There is more than one reason to run a Common wire, also known as a return wire or C-wire.
In today’s world, newer devices like thermostats and user interfaces often offer features such as clocks, backlit displays, and WiFi, among others. These controls use circuit boards and integrated circuits. These circuits require a path for electricity to return to its source, and this path is known as the Common wire.
One might ask, what is a “source”? A transformer is an example of a source. It contains a primary and a secondary voltage. Typically, in a residential system, the control circuit voltage is transformed down, from 120VAC to 24VAC.
When adding a new device, if you are fortunate there are plenty of extra wires and it is a simple task to install and wire the new device. If you are not so fortunate to have plenty of wire, then you must assess the situation and determine the best way to get the Common wire back to the source.
In some cases, only one wire may be needed; in others, you may need multiple wires. If you need more than one, more than likely the only option is to pull new wire. If only one additional wire is needed, you may be able to get away with using a device called Add-A-Wire.
These Add-A-Wires can be installed near or inside your furnace, and are sometimes called a "diode Y". A diode Y basically splits one thermostat wire into two wires. In most cases, it takes the Y wire and splits it so you have a Y and a G, which frees up a wire to use as the Common wire. When adding your new thermostats or user interfaces, be sure to properly assess the situation to determine how many extra wires you need to add, so you can add the Common wire the proper way.