A flare nut is a specialized brass fastener used to secure the copper line set to the service valves of the condenser and the connections of the air handler. The end of the copper pipe is "flared" into a bell shape, and the nut compresses this flare against the fitting to create a mechanical, leak-proof seal. This eliminates the need for brazing or soldering, making it a favorite for diy-1 enthusiasts. Over-tightening or under-tightening these nuts is a frequent source of refrigerant leaks in wall mount installations. Always use a torque wrench to meet the manufacturer's specific foot-pound requirements for your multi-zone wall mounts.