Low Pressure Longitudinal Seams

Low Pressure Fittings for Residential and Light Commercial HVAC Applications


Longitudinal seams which run horizontally on the duct sections are important because these locks must hold the duct pieces securely and tightly and should not leak under pressure. They can be readily and swiftly put together on the job or in the shop.

The common type of longitudinal seam is the Pittsburgh Lock Seam. Originally formed in the brake or press brake, today roll forming machines are used to form the pocket on one piece and the flange on the other piece. After one piece is inserted in the pocket, the “tail” in hammered over to close the lock.

The Button Punch Snap Lock is a recent innovation. Originally, the continuous snap lock was used on light gauge stove and furnace pipe to permit shipping nested. The pipe section was then snapped together.

The button punch spaces the buttons on approximately two-inch centers along the flange to be inserted in the pocket. The continuous sharp fold on the pocket permits the button flange to be snapped into the pocket.

The pocket and flange must be formed in a machine suited to the gauge of metal being formed. If this is not adhered to, the pocket will be loose and stiffness and air tightness will be lessened. A final note: Button punch snap lock is not recommended for aluminum.

Low Pressure Longitudinal Seams

Low Pressure Longitudinal Seams

Duct Reinforcement

Duct reinforcement is usually needed for heating, ventilating and air conditioning (“HVAC”) duct and, even more likely, for an integrally reinforced rectangular duct system.

It is accepted that longer and larger cross-section HVAC ducts require transverse reinforcement spaced along their lengths.

The specific reinforcement requirements depend on many factors, including joint type, system air pressure, gauge of material and finished duct size, and are set by industry standards and building codes.

An example: The manual, HVAC Duct Construction Standards, Metal & Flexible, published by SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association, Inc.). The specific SMACNA standards require systematic duct reinforcement. When such reinforcement is needed, it should be in accordance with SMACNA or other applicable standards necessary.

Duct reinforcement may be necessary either at the ends of a duct section—in the form of duct section connecting joints—and/or at spaced intermediate locations along the length of a given duct section. If short duct sections are employed with the corresponding increase in the number of inherently-reinforced joints, additional intermediate reinforcement may be reduced or eliminated.

Many forms of intermediate duct reinforcement are in common use today. The most common form of reinforcement is conventional angle, zee, or channel iron. It is pre-fabricated into rectangular reinforcement shapes or brackets, then positioned around the duct, transversely, to the direction of air flow.

The duct sub-sections, e.g. the L-shaped duct components, may be nested and shipped (knocked down) to the job site for final assembly and installation. This reduces shipping costs and shipping damage.

Low Pressure Longitudinal Seams