A Full Guide To HVAC Thermostat Wiring (With Diagrams) (2024)

A Full Guide To HVAC Thermostat Wiring (With Diagrams) (1)

Written By

Josh Mitchell

A Full Guide To HVAC Thermostat Wiring (With Diagrams) (2)

Expert Reviewed By

Dean Zoet

Last Updated On

July 3, 2024

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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

  • 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.

How Do Thermostats Work?

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]

TL;DR: Thermostats sense the ambient temperature and then send electrical signals to the HVAC appliance to either turn on or off on the preset temperature of your choice.

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.[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.

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.

TL;DR: Thermostats sense the ambient temperature and then send electrical signals to the HVAC appliance to either turn on or off based upon the preset temperature of your choice.

How Many Wires Are Used in a Thermostat?

A Full Guide To HVAC Thermostat Wiring (With Diagrams) (3)

  • 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.

Essentially, the more advanced the functionality of your HVAC system, the more wires your thermostat will have.

TL;DR: Thermostats can utilize anywhere between 2-18 wires.

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.

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 & 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.

TL;DR: R wire powers the thermostat.

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.

TL;DR: G wire controls the fan

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.

TL;DR: Y wire controls cooling 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.

TL;DR: Y2 wire controls cooling system with two state cooling

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.

TL;DR: W wire controls heating system. AUX controls secondary heating system.

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.

TL;DR: W2 wire controls second state heating . AUX2 controls secondary second-stage heating system.

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.

TL;DR: O/B controls switching between heating and cooling mode (often found in heat pumps).

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.

TL;DR: C wire closes the circuit.

Not So Common Thermostat Wires

  • 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 with three 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.

TL;DR: More complicated and industrial grade thermostats can have additional wirings.

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:

  • 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.

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:
  • Basic heating systems (furnaces) with no fan or cooling that can only be signaled to turn on and off.

Wires:
  • R wire (power) connected to the R power terminal to power the thermostat.
  • W wire (heat) connected to the heating control to signal the heating to switch on and off.

TL;DR: These are the simplest thermostats out there. They offer the most basic temperature control for heating systems.

3 Wire

Commonly used in:
  • Heating-only systems with fan control.

Wires:
  • R wire (power) connected to the R power terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the fan control to operate a blower in your HVAC system.

The addition of the g-wire connection allows independent control of the fan system in this slightly more complicated configuration.

TL;DR: 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:
  • Systems that provide heating and cooling
  • Central heating, heat pumps, or air conditioning

Wires:
  • R wire (power) connected to the R terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the G terminal fan control to operate a blower in your HVAC system.
  • Y wire (cooling) connected to the compressor/refrigerant system.

The addition of the cooling wire allows it to activate the refrigeration equipment so we can now heat and cool a room.

TL;DR: 4 Wire thermostat is the least require 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:
  • A wide range of HVAC units, heat pumps, and furnaces, including those with a new smart thermostat (these need five wires for continuous thermostat control).

Wires:
  • R wire (power) connected to the R terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the fan control to operate a blower in your HVAC system.
  • Y wire (cooling) connected to the compressor/refrigerant system.
  • C wire (common) wire to complete the circuit and keep power flowing.

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.

TL;DR: These are often used for modern smart devices.

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:
  • Two staged HVAC systems or reversing valve systems that can switch between heating and cooling.

Wires:
  • R or RC wire (power) connected to the R power terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the fan control to operate a blower in your HVAC system.
  • Y1 wire (cooling) connected to the Y terminal compressor/refrigerant system.
  • C wire (common) wire to complete the circuit and keep power flowing.
  • Y2 wire (stage 2 cooling) connected to the 2nd stage cooling system.

OR

  • O/B wire (orange/blue) connected to the reversing valve to switch between heating and cooling.

The 6th wire will depend on your HVAC setup and whether you have a reversing valve system:

  • A Y2 wire allows the HVAC system to provide second-stage cooling.
  • An O/B wire allows reverse valve heating or reverse valve cooling to change the direction of refrigerant, allowing for more efficient heating and cooling.

TL;DR: These 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:
  • Heat pumps and HVAC systems with auxiliary heating or emergency power supplies.

Wires:
  • R or RC or RH wire (power) connected to the R power terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the fan control to operate a blower in your HVAC system.
  • Y1 wire (cooling mode) connected to the compressor/refrigerant system.
  • C wire (common) wire connected to the C terminal to complete the control board circuit and keep power flowing.
  • O/B wire (orange/blue) connected to the reversing valve to switch between heating and cooling.
  • W2, AUX2, E (second stage heating mode) connected to the secondary or emergency heater.

The additional W2 wire offers higher-capacity heating controlled through the electronic thermostat.

TL;DR: These are installed for complicated systems that involve emergency or auxiliary heat.

8 Wire

This thermostat configuration is more complicated, and the exact wiring can vary.

Commonly used in:
  • More advanced heating and cooling systems that run continuously to maintain the temperature.

Wires:
  • R wire (power) connected to the R power terminal to power the thermostat.
  • W wire (heat), connected to the heating control.
  • G wire (fan) connected to the fan control to operate a blower in your HVAC system.
  • Y1 wire (cooling) connected to the compressor/refrigerant system.
  • Y2 wire (second stage cooling) connected to the 2nd stage cooling system.
  • C wire (common) wire to complete the circuit and keep power flowing.
  • O/B wire (orange/blue) connected to the reversing valve to switch between heating and cooling.
  • W2, AUX2, E (second stage heating) connected to the secondary or emergency heater.

The addition of the second-stage heating transformer and cooling transformer control allows the temperature to be more easily maintained by the digital thermostat.

TL;DR: These are installed for complicated systems that involve second stage heating or cooling.

ACLAB Note:

The yellow and common wire can be connected to a condensate pump safety switch if the condensate from your system needs to be pumped outside. This is usually wired in series with your cooling system, so that the AC will not turn on if the condensate pump fails.[3]

People Also Ask (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.[4]

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.[5]

A Full Guide To HVAC Thermostat Wiring (With Diagrams) (2024)
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