If your home is not equipped with an AC system, you have several options available depending on the size, location, and build of your home. If your home does not currently have a cooling unit, it is possible to add one onto your existing air handling system. However, your existing unit must be a furnace. If you have a boilera radiant heateror a space heater, this option will not be possible.
Advantages to Adding AC to an Existing Furnace You will have to add a conduit to connect your indoor and outdoor units and supply.
Wire colors with HVAC wiring are non-standardized, so the colors themselves don't help much. Your furance/air handler should have a control.
If you have heat and AC, you'll need 18/5. If you find a C-wire in your furnace, then the other end is probably stuffed into the wall behind your thermostat. As you can see in the photo above, this furnace does not have a C-wire hooked up.