Before soldering an evaporator coil, why must the end of the evaporator lines be opened to the atmosphere?

Opening the end of the evaporator lines to the atmosphere before soldering is essential primarily to manage the internal pressure and ensure safety. The refrigeration systems often use nitrogen to pressurize the coils to prevent moisture and contaminants from entering the system during transport and storage. This nitrogen charge keeps the atmosphere inside the evaporator lines dry, which is crucial because moisture can lead to corrosion and reduce the efficiency of the refrigerant system.

Opening the lines to the atmosphere before soldering allows the nitrogen to escape, ensuring that when you apply heat, the solder can flow properly without the risk of creating a pressure buildup that could cause leaks or other issues in the system. If you were to solder while the evaporator lines were still under nitrogen pressure, it could lead to dangerous situations, including potential splatter of molten solder or a breached line.

The objective in refrigerant system maintenance and installation is to have a clean, dry environment inside the system. Therefore, this particular practice is a preventative measure that protects the integrity and efficiency of the refrigeration system.