Thermostats – Do I Need a Common Wire (C-Wire)?

Smart thermostats often require a continuous flow of power to maintain their displays and network connectivity. They usually need a common wire or C-wire to provide this continuous power. Power flows from the red wire, but not continuously so the common wire is necessary to complete the process. When everything is complete, the thermostat will have a continuous supply of 24-volt energy.

The common wire is usually blue or black, but that is not guaranteed. The previous homeowner may have made the wires whatever color they wanted or they could have run any wire they had available, so never rely on color alone. Regardless of color, the common wire is connected to the C port inside your thermostat. If you are unsure, it is always best to consult with an electrician or HVAC expert.

Why does a Smart Thermostat require a C-Wire?

A C-wire is required for many smart thermostats because they need a constant flow of electricity from the furnace in order to keep the smart components running. Smart thermostats usually have Wi-Fi in them so they can communicate to the internet, which allows you to monitor or change the temperature when you are away. In addition, smart thermostats also have a lit color display, which uses power. Unfortunately, batteries will not provide enough power for a long enough period of time to be useful, thus the need for the C-wire power from the furnace or boiler.

Not all smart thermostats require a C-wire. Some of the newer models include circuitry that captures the intermittent power from the red wire to charge an internal battery. This means they can run entirely without a common wire, which makes installation a lot easier. For instance, the Nest Thermostat and the Nest E Thermostat can usually run without a common wire — see below for more information. Other thermostats, like the Ecobee, come with an adapter that can be used if you already have four wires, but no common wire — Ecobee thermostats are also discussed below.

How to Tell if You Have a C-Wire

The best way to figure out if you need to adjust to a c-wire system is to take your thermostat off the wall and see what it looks like. If your wiring looks anything like the diagram below, you are okay with any smart thermostat you want. You have the proper blue c-wire running into the C slot. This wire sends a constant flow of energy to your system and requires no changing.

Thermostat wiring - with a blue common wire

Even if you don’t see a C-wire that doesn’t mean you don’t have one. The C-wire could be rolled up and hidden in the wall if your existing thermostat doesn’t need it. You may want to take a look behind your current thermostat bracket to see if there are any additional wires. BE VERY CAREFUL that the wires don’t accidentally fall into the wall while you are investigating — carefully wrap them around something like a screwdriver to make sure they don’t get pulled into the wall.

No Thermostat C-Wire? No Problem!

If your home is old or you have an older heating system, then you may not have a common wire. That doesn’t mean you can’t use a smart thermostat. You have a few options if you want to use a smart thermostat without a C-wire.

WARNING: Always turn the power to your HVAC system off when doing work on it, including adjusting thermostat wires. Yes, the thermostat uses low amperage and low voltage power, but it is still important to turn off power to your HVAC system when running and connecting the wires to prevent injury or damage. Safety first.

Option 1 — Run a New Thermostat Cable

The absolute best (and future-proof) solution is to run a new 8-wire thermostat cable. The cable itself is relatively inexpensive, but hiring an electrician could cost quite a bit depending on how complicated the run is. There are 5-wire and 7-wire thermostat cables, but if you have to run a new cable you might as well run the 8-wire cable even if you don’t need all of the wires so that you never have to run another cable.

The easiest way to run a new cable is to follow the same path of the existing cable — you may even be able to securely tape the new wire to the old, then use the old wire to pull the new cable. Of course, electricians do this type of work all of the time and may have an easier and less frustrating time than a do-it-yourselfer.

Keep in mind that you should leave your thermostat in its current location. The engineer or HVAC installer who did the original work chose the current location carefully to make sure your system works efficiently. Only move your thermostat if you have a compelling reason, like if the sun shines on it for part of the day.

Option 2 — Run a Single Wire

If you have a simple, short, and accessible run between your thermostat and your HVAC system you may consider running a 2-conductor 18-guage wire. The cost of the cable is less expensive, but the labor is more challenging since you have to follow the existing wire instead of using the old wire as a pull string. This is only recommended if the run is very easy.

Option 3 — Use an Adapter Kit

In addition to running a c-wire, some companies also sell adapters to take your four wire setup to a five wire setup. To use an adapter kit, you must have a minimum of four wires currently present. Some smart thermostats come with an adapter for missing c-wire systems, while others sell a separate extender kit.

One of the most popular methods for adding a c-wire is using the Venstar Add-A-WireTM adapter. This adapter mounts directly to the furnace and requires you to match up and twist the wires together. This method is simple and easy for beginners and inexperienced homeowners to do. Of course, be sure to turn off power to your furnace or boiler ahead of time.

Venstar Add-A-Wire adapter

The adapter comes with a complete installation guide, and Venstar has an official installation video online. Many experts recommend using this method instead of running new HVAC wiring if you have four wires because it is much easier than trying to snake a new wire through the walls.

Option 4 — Use a Thermostat That Does Not Require a C-wire

The simplest way to avoid work and complications is to find a thermostat that does not require a common wire.

One popular option is the Ecobee Thermostat. Even though Ecobee does not require a c-wire, you still need to have a 4-wire thermostat cable because they require a power extender kit. You will connect the wires from your furnace to the power extender kit and then to the thermostat. Once you’ve correctly connected the wires, you can mount the extender kit to the furnace and head back to the new thermostat.

You can find more information about the Ecobee Power Extender Kit (PEK) on their website. The PEK comes included with both the Ecobee 3 Lite and the Ecobee 4 so you don’t have to make a separate purchase.

Does the Nest Thermostat Require a Common Wire?

Unfortunately, the answer to this question is, “it depends.” I have a single-stage boiler with only red and yellow wires, but the Nest works just fine. However, my HVAC technician did tell me that he has seen problems with Nest thermostats that don’t use a common wire.

Nest used to claim that their thermostats would work without a C-wire because the circuitry uses the voltage provided by the R-wire to charge an internal battery when needed. This is not a problem when the HVAC system is running in either heat or cool mode. However, if the system is not being used the Nest still needs to get power so it will pull short bursts of power from the boiler or furnace. Unfortunately, these short pulses or bursts can cause some problems with the boiler/furnace’s circuitry. For more information, consult with the Nest website.

If you notice strange noises from your system’s power supply after installing the Nest or if you start to experience power trouble then you may want to invest in running a new 8-wire thermostat cable, which should future-proof your home. Or, it you already have a 4-wire thermostat cable, you can use the Venstar Add-A-WireTM discussed above.

Thermostat Wires Color Coding

Each wire in the thermostat cable serves a different purpose. The thermostat uses the low voltage wires to send an electrical signal to the furnace or boiler, which is a way of providing commands to turn the heat or air conditioning on and off. In general, the following standards are used for wiring thermostats. However, be careful because the person who wired your thermostat may not have followed this conventional logic. You should always trace the wires to the source before connecting to your expensive new thermostat.

Code Typical Color Function
C Black or Blue Common: Provides 24V of power to the thermostat if required. Most digital or WiFi thermostats require power to operate.
R Red Power: Provides power from the transformer on the furnace or boiler.
Rc Red or Jumper Power for Cooling: Provides power for the cooling cycle. If the heating and cooling systems share a transformer then a jumper may be connected between the Rh and Rc terminals.
Rh Red Power for Heating: Provides power for the heating cycle. If the heating and cooling systems share a transformer then a jumper may be connected between the Rh and Rc terminals.
G Green Relay for Fan: Provides power/signal for turning the interior fan on.
W White Relay for Heating: Provides a signal to the heat source to initiate.
W2 Varies (Brown) Relay for Heating Stage 2: Provides a signal to initiate the second heating stage. Used mostly on heat pumps.
O/B Orange, Dark Blue, Black Reversing Relay: Used on heat pumps to trigger the reversing valve, which is located on the outdoor heat pump condenser.
Y Yellow Cooling Compressor Relay: Used to signal the air conditioner compressor, which is located outside.
Y2 Varies (Light blue) Cooling Compressor Relay Stage 2: Used to signal the second stage of the air conditioner compressor, which is located outside.
E Varies Emergency Heating on Heat Pumps: Used to trigger the emergency heat source in case the heat pump’s condenser isn’t working.
X or Aux Varies Back-up Heating on Heat Pumps: Used to initiate an auxiliary heat source, which is usually located in the air handler.
A Varies Sends signal that the heating or cooling cycle is active.
S1 or S2 Separate shielded Wire Connection to an outdoor temperature sensor.

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