What should be done before opening a system that has had all liquid refrigerant recovered?

Before opening a system that has had all liquid refrigerant recovered, it is crucial to recover any remaining vapor from the system. This step is important because even after liquid refrigerant has been removed, vapor can still be present, and it can create pressure within the system. Opening the system without adequately managing the remaining vapor can lead to unsafe conditions, including potential leaks or pressure release that could cause harm.

By ensuring all vapor is recovered, you maintain a safe working environment and help prevent accidental releases of refrigerant, which are harmful to both the environment and personal safety. This practice aligns with safety protocols and regulatory requirements regarding the handling of refrigerants.

The other options suggest actions that could compromise safety, such as releasing refrigerant into the atmosphere, which is not only illegal but poses significant environmental risks. Opening the system immediately without managing vapor can lead to dangerous pressure buildup, while cooling it down first may not effectively address vapor presence, making those options less appropriate.