Thermostat Wiring 101: How to Wire a Thermostat for Your Home | Fixr (2024)

Removing and wiring your thermostat is a simple DIY job for any homeowner and an essential skill to possess. If you’re having problems with your thermostat and need to replace it or want to upgrade to a smart thermostat, knowing how to wire it is essential. There are many different types and brands of thermostats available depending on your HVAC system.

Let’s identify your type of thermostat, the different types of wires, and what each wire controls.

Types of thermostats

Low voltage (24v)

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Image source: Grainger

The most common type of thermostat is low voltage (24-volt), which is used with furnaces, boilers, or heat pumps. Low voltage thermostats are more popular because they can reduce energy consumption, saving you money on your electricity bills in the long run.

Different types of low voltage thermostats

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Typically, low voltage thermostats control both heating and air conditioning, affecting multiple rooms in your home. Some mechanical thermostats, most digital or programmable thermostats from brands like Honeywell, and smart thermostats such as the Nest are low voltage, typically featuring between 2 and 9 different colored wires.

Line voltage (120v/240v)

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Image source: JBJ Supply

Line voltage (120v or 240v) thermostats are typically used with electric heaters like baseboards and wall vents that can be seen in older homes, condos, and city apartments. Typically, line voltage thermostats feature 2-4 thicker wires (single pole or double pole) which control the heat only. Four wire (double-pole) thermostats can be shut off whereas two-wire (single pole) thermostats can only turn to their lowest setting.

If you have a line voltage thermostat and want to upgrade to something modern like a smart thermostat, you may have to purchase an adapter or look for a Wi-Fi thermostat, which offers compatibility with most electric heat and baseboard heating systems.

Types of wiring

The most common types of 24v thermostats feature 2 to 5 wires, but there could be up to 9 wires. Here’s what each wire system represents depending on which type of thermostat configuration you have.

Two wires

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Most two-wire systems are primarily used for line voltage thermostats that provide heating only and feature one red and one white wire. Red wires or R-wires, labeled either as “R”, “Rc” or “Rh” on the terminals are for their power source and white wires or W-wires are for heating. If your thermostat is for air conditioning only, you'll have an Rc wire. But if you have both heating and cooling, you'll have both Rc and Rh wires.

Three wires

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Three-wire systems are most common for heaters, boilers, and hot water heaters. These systems include one red wire, one white wire, and one green wire. These green or “G” wires in a three-wire system are used as the common wire.

Four wires

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For most battery-powered thermostats, you’ll typically see a system with four wires. These consist of red, white, green, and blue wire. While the red and white wires control power and heating, the green wire controls fans, and the blue “C” or “B” are either your common wire or for your heat pump.

Five wires

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The most common wiring system usually features five wires. These include red, white, blue, green, and yellow wire. The yellow Y-wire is typically for your cooling system and the blue, or sometimes black, wire is the common wire. Your Y wires may connect to a Y1 or Y2 terminal if you have two compressors.

Common "C" wires

Common “C” wires are connected to the transformer and provide a continuous flow of power to the thermostat. These wires are typically either blue or black and connect into the “C” terminal. They can be hard to find, so make sure to look either inside the furnace or inside the wall. If you have a line voltage thermostat, you likely won't have a common wire.

Thermostat wiring color codes

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How to replace your thermostat

Remove the faceplate

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Image source: Home Depot

First things first, make sure to shut off the power on your fuse box for the thermostat, air conditioner, and furnace. Then, carefully remove the faceplate of your thermostat and take a photo so you have an easy-to-follow wiring diagram.

Look for the common wire

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Next, look for the “C” or common wire, which provides continuous power to your thermostat. Technically, the red wire provides power but not continuously on its own. Sometimes common wires are tucked behind the thermostat.

Disconnect the wires

Once you’ve located all the wires, taken a photo, and labeled where everything goes, it’s time to disconnect the wires and the mounting screws. Make sure to tape the wires down so they don’t fall inside the wall. Remove any anchor screws and the backplate of the existing thermostat.

Install a new thermostat

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After removing the old thermostat, it’s time to screw in the new one. Place the new thermostat control board against the wall, attach the backplate with the mounting screws, and reconnect all of the new wires into their corresponding terminals.

When everything’s reconnected and mounted to the wall, go back to the fuse box and turn everything back on. Give your thermostat a few minutes to activate and re-check your wire connections or hit the reset button if your heating and cooling system doesn’t kick in.

Thermostat wiring: in summary

Whether you’re having issues with your thermostat or want to upgrade to a newer one, knowing how to wire a thermostat is an easy and essential home improvement task. As long as you keep track of what type of thermostat you need and the wires are connected to their respective terminals, you’ll be in good shape.

Hire a local pro to install your thermostat
Thermostat Wiring 101: How to Wire a Thermostat for Your Home | Fixr (2024)

FAQs

Where does white wire go on a thermostat? ›

The white wire on your thermostat connects to your heating system. It connects your thermostat to your air handler or furnace. The white wire connects to terminal W in most industry-standard thermostats. If your HVAC system has multiple heating stages, then you may find more than one white wire.

Where does the brown wire go on a thermostat? ›

A brown wire is mostly likely a heat pump wire and should be connected to the O/B connector on the Nest thermostat. If the B wire is blue, it should most likely be connected to the C connector.

Where does the red wire go when installing a thermostat? ›

7-Wire Thermostat

Red, R wire: This wire is responsible for delivering power to your heating and cooling system. Attach it to the R terminal. Green, G wire: This one connects to your HVAC fan. White, W wire: It is a heating wire and connects to the W or W1 terminal.

Where does blue wire go on a thermostat? ›

Attach the white wire to the W terminal (Heat). Attach the yellow wire to the Y terminal (AC). Attach the blue or black wire to the C terminal (Common wire) Not all systems use a common. Some systems are heat only and do not have AC and some systems use more wires like heat pumps with emergency heat.

Where does the orange wire go on a thermostat? ›

The orange wire runs from the thermostat to the heat pump. If your home doesn't have a heat pump, the thermostat either won't contain this wire color or may have it but won't use it.

How many wires are connected to a thermostat? ›

The number of wires needed for a thermostat depends on the type of system you have and the features of the thermostat you are installing. A basic thermostat that would be used with an HVAC typically requires 4 wires (power, ground, heating, and cooling), but a more advanced thermostat may require additional wires.

What happens if you wire thermostat wrong? ›

Wiring a thermostat requires you have electrical knowledge to do it accurately. There's a risk of electric shock if you don't know what you're doing with the wires, and you might also end up causing electrical damage to the circuit breaker panel.

Where does the white wire go? ›

The white wire is the "neutral" wire, it takes any unused electricity and current and sends it back to the breaker panel. The green wire (or it can sometimes be uncolored) wire is the "ground" wire, it will take electricity back to the breaker panel, then outside to a rod that's buried in the ground.

Does R go to RC or RH? ›

If you have two wires, R or RH go into the R terminal and RC into the RC terminal. If you have more than one wire (for example, you have a wire labeled R and another wire labeled Rc), remove any jumpers between the R and Rc terminals or push the switch to open the RC terminal to insert a wire.

What color does the brown wire connect to? ›

Brown is your hot wire so you want to connect that to your building's black wire. The blue is negative or return, so that will go to white. Green with yellow stripe is the ground and will go to the building green.

What is the common wire on a thermostat? ›

A C-wire, or a common wire, runs from your low-voltage heating system (24v) and carries continuous power to your thermostat. In today's electric market, most newer heating and cooling systems have C-wires, which guarantees compatibility for the installation of all smart thermostats.

What are the 5 thermostat wires? ›

This configuration includes a 24-volt red wire for power, a white wire for the heating elements, a yellow wire for the AC unit, a green wire for the fan component, and a common wire which is most likely blue but could be a different color.

What color is the C wire? ›

Blue Or C Wire

The blue wire—also called the C, or “common,” wire—is required to power any thermostat and ensure proper system function. Your HVAC system and thermostat configuration will determine what color the C wire will be. The C wire in homes with a heat pump is always blue.

What is W1 and W2 on a thermostat? ›

W/W1 and W2. In a conventional system, W/W1 controls your heating system and, where applicable, W2 controls your heating system's second stage, which helps warm the home faster. If you have a heat pump system with auxiliary heating, you'll normally connect the AUX/AUX1 or W2 wire to the ecobee's W1 terminal.

What are the four wires on a thermostat? ›

A 4-wire thermostat system typically consists of four wires: red (power), white (heat), yellow (cool), and green (fan). These wires allow for basic heating and cooling functions and are commonly found in older HVAC systems or simpler setups.

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