Does it have 4 wires or 5? This question can be important when it comes to upgrading your home HVAC system to run on a smart thermostat. Many older systems use 4 wires, but you’ll probably need a fifth wire, known as a common wire (or C-wire), to successfully install your new device.
Below are the basics of what you should know about C-wires1: what it is, why you need one and how to resolve the issue if you don’t have what you need.