How to identify a thermostat wire

You’ll need to check your current thermostat’s wires to tell if your system is Nest compatible and to help with Nest thermostat installation.

But sometimes a thermostat’s wire connectors have two labels or no label at all. In rare cases, a wire may be in the wrong thermostat connector, which can cause your system to malfunction. 

There are several ways to identify a thermostat wire:

Take a picture of your current thermostat’s wiring

Always take a picture of your current thermostat’s wires before removing it from the wall and installing your Nest thermostat. This picture will be an essential reference for identifying wires and correctly installing your Nest thermostat.

Read this article if you need help removing your thermostat's cover.

Contact a Nest Pro installer

A trained professional will be able to quickly tell what kind of system you have and identify the wires. They can also install and set up your Nest thermostat for you, and answer any questions you have. You can contact a Nest Pro in your area with the online Pro Finder.

Check your system’s control board wires

Checking your system's control board is a very reliable method for determining what an unknown wire is. The control board’s connectors typically have labels that can tell you what each wire is when other methods aren’t convenient or don’t work. 

The control board is usually inside your furnace or HVAC equipment, so you will probably have to open or remove a panel to see inside.

How to check your system wires

Important: These are advanced troubleshooting steps. Your HVAC system uses high voltage, which can be dangerous. Make sure to turn off the power to your HVAC system completely. 

Contact a Nest Pro if you need any help.

1. Turn off the main power to your HVAC system at your breaker box

Before you turn off the power, make sure each wire coming to your thermostat is a different color. If two or more wires are the same color, you won’t be able to tell them apart at the other end. You’ll need to contact a Nest Pro.

Your HVAC system can have multiple breakers, so make sure to turn them all off before moving on. Once the power to your whole HVAC is off, check your furnace and fan. Make sure there are no lights that might indicate your system still has power.

  • If you still see lights on your furnace or fan, your system might still have power. Turn off the main power to your home at the breaker.

2. Test that you've correctly turned off power

Try to turn on each part of your HVAC system.
  1.  Turn the temperature up on your thermostat at least 5 degrees to try to turn on your furnace.

  2. Wait at least 5 minutes to see if your system turns on ( you’ll need to wait because many systems have a built-in delay).

  3. Turn the temperature down on your thermostat at least 5 degrees to try to turn on your AC.

  4. Wait at least 5 minutes to see if your system turns on.

  5. You’ll know that the power is off if your system doesn’t turn on. You can listen for your system to turn on, or you can feel for hot or cool air coming through the vents in your home.

3. Remove your system's panel

Once you’re sure you’ve turned off your system’s power, look for a removable panel on your system. It may have a handle or latch, or you may need to remove some screws. Be careful not to touch any of the components inside your furnace, as some HVAC equipment is manufactured with large capacitors (electrical components that store charge like a battery).

To learn more about removing your system’s panel, consult the user guide for your HVAC system or contact a Nest Pro in your area.

4. Look for the wire connectors

When you’ve removed the panel, look for the control board. It looks like a circuit board and usually has lots of wires attached to it. You should see several sets of connectors on the control board with wires connected to them.

  • If you need help finding your HVAC system’s control board, consult the user guide for your HVAC system or contact a Nest Pro in your area.

Look for connectors that have labels that are the same as your thermostat. For instance, you might see labels like R, W, G, Y, C or O/B. You should see wires attached to these connectors that are the same color as the wires that connect to your thermostat.

Look at the labels where these wires connect to the control board to determine what each wire attached to your thermostat should be labeled.

You may see labels like 1, 2, 3, 4, and A, B, C, D, or you may see no labels at all. These systems are incompatible with Nest Thermostats. See the following article if you need more information about thermostat wire labels.

5. Take a picture of the wire connectors

Take a picture of the wires and connectors on your system. Make sure the connector labels are clearly visible.

6. Close your system's panel

Some systems have a safety feature that will keep them from turning on unless the panel is fully closed. If your system doesn’t turn on after you’ve turned the power back on, double check that the panel is shut properly and that any latches are secured.

7. Use the Nest app to get a wiring diagram

Begin setup with the Nest app to get a custom wiring diagram. 

  1. Open the Nest app.
  2. If this is your first Nest product, tap Add Nest add product icon. .
    1. If you already own a Nest product, tap Settings Nest settings icon in the top right corner of the app home screen. Then select Add product.

The app will ask you a few questions. Use the picture of your system’s connectors to enter wires into the Nest app. 

More help with wire labels

Thermostat wires with two labels

thermostat double label
  • Some thermostats have wire connectors with two sets of labels: one for conventional systems, one for systems with a heat pump. You need to know whether you have a heat pump or conventional system so you know which thermostat labels to enter into the Nest compatibility checker or the Nest app during thermostat installation.

  • Putting wires into the wrong Nest thermostat connectors during setup can cause your system to malfunction or possibly damage it. 

Important: The side that the wires come out of the connectors doesn’t matter. In the picture above, you might assume that this thermostat is connected to a conventional system because the wires are coming out of the conventional side of the connectors. In fact, this thermostat is actually connected to a Heat Pump system.

Use wire color as a guide

You can use most of the other methods in this article to determine which labels to use: check your system’s manual, contact a Nest Pro, or look at your system’s control board.

While thermostat wire colors can vary, you can also sometimes use color as a guide  if a wire is in your thermostat’s O/B - W terminal.

  • If the wire in the O/B-W connector is orange: you probably have a heat pump. ( see the picture above).
  • If the wire in the O/B-W connector is white: you probably have a conventional system.
  • Once you know what type of system you have, enter the labels for conventional system or heat pump into the compatibility checker if you’re buying a Nest thermostat, or the Nest app to get a custom wiring diagram for installation.

Short “jumper” wires 

Jumper wires connect two thermostat terminals together (often Rc and Rh), but they aren’t “two label” wires. Jumpers aren’t connected to the system and don’t run back into the wall.

Nest thermostats don’t use jumper wires, but if your current thermostat
has a jumper wire your system may still be Nest compatible. However,
do not enter a jumper wire into the compatibility checker or the Nest app
to get a wiring diagram, and don't connect it to your Nest thermostat.

Learn about jumper wires

nest thermostat generation 3

Thermostat wires with no label

If one or more wires in your current thermostat doesn’t have a label, don’t guess. You’ll need to know what wires are connected to your current thermostat before you can install your Nest thermostat. 

You can use most of the other methods in this article to determine which labels to use: check your system’s manual, contact a Nest Pro, or look at your system’s control board. 

Important: Most systems don’t use all the wires in the bundle coming out of the wall. If any wires were not originally connected to your current thermostat (and don’t have a label), they typically aren’t connected to your system. Do not connect these wires to your current thermostat or Nest thermostat.

How to tell if a B wire is really a common or C wire

Wires labeled B typically should go into a Google Nest thermostat's O/B connector. But for some systems the wire in the thermostat's B connector is actually a common or C wire. Common wires should go in the Nest thermostat's C connector.

Because the common wire is used to provide consistent power to the thermostat, it’s important to determine what function a wire labeled B serves before connecting it to a Nest thermostat.

WARNING: If a common wire is inserted into any connector other than C on your Nest thermostat, you may blow a fuse on your HVAC system and/or damage the thermostat. If you're at all uncertain what type of wire you have, we strongly recommend that you find a Nest Pro installer near you.

  • Check if your thermostat has a C connector
    Whether or not there is a wire attached to it, if your thermostat has a separate connector labeled C in addition to a connector labeled B, then the wire in the B connector should be connected to the Nest thermostat’s O/B connector.
  • If you have a heat pump
    If you have a heat pump and your thermostat doesn’t have a separate wire labeled O, then the wire labeled B should be connected to the Nest thermostat's O/B connector.
  • Read your current thermostat’s manual.
    If you have the manual for your current thermostat (or can find it online), it may describe the function associated with the B connector and help you determine if it’s meant for a heat pump wire or a common wire.
  • Check the B wire color
    The colors of thermostat wires often don’t adhere to industry standards, but they can sometimes provide a clue in certain situations. If you’re thermostat doesn’t have separate connector labeled C and you’re not sure what type of system you have:
    • 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.

      Note: This method is not foolproof and should be cross-checked with another method.
  • Check your system’s control board
    Looking at your system’s board is the most certain way to determine what the wire is. Follow the instructions in the next section

What thermostat wire labels mean

Here are some of the labels you might see on your thermostat and what they do. After you know what wires you have, set up your Nest thermostat in the Nest app.

The Nest app will give you a custom wiring diagram to help you install the Nest thermostat. Go to the following article for step-by-step installation instructions.

How to install your Nest thermostat

Note: If you don’t see your wiring labels in this article, contact a Nest Pro.

Common thermostat wires 

Label Description
R

The R wire is the power wire for your heating and cooling system.

If you only have one R wire (no Rh or Rc), you can connect your R wire to
either Rc or Rh on the Nest thermostat.

Note: Do not connect any Jumper wires to the Nest thermostat.

Rh
  • If you don’t have an Rc wire, the Rh wire is the power wire for both your
    heating and cooling systems.
  • If you have an Rh and an Rc wire, the Rh wire is the power wire for your
    heating system.

Some HVAC systems, called dual transformer systems, use separate
power sources for heating and cooling (Rc and Rh). If you have both an
Rh and an Rc wire, you have a dual transformer system.

If you have a dual transformer system, we strongly recommend a Nest Pro installation to prevent damage to your system.

Rc
  • If you don’t have an Rh wire, the Rc wire is the power wire for both your
    heating and cooling systems.
  • If you have an Rc and an Rh wire, the Rc wire is the power wire for your
    cooling system.

Some HVAC systems, called dual transformer systems, use separate
power sources for heating and cooling (Rc and Rh). If you have both an
Rh and an Rc wire, you have a dual transformer system.

If you have a dual transformer system, we strongly recommend a Nest Pro installation to prevent damage to your system.

W or W1 The W or W1 wire controls your heating system.
Y or Y1

In most systems, the Y or Y1 wire controls your cooling system.

If you have a heat pump, your Y or Y1 wire controls your compressor. Your
compressor is responsible for heating and cooling your home.

         What to do when your thermostat has two labels 

G or G1 The G or G1 wire controls your fan. Your fan pushes the warm or cool air
through your vents into the rooms of your home.
O/B

Heat pump systems use a changeover valve controlled by the O/B wire.
Your changeover valve tells your system when to switch between heating
and cooling.

          What to do when my your system is heating when it should cool

Note: O/B wires are typically orange, but you should not rely on Orange wire label
color alone to determine the function of a wire.

Some systems have separate O and B wires. 

E

Some thermostats have an E connector. The E wire turns your
emergency heat on or off. Emergency heat is usually used when it’s too
cold outside for your regular heater to keep your home warm.

          Learn more about emergency heat

Y2

The Y2 wire controls the second stage of cooling in conventional
systems, which can help cool your home faster.

In heat pump systems, the Y2 wire controls the second stage of your
compressor, which can help heat or cool your home faster.

AUX

Heat pump systems sometimes have auxiliary heat to help heat your
home more quickly or to help heat your home when it’s too cold outside
for your heat pump to run.

          Learn more about auxiliary heat

C

The C wire is also known as the common wire. This wire connects your
system to the common ground and can help provide power to your
thermostat.

Note: The Nest thermostat doesn’t
require this wire for most installations.

          Learn more about common wires

Tip: Common wires are typically blue, but you should not blue
rely on color alone to determine the function of a wire.

W2

The W2 wire can control different types of heat based on the type of
system you have.

          What to do when your thermostat has two labels 

If you have a conventional system

In some cold areas, you might have a furnace with different stages of
heat, low and high, to help heat your home quickly when it’s very cold
outside. The wire in the W2 connector controls the second stage of heat.

          Learn more about multistage systems

If you have a heat pump

If you have a heat pump, the W2 wire may control your auxiliary heat.
Auxiliary heat helps heat your home when it’s too cold outside outside
for your heat pump to work efficiently.

          Learn more about auxiliary heat

In some heat pump systems, the W2 wire may control a separate furnace
to help heat your home. These types of systems are called dual fuel systems because the heat pump and the furnace use different heat sources.

          Learn more about dual fuel

Other thermostat wires 

If you see these wires attached to your thermostat, you may need to contact a Nest Pro to
help verify the function of your thermostat’s wiring. If installed incorrectly, these types of
wires can damage your thermostat or your system.

           Find a Nest Pro >

Wire label Description
SS1, or S2

The Nest thermostat does not need these wires. Install your Nest thermostat without these wires.

Wires labeled S or S1 can be indicator lights or sensor wires. The Nest thermostat uses its LCD display instead of an indicator light and gets sensor information, like outdoor weather conditions, over Wi-Fi.

L

The Nest thermostat does not need this wire.

Wires labeled L can be indicator lights. The Nest thermostat uses its LCD display instead of an indicator light.

G2 or GM

Requires Nest Pro installation.

Some heating and cooling systems have fans with multiple speeds. A G2
or GM wire will control the second speed of your system’s fan.

      Learn more about multi-speed fans

G3 or GH

Requires Nest Pro installation.

Some heating and cooling systems have fans with multiple speeds. A G3
or GH wire may control either the second or third speed of your system’s fan. Your Nest Pro will be able to tell which speed this wire controls.

       Learn more about multi-speed fans

O

May require Nest Pro installation if you have both an O wire and a B wire.

O and B wires control when your heat pump cools or heats your home.

B

May require Nest Pro installation if you have both an O wire and a B wire.

O and B wires control when your heat pump cools or heats your home.

In some installations the common wire may be disguised as a B wire.

        How to tell if your B wire is a common wire

        Learn more about common wires

Tip: Common wires are typically blue, but you should not blue
rely on color alone to determine the function of a wire.

W3

May require Nest Pro installation.

In some systems, the wire in the W3 connector controls your third stage
of heat. This wire might also control your emergency heat. If you’re not sure what type of heating you have, contact a Nest Pro.

         Learn more about multistage systems

         Learn more about emergency heat

H or HUM or 
HUM1

Requires Nest Pro installation.

This wire will control your in-home humidifier. If your system has a humidifier wire, or more than one humidifier wire, you’ll need a
professional installation.

H2 or HUM2

Requires Nest Pro installation.

Some humidifiers use two wires to turn on or off. Only one humidifier
wire can be connected to your Nest thermostat. Your Nest Pro will determine how to install your humidifier.

D or DH or DHUM 
or DEHUM1

Requires Nest Pro installation.

This wire will control your in-home dehumidifier. If your system has a dehumidifier wire, or more than one dehumidifier wire, you’ll need a professional installation.

DH2 or DHUM2 
or DEHUM2

Requires Nest Pro installation.

Some dehumidifiers use two wires to turn on or off. Only one
dehumidifier wire can be connected to your Nest thermostat. Your Nest
Pro will determine how to install your dehumidifier.

Other

Requires Nest Pro installation.

If you see a wire label not listed here, or you see a wire labeled Other,
you’ll need to contact a Nest Pro to check compatibility and have your thermostat installed.

            Systems that are incompatible with the Nest thermostat

 

Incompatible thermostat wires 

If you see these wires attached to your thermostat, your system is not compatible with the Nest thermostat.

Note: This is not a complete list of all incompatible wires since there are so many possibilities. You may have an incompatible system if you don’t see your current thermostat’s wire labels listed here.

Wire Label Description
Y3 Y3 wires indicate a system that has three stages of cooling. The Nest
thermostat does not support three stages of cooling.
1 2 3 4

Wire labels such as 1 2 3 and 4 indicate a proprietary system.

Proprietary systems usually only work with thermostats made by the same manufacturer.  But some proprietary systems may also have the option to use standard heating system wires. Contact a Nest pro and have them check if your proprietary system can be rewired to be Nest compatible.

A B C D

Wire labels such as A B C and D indicate a proprietary system.

Proprietary systems are designed by the manufacturer so that they can’t work with other thermostats.

         Learn more about proprietary systems

V or VR These types of wires are not compatible with the Nest thermostat.
Water or H2O These types of wires are not compatible with the Nest thermostat.
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