Which factory seam is acceptable on ductwork?

Which factory seam is acceptable on ductwork?

• A. Spiral seam
• B. Pittsburgh
• C. Lap seam
• D. Butt seam

Answer: (B)

The main idea is that duct joints in factory-built systems should be tight, durable, and able to maintain cross-sectional area under pressure. The Pittsburgh seam achieves this by forming a locking interlock along the edge of the sheet metal. This lock seam is created with specialized equipment in the factory, producing a continuous, strong joint that resists air leakage and stays true to size throughout operation. Because of its interlocking design and controlled fabrication, it seals well and remains secure even with temperature changes and airflow vibrations, which is essential for factory-made ductwork. Lap seams rely on overlapping edges and sealing them, which can leave gaps if the fit isn’t perfect or if the sealant deteriorates. Butt seams join edges directly without an interlock, making them more prone to leaks and distortion under pressure. Spiral seams are typical of spiral ducts or continuous rolling processes and aren’t the standard factory seam for rectangular or sheet-metal ducts. So the Pittsburgh seam stands out as the acceptable, reliable factory joint.

The main idea is that duct joints in factory-built systems should be tight, durable, and able to maintain cross-sectional area under pressure. The Pittsburgh seam achieves this by forming a locking interlock along the edge of the sheet metal. This lock seam is created with specialized equipment in the factory, producing a continuous, strong joint that resists air leakage and stays true to size throughout operation. Because of its interlocking design and controlled fabrication, it seals well and remains secure even with temperature changes and airflow vibrations, which is essential for factory-made ductwork.

Lap seams rely on overlapping edges and sealing them, which can leave gaps if the fit isn’t perfect or if the sealant deteriorates. Butt seams join edges directly without an interlock, making them more prone to leaks and distortion under pressure. Spiral seams are typical of spiral ducts or continuous rolling processes and aren’t the standard factory seam for rectangular or sheet-metal ducts. So the Pittsburgh seam stands out as the acceptable, reliable factory joint.