Communication wire, specifically 14-gauge 4-conductor wire, is used to connect the indoor air handler to the outdoor unit. This wire carries both the low-voltage signals and the power needed for the Indoor Air Handler to operate. It is "the central nervous system" that allows the thermostat and remote to tell the condenser exactly what to do. Most of our Installation Kits include this specific wire to ensure compatibility with brands like Cooper&Hunter and OLMO. Using the wrong gauge or type of wire can lead to communication errors (like E1 codes) and system malfunctions. The wire is typically run alongside the copper Line Sets and is often protected by a Decorative Line Cover. It is rated for outdoor use and is flexible enough to be pulled through conduits or wall cavities. For larger Multi-Zone Systems, you will need a separate run of this wire for every indoor head. Proper stripping and termination of these four wires are essential for a successful startup.