Round metallic ducts must be mechanically fastened with how many sheet metal screws or rivets?

Round metallic ducts must be mechanically fastened with at least three sheet metal screws or rivets to ensure proper assembly and to maintain airtight conditions within the ductwork. This fastening method is crucial for preventing air leaks, which can significantly impact the efficiency of heating, ventilation, and air conditioning systems.

Using three fasteners provides a strong connection that distributes the mechanical load evenly across the joint. This is particularly important as round ductwork is often subjected to varying pressures and flow conditions. A joint secured with the correct number of fasteners will help maintain structural integrity even under these conditions.

Furthermore, standard industry practices and guidelines suggest that three fasteners are sufficient for ensuring a reliable seal while allowing for ease of installation. When assembling ductwork, adhering to these standards is vital for the system's overall performance and longevity. This practice is commonly found in building and mechanical codes, emphasizing the importance of following established guidelines for duct construction and installation.