What should not be used to join ducts if it protrudes into the inside of the duct?

Joining ducts requires careful consideration of the materials and techniques used, especially regarding how they affect airflow and system efficiency. Using screws or similar fasteners is not advisable if they protrude into the inside of the duct. This is because protruding fasteners can disrupt airflow, create turbulence, and accumulate debris, which may lead to blockages or reduced efficiency in the ventilation system.

Moreover, any protrusions can become points for air leakage, as they can compromise the airtight nature of the ductwork, leading to energy loss and increased operational costs. The focus in HVAC systems is to ensure smooth, unobstructed airflow, and using screws that intrude into the duct’s air passage counteracts this goal.

In contrast, glue, while not ideal for all joint types, or rivets, which can provide a flat surface, do not necessarily lead to the same concerns regarding airflow disruptions when they are applied correctly. Therefore, the recommendation against the use of screws or similar fasteners that intrude into the duct is rooted in promoting efficient and effective airflow management in HVAC systems.