Smart Thermostat C Wire Explained – Thermostat Wires explained?

Smart thermostats have been the recent craze of homeowners who are allured by automating the cooling and heating system. Besides, its biggest USP is connecting it with the WiFi network that enables you to connect with the thermostat without any hassle. For this, you can either use an app or login in to the official website portal from the computer.

Well, these are not the only reason for choosing a smart thermostat. Apart from its feature of saving on your energy bills and the convenience it offers, smart thermostats are also quite easy to install. You do not need to be a techno-wizard, but instead, you are technical savvy, then you can add and install this piece of technology in your home.

However, before we proceed to elucidate the installation process of the thermostat, we must know the thermostat’s wiring, especially the C-wire. Most smart thermostats need a C-wire for displaying the screen in addition to its internal processor and getting it connected wirelessly.

This article will elaborate on C-wire. In addition, we would also give a detailed view of its working, know if you have C-wire. Besides, we will also elucidate on options that can be used instead of a C-wire and how you can install a smart thermostat.


About C-Wire

C-Wire is the abbreviation for Common Wire that enables the wire to flow with a constant power of 24 VAC (Volts AC) to that of the smart thermostat. However, it doesn’t provide any source of power to your smart thermostat.

These wires offer a source of power which includes the R wire, also known as hot wires. These are labeled as Rc for cooling, Rh for heating, and Rh/c when it has both cooling and heating. Moreover, few thermostats only have a single wire that is labeled R.

The power isn’t continuous from the R wire, and here is where you require a C-Wire. It helps complete the circuit and get the R wire connected back to your HVAC system’s control board. It provides continuous power flow that helps in running the WiFi and display screen 24/7.

What If I Don't Have C-Wire

Color of C-Wire

There isn’t any standardized color for a C-wire. Thus, you cannot expect a particular color for your C-wire that is the same for all HVAC systems, and these are either black or blue. However, if the installer has rewired it themselves during installation, the color of the C-wire may vary.

The commonest colors and labels that are used for thermostat wires are as follows:

  • Red stands for R, a 24 VAC power that flows from the transformer to the Furnace.
  • Black or Blue which is a C wire or Common Wire
  • Red which is for Rc rendering 24 VAC power for cooling
  • Green stands for Fan
  • Red which is Rh stands for 24 VAC power rendered for heating
  • Yellow Y is for air conditioner
  • White W is for Heat

How can I know if my system has a C-Wire?

Looking at the labels on the wire will help you know if the thermostat has a C-wire or not if you can open the thermostat and herein see wires connected to the labeled terminals. There is one terminal labeled as C. Connecting your wire here in the terminal will enable you to know if the wire is a C type. If you cannot locate a C wire connected with the C terminal, then there are chances you can rather connect or install a C-wire instead.

Often a C wire is not required for a thermostat, and the installer will connect the C wire to the HVAC control board and disconnect it to the thermostat. Herein, move ahead to your HVAC control board to locate if any wire is still attached with the thermostat’s C terminal.

If a wire is still connected to the C terminal in your HVAC control board, then check out for the following. Which are as follows:

Check out its color

Is there any wire that is tucked on the behind of the thermostat of the same color

In the case there is a matching wire, then you can locate a C wire

However, if no wires are connected with the C terminal in the HVAC control board, it doesn’t have one.

A-C wire is, however, not imperative for cooling and heating systems for your thermostats. If your system is older and offers heating or cooling, you do not need a C wire.

Besides, the lack of C wire doesn’t mean it cannot be installed with the smart thermostat system. There are numerous options if you lack a C wire and want to use a smart thermostat.

What If I Don't Have C-Wire

If your system lacks a C wire

Note: Below listed are options that need wiring connected to your HVAC system. Besides, these thermostats are low voltage in power; it is imperative to switch off the system’s power before you connect it or run wiring.

If you are obscure about how to wire a thermostat, we suggest you hire a professional or expert who holds extensive experience in it. However, if you are experienced in wiring and like to do it yourself, it is not so complicated.

Read below to find different options detailed with step-by-step instructions if your system lacks a C wire.

Related Article– What if I don’t have a C-Wire?

Option 1 – Attach a C wire to the Furnace

If your thermostat and Furnace include a C terminal, you can run the wire between the thermostat and Furnace.

  • Switch off the power from your Furnace and thermostat available in the circuit breaker
  • Ensure you turn off the power and then turn on the thermostat
  • Remove the panel located on the Furnace and find the thermostat terminals and wire
  • Attach an 18 AWG wire with the C terminal on the Furnace
  • Remove the faceplate of the thermostat
  • Run the C wire between the Furnace and the new thermostat. It may look a little hard work based on how far the thermostat is from your Furnace.
  • Attach the wire with the C terminal of the thermostat
  • Place the thermostat face place again inside of it
  • Place the access panel on the Furnace
  • Switch on the furnace power and connect the thermostat again with the circuit breaker.

Option 2 – Connect a Thermostat Cable

Another option for option 1 is running a new 18/5 wire from your thermostat to the Furnace. If you have a C terminal in the HVAC control board, then this option is the best.

An 18/5 wire is a bundle of five different colored wires or 18 AWG wires. If there is only a heating system but no AC, you can opt for an 18/3 wire. However, if you are going to connect a new wire, you may opt for an 18/5 to install an AC later.

Using an 18/5 wire is easier to install than that of option1, as you can use an old wire to pull newer wires from the wall and back inside the thermostat.

  • Switch off the power from the Furnace and disconnect the circuit breaker from the thermostat
  • Ensure the power is off
  • From the Furnace, remove its access panel
  • Locate thermostat terminals and wires
  • Now disconnect the older wire and connect the new ones into the right terminals in the HVAC control panel. You can make use of the label or color in the chart to connect the wires.
  • Remove the faceplate of the thermostat.
  • Connect the wires from the thermostat to the HVAC control board
  • It is easy to run the 18/5 wire with the thermostat as it offers a secure connection of the new wire with the old ones. Besides, you can use the old wires to again pull the new wire in the same route into the thermostat
  • Connect the new wires into the right terminals in the thermostat
  • Place the faceplate back into the thermostat
  • Place the access panel into the Furnace
  • Turn on the power of the Furnace and switch on the thermostat back with its circuit breaker.

C wire colour

Option 3: Use a G wire in place of a C wire

It is considered the easiest and most cost-friendly method to add to your system a C wire. You must opt for the method only in the following circumstances:

  • If your thermostat is directly wired to your zone panel and Furnace
  • If there is no other equipment that is wired with the Fan like a purifier, humidifier, or indoor air quality system (IAQ)
  • If the thermostat runs the Fan when it is in cooling mode, you do not require a fan for heating.
  • If you meet the conditions mentioned above, then opt for the following options. If not, then you can opt for another method. It is necessary to note that you will not be able to control the Fan manually with the option. However, if you can operate it normally, then the thermostat is in auto mode.
  • Switch on the power of the Furnace and disconnect the circuit breaker from the thermostat
  • Ensure the power is switched off
  • Remove access panel in the Furnace
  • Locate the wires and terminals in the thermostat
  • Disconnect from the terminal the G wire on the HVAC control board
  • Attach the G wire inside the C terminal available on the HVAC control board
  • Attach a jumper wire to the Y and G terminals
  • Remove the faceplate
  • Disconnect the G wire
  • Connect G wire in place of the C terminal in the thermostat
  • Place the faceplate and access plate back on the Furnace
  • Turn on the furnace power and connect the circuit breaker with the thermostat.

Option 4 – Opt for an AC wire adapter kit

A-C wire adapter kit includes a 4-wire system that allows the thermostat to work with the C wire. You require at least four wires to use the adapter kit that is present on your HVAC system and thermostat.

Venstar Add-A-Wire is considered the most inexpensive and cost-friendly kit that a novice homeowner can use.

The kit comprises of the following:

  • Diode Y, which is a blue-colored wire split into the yellow and green wire
  • Venstar Add a wire kit
  • Electrical Caps
  • Manual with instructions for installation
  • 2 wire nuts

The tools include:

  • Drill bit
  • Electrical Tape
  • Needle nose pliers

Method to install a Venstar Add a Wire Kit with your 4-wire system

  • Switch off the power on the Furnace and disconnect the circuit breaker on the thermostat
  • Switch on the thermostat and remove the faceplate from the thermostat
  • Remove from the terminal the G wire and connect it in place of the C terminal
  • Remove from the terminal the Y wire and reconnect the blue wire in the place of Diode Y along with a wire nut
  • Connect the green wire to the G terminal from the Diode Y on the thermostat
  • Connect the yellow wire to the Y terminal from the Diode Y on the thermostat
  • Place the faceplate back inside the thermostat
  • Remove from the Furnace the access panel and locate the terminals and wires in the thermostat.
  • Now mount Venstar nearby to the HVAC control board
  • Connect the red wire to the R terminal and brown wire to the C terminal from the Venstar device on the HVAC control board
  • Remove from the terminal G wire and connect it back inside the C terminal
  • Connect the Y wire into the blue wire with the wire nut
  • Connect the yellow wire to the Y terminal and green wire into the G terminal on the HVAC control board
  • Pace the access panel into the Furnace
  • Switch on the furnace power and connect the circuit breaker to the thermostat.
  • It has to be noted that the Venstar Add A Wire Kit is compatible with a 2 and 3 system of wire. Read the manual to know in detail the online installation process.

Option 5 – Opt for a Smart Thermostat that includes a Power Extender Kit

Ecobee Smart Thermostats and Ecobee lite comprise a Power Extender Kit (PEK) from the package, which will allow you to use the smart thermostat without a C wire.

Herein, the PEK is compatible with HVAC systems that comprise:

  • 3 wires which are G, Y/Y1, and R, including Rh or Rc
  • Or 4 wires which are G, R including Rc or Rh, W/W1 and Y/Y1

Follow the below-mentioned process to install a PEK, which are as follows:

  • Switch off the power of the Furnace and disconnect the circuit breaker from the thermostat
  • Locate the terminals and wires from the thermostat by removing the access panel from the Furnace
  • Disconnect and now label the wires as per G, Y, R, and W wires
  • Connect these wires to the terminals in the PEK
  • Connect the labeled wires from the PEK with the corresponding terminals in the HVAC control board
  • Now mount the PEK into the Furnace
  • Place the access panel and remove the faceplate
  • Disconnect the label wires from the older thermostat
  • Remove the mounting plate from the old thermostat and install the new one
  • Connect the wires with the corresponding terminals in the new thermostat
  • Place the faceplate and turn on the Furnace
  • Connect the thermostat back with the circuit breaker

Option 6 – Opt for Power Stealing Thermostat

If you do not have a C wire to use your smart thermostat system, that means it doesn’t need one. Herein, these smart thermostats use a power stealing process in the lack of C wire. A power stealing thermostat assists in stealing the power that is used for deactivating to activating the cooling and heating of the HVAC system from the circuits.

If the thermostat is inactive or off, the power stealing system draws current from the circuits that are not used. The power is used for drawing extra power whenever the HVAC system is still active.

The current here is used for powering the thermostat, like keeping it connected with the WiFi, powering display power, and recharging the capacitor or battery. Herein, the thermostat should draw sufficient power to complete the functions.

In the case excess power is drawn, it can lead the thermostat to turn on again. On the other hand, if there is no sufficient power, the thermostat will run out of battery power and shut down. It is difficult to determine the right amount of power to use the power stealing system of thermostats not to work erratically. Additionally, the circuits may reason to break down the thermostat after its servicing.

The product has numerous negative reviews wherein people have shared their experience on the internet, highlighting the performance of the power stealing thermostat. Thus, we recommend you to choose and pick on the methods and options mentioned above.

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I spend each darn end of the week taking a shot at home tasks and trimming down the “nectar do” list. This is where I share the majority of the undertakings I’ve done and things I’m learning.

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