Can You Use Nest Thermostat Blue Wire as C Wire?

Are you upgrading from an old thermostat to a new programmable nest thermostat? If so you may have noticed that from your thermostat wiring there is an unused blue wire and sometimes when you try to run your thermostat, you could be getting a ‘low power’ error message.

Generally, you can use the unused blue wire as your C wire when you are wiring your new thermostat but just make sure that the blue wire is connected to the C terminal on the terminal block of your control board in the furnace compartment.

simple nest set up

How do you know if the unused blue wire is the C-wire?

You can know for sure whether or not  the blue wire is the C wire by checking the terminal it is connected to on the control board. 

The blue wire should be connected to the C terminal if you are to use it as a C wire. If you have no access to the furnace or circuit control board ( in case it is installed in a place where you can’t easily access it) then you can still test the blue wire to check whether or not it’s connected to the C terminal. 

Use a multimeter to do this. Use Y wire and the blue wire and if there is 24v going through then this is definitely a common wire

Table 1 below shows Nest thermostat wire colors and the functions of each wire.

Nest thermostat wire color code and functions

WireRepresented colorFunction
YYellowCompressor Relays (usually stage 1)
Y2GreyCompressor relays usually stage 2
GGreenFan relays
OOrangeChange over valve
RRedPower from the transformer
WWhiteHeat relay stage 1
BBrownHeat relay stage 2
CBlueCommon wire
Table 1

Here is the function of each of the wires in table 1.

Yellow wire: This is denoted by Y . This wire goes into the Y terminal and controls the cooling for your air conditioner. If you have a heat pump system, this wire will control the cooling  and the heating of your home.

Red wire : This is usually represented by R and  supplies power for heating and cooling. If you don’t have the R terminal you can connect this wire to either the Rc or Rh terminal. Remember, when you are dealing with a Nest thermostat, do not use jumpers, as the Nest will automatically jump the necessary wires.

White wire: This is the W wire and controls the stage 1 heating for your system. It should go into the W1 connector on your nest.

Brown wire: This is the ‘B’ wire.This wire is used for controlling secondary or auxiliary heating when you are using  a heat pump.  This is also useful if your system is a 2 stage heating system (furnace). When the outdoor temperature drops too low for your heat pump  to provide sufficient cooling this wire will energize the auxiliary heating.

Gray wire:  The gray wire controls the 2nd stage relay for cooling in a conventional air conditioning system. But if you have a heat pump this is the wire that controls the 2nd stage compressor for both heating and cooling. It will usually go into the Y2 connector of your nest.

Green: This is  the G wire (fan relay) and controls the fan that blows air throughout the ducts and out of vents into your home

Orange: This is represented by O. this wire controls the change over valve on a heat pump system. So this wire is only useful if you have a heat pump system. The change over valve switches between the heating and the cooling of your heating system. And when heat and cool are reversed, this is  part which should be monitored for troubleshooting purposes.

Where does the blue wire go on a nest thermostat?

The blue wire is the one that powers up the thermostat. Even though some Nest thermostat may work without the common wire, soon or later they will start giving power related problems and for that matter, you will need to connect the blue c wire to your thermostat. It  should typically go onto the C connector of your Nest thermostat. 

What is the e wire on a nest thermostat?

The e wire on a nest thermostat controls the emergency heat. It turns on and off the emergency heat. For example when the outdoors temperature drops too low for a regular heater to provide sufficient heat,  emergency heating can be turned on to provide supplementary heating.

 In some thermostats you will find terminals labeled w1 or w2 while on other thermostats it will be E or emergency heat. 

In other words, E can also go on the W connector  when you don’t have an e conector on a thermostat.


Is it possible to use a g wire as C wire on a Nest thermostat?

Yes, you can use the fan wire for the c wire. But note that you won’t be able to use the fan independent of heating and cooling. You will need to pull the wire from the G terminal on the thermostat and on the control board. Then make a jumper between the G and the Y terminal.


We hope that you now  understand what the blue wire on your nest thermostat is and what it is used for. If you still can’t figure out how to wire your thermostat we would advise you to seek help from local HVAC pros. We appreciate your time reading this post.

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