Since you’re on this page and going through a guide to C Wire, I assume you are finding a way to install C-Wire for 
your Smart Wireless Thermostat. I have prepared a guide for people who prefer DIY method of installing smart thermostats at homes.
What is C-Wire and What it is used for? – C stands for “Common” wire
C-Wire is one additional wire in latest smart thermostats models. It merely provides 24vac power to thermostats to operate properly. C-wiring was not available in older mercury-based thermostat models. However, It is used to power up additional smart features that are available in latest wifi thermostats. These additional features include but not limited to fancy touch screens, smart sensors, wireless connectivity etc.
C-wire facilitates a connection with ‘air-handler’ or also know as ‘furnace’ to enable communication between opposite nodes. Normally in older models, there are 2 to 4 nodes that complete the wiring of thermostats. however, in a case of latest wifi thermostats, there is a fifth node( wire) that provides additional power for fancy functionalities.
I have developed a buying guide for wifi thermostats here, you might want to check it out if you are looking one for your home or office.
If you are planning to install thermostat yourself, I have explained below how to check if your thermostat has a C- wire already or if you need one and don’t have it, I will explain how to add it yourself.
Your thermostat have C Wire?
To find out whether or not your thermostat has c wire, first of all, we need to uncover a front case of it without removing any wire. Connected wires will be in front of you, If you count them to be 5, congratulations, C wire is available and that is most likely to be ‘blue’. But if you find them to be only 4, C wire is not available. Refer to the image below.
What if I don’t have C Wire?
There are two ways to go by if no C wire is installed.
- Run a new wire( preferably Blue) from thermostat to air handler/furnace
- Update to smarter thermostat solutions that does not need C wire in the first place.
Before going any further, let explain the color of the wires and where they should be connected
Wires and connectors they should connect to
- White – The white wire connects to the additional heat on your system.
- Yellow – The yellow wire is meant to connect to a compressor.
- Green – The green wire connects to the fan.
- Orange – Orange wire is connected to a heat pump (if you have one).
- Red – There could be two Red wires, one for ‘heat’ and other is ‘Cold’. These are abbreviated as “RH” & “RC” respectively.
- Blue – Blue wire is known as “common wire” or “C” wire.
These colorful wires don’t necessarily are connected to where they should be. Consider a scenario where technician installed these wires without keeping the wires in the account and connected them anyway. If wires are connected correctly regardless of the order of the color, it will still work fine. But it will be a time taking task to find which wire is connected to which part of a thermostat.
Ideally, wires should be connected as I have mentioned above, but you may not find it in recommended order.
Recommendation :
No matter how technically sound you are, if you are not qualified technician you are bound to mess up. It is not wise to install wire that can electricute you. We therefore, recommend to hire a pro for installing and configuring your thermostat. Just for saving few bucks you can potentially harm yourself and can burn out entire thing for good.
Good luck !