11/01/24. Known as the common wire, (C-Wire) provides WiFi thermostats continuous power from heating (e.g. furnaces) and cooling systems (e.g. Air Conditioning systems).
Honeywell (and others such as Trane) use a C wire on models that use WiFi or voice recognition or Z-wave so they can have a 24V connection. All thermostats use low voltage from the other connectors and typically use the batteries as a backup when there's no C wire.
Do all Smart Thermostats Require a C-Wire? Though all smart thermostats can use a C-wire and arguably may be more reliable with the use of one, not all smart thermostats require a C-wire to function.
How To Find Your C-Wire. If your home operates on a low-voltage heating system, you should be able to locate the C-wire behind your current thermostat. You should be able to pop the cover off of your thermostat with ease.
Troubleshoot or connect a C wire
Remove the thermostat display and make sure that the C wire is fully inserted then turn the power back on. If the C wire still isn't getting power, we recommend contacting a Pro. On your thermostat, check for codes that can help you understand what's wrong.
A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.
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.
A standard USB Type C connector houses 16 data transfer pins, 4 power pins, and 4 ground pins for a total of 24 pins.
Q: what is a C wire
A: C stands for common. The common wire is the ground or negative wire. To complete the circuit that provides power to the thermostat you need a positive and negative wire from the 24V transformer.
Add an Adapter
Some smart thermostats include an adapter in case there's no C-wire in your home. You can also find thermostat adapters at a hardware store or online. If you have previous electrical experience, you can follow the instructions included with the adapter to install it.
A "C" wire is the common wire that completes the electrical circuit from your transformer to your thermostat. "R" and "C" complete the 24v AC circuit to power the smart thermostat. Older "dumb" thermostats don't need power to operate things like wifi.
The most common configuration is five wires. However, you could see as few as two and many as ten. Make a note of any present wire not connected to a terminal. Do not label these wires.
In many recently-constructed homes, the HVAC c-wire is already installed. However, in homes that are only a few years old or older, the wire may need to be installed as part of the new smart thermostat installation.
But if you look closer, you will see it's more oval in shape and slightly thicker to accommodate its best feature: flippability. Like Lightning and MagSafe, the USB-C connector has no up or down orientation. Line up the connector properly, and you never have to flip it over to plug it in; the "right way" is always up.
Type C connectors differ in appearance compared to the previous USB types, as they are asymmetrical and have a small and thin appearance. One major difference is that the USB C is reversible; meaning it does not matter which way the connector is facing.
USB-C, or USB Type-C, is a 24-pin connector (not a protocol) that supersedes previous USB connectors and can carry audio, video, and other data, to connect to monitors or external drives. It can also provide and receive power, to power, e.g., a laptop or a mobile phone.
Known as the common wire, (C-Wire) provides WiFi thermostats continuous power from heating (e.g. furnaces) and cooling systems (e.g. Air Conditioning systems). C-Wires are required on all Resideo WiFi thermostats. For full thermostat wiring assistance: Click here.
If there is no hidden C-wire, you can use a G-wire instead. However, you will not be able to use your fan independently when the heating or cooling isn't running. Also, many HVAC systems are incompatible with this solution, including: HVAC systems using electric heat.
Faulty Wiring
If the wiring between the thermostat and the air conditioning system is faulty, the system may not turn on. This can happen if the wiring is old or if it has become damaged over time. To fix this issue, you may need to call a professional HVAC technician to inspect and repair the wiring.
The thermostat is located behind the control panel. Here's how to test and/or replace the thermostat: Step 1: Remove grille and control panel from unit. Thermostat has special sensing bulb attached to it; this part extends from thermostat into evaporator coil area.