A flare nut is a brass fitting used to secure the copper Line Set to the indoor and outdoor units. It works by sandwiching the flared end of the copper pipe against the brass service valve for a mechanical seal. We provide high-quality, pre-flared lines in our Installation Kits. It is critical to use a Torque Wrench to tighten these nuts to the manufacturer's exact specs. Over-tightening can crack the flare, while under-tightening will lead to a slow refrigerant leak. You can find the required torque values for all Cooper&Hunter units in our Technical Data. This small nut is the primary connection point for your entire Ductless Mini Split. Ensuring these are secure is the most important step in a leak-free setup.