Line sets are the copper lines that run from the evaporator coil (usually located on top of the furnace) to the outside condenser unit. The smaller copper line is the liquid line When the air contioner is running, the liquid line is warmer in tempertaure. The bigger copper line, that is covered in insulation, is the suction line. Under the insulation, the copper line is cooler in temperture. Inside of these copper lines is freon. There usually is a thermosat wire that runs next to the copper lines. The thermostat wire is sending the power out to the condenser. Since the copper lines are going out there, the thermostat wires are just grouped with them.