What type of refrigerant pipe joint construction is required for ducts carrying air to occupied spaces?

The requirement for ducts carrying air to occupied spaces is that they must be able to withstand significant temperatures without leakage, specifically up to 700 degrees. This is crucial to ensure the safety and comfort of occupants within those spaces, as well as to prevent potential fire hazards. When materials and joints can resist higher temperatures, it minimizes the risk of failure under extreme conditions, maintaining airflow integrity and preventing the introduction of contaminants or hazardous gases into the occupied areas.

The other options, while they describe various types of joints and their characteristics, do not align with the strict safety and performance criteria necessary for ducts related to air delivery in occupied areas. Flexible joints may allow for movement and vibration without breaking; however, they may not provide the necessary heat resistance. Welded joints are strong and can be effective in many applications, but they are not specifically required in the context of ducts for air distribution. Standard threaded joints can be practical in many situations, yet they may also lack the high-temperature resistance required for safety in occupied spaces.