A flare nut is used to create a leak-proof connection between the copper line set and the mini-split unit. These are included with the Bravo Milano Series 2 Zone 18k (9k+9k) and other kits. Using a torque wrench on these nuts is essential to prevent refrigerant leaks. You can find replacement nuts and tools in Other Accessories. They are designed to withstand high pressures without failing. Quality flare nuts are the key to a long-lasting, DIY-installed system.