A Full Guide To HVAC Thermostat Wiring (With Diagrams) (2025)

• Depending upon how complex your HVAC system is, the number of thermostat wiring can differ.

• You can have 2 Wire thermostat that that only control heating all the way to 8-wire and beyond that control, heating, cooling, fan, reversing valve, emergency heat, second stage or even third stage heating or cooling etc.

• Most thermostats can easily be installed yourself. However, if in doubt, you should always consult with an HVAC consultant.

Thermostats are the component that makes your heating or cooling system work. They help control cooling appliances (ACs & 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.[1]

Related Article: How Cold Should the Air From The AC Be?

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.[2]

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.

Thermostats typically have between 4 and 8 wires.

However, the exact thermostat wiring depends on the functionality of the HVAC system.

• Basic thermostats use 2 or 3: R, W, G. These control the power, heat and fan.
• Standard thermostats use 4 or 5 wires: R, W, G, Y, and sometimes C. These control the power, heat, fan, and cooling.
• More advanced thermostats use 6-8 wires. The extra wires control additional HVAC system features and settings, signaling a two-stage cooling or heat pump system.
• High-end HVAC system thermostats can have 12-18 wires, often including a HUM wire to control humidity, DEHUM for dehumidifying, and separated RH and RC wires to power the heating and cooling subsystems.

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.

The key to thermostat wiring is understanding the thermostat wire color codes.

Here’s a breakdown of the most common wire options:

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.

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.

ACLAB Note:

If yourAC fan is not spinning, there could be an issue with this 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.

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.

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.

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.

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.

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.

• L wire
  The L wire is a system monitor wire commonly used in an old thermostat to activate indicator lights when there’s an error. These are less common in modern thermostats because LCD error displays have rendered them unnecessary. The L wire is usually black.
• S, S1, or S2 Wires
  The S, S1, and S2 are used in more complex heating and air conditioning systems withthree or more stages to activate multiple heating and cooling mechanisms. These thermostat wires are usually green.
• G2, G3, GL, GM, or GH
  These thermostat wires control multiple fan speeds in more advanced heating and air conditioning systems with a smart thermostat. Every fan wire is usually green - which can get confusing.
• Y3 Wire
  The Y3 wire is used to activate a third-stage cooling system in your air conditioning system. It’s typically a yellow or brown wire.
• W3 Wire
  The W3 wire activates the 3rd stage heat mode, giving extra heat if needed. It’s most common in advanced heating systems with multiple settings, and the wire is usually brown or red.
• X & B wires
  X&B wires work the same way as O/B wires in reverse valve cooling, switching from cooling to heating or vice versa. However, X&B wires also switch power between heating/cooling mechanisms.

  These wires are brown, orange, or blue and are only used with transformer heat pumps.

• H or D wires
  HUMID or DEHUMID wires (sometimes called H or D wires) send signals from a humidity sensor to your HVAC system to turn on the dehumidify or humidify mode. These two wires are gray and only used in HVAC systems that control temperature and air quality.

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:

• 2, 3, and 4-wire systems are for conventional single-stage heating or cooling systems
• 5-wire systems allow you to control cooling air conditioning systems
• 6 and 7 wire systems are for two-stage heating and cooling systems
• 8-wire systems are for complete multi-stage heating and cooling control
• 9+ wiring systems are for complex HVAC systems with a range of settings.