These are the pipes through which refrigerant is circulated throughout the system. The tubing used in household units is made of copper. Aluminum and steel are seldom used. Copper is relatively soft, flexible, and easy to bend and flare. The nine most common sizes in use are 3/16″, 1/4″, 5/16″, 3/8″, 7/16″, 1/2″, 9/16″, 5/8″, and 3/4″ outside diameter (OD). Handle the tubing with care to prevent damage.