note:
The additional W2 wire offers higher-capacity heating controlled through the electronic thermostat.
8 Wire
This thermostat configuration is more complicated, and the exact wiring can vary.
Commonly used in:
Wires:
The addition of the second-stage heating transformer and cooling transformer control allows the temperature to be more easily maintained by the digital thermostat.
FAQs
What is C wire for thermostat?
The C wire closes the circuit and provides a continuous power supply to the thermostat so it can run without interruption.
It’s essential for a digital thermostat and enables digital displays or Wi-Fi connectivity for smart thermostats.
How many volts is a thermostat?
HVAC thermostats are low-voltage thermostats that operate on 24 volts (a very low voltage), just enough to power the thermostat’s functions and controls - meaning there’s very little risk of electric shocks.
However, a line voltage thermostat can use 120-240 volts.
Are thermostats AC or DC?
Standard thermostats receive power from a transformer that reduces the standard voltage of 120 or 220 volts down to 12 or 24 volts, which is low voltage to run the controls of the AC/Heating unit.
Line thermostats are dangerous. These thermostats use 120 or 220 volts to power the controls on the AC/Heating unit. Recommend calling an experienced HVAC technician to repair these systems.
