Low Pressure Longitudinal Seams

Low Pressure Fittings for Residential and Light Commercial HVAC Applications


Longitudinal seams on sections of duct travel horizontally. These seams are vital since they function as a lock that holds the pieces of duct tightly and securely in place, preventing leaks under pressure. These seams may be made quickly in the shop or on the job as needed. At Spiral Manufacturing Co., Inc., we are your source for low pressure longitudinal seams.

Types of Low Pressure Longitudinal Seams

The Pittsburgh Lock Seam is a type of commonly used longitudinal seam. These seams were once manufactured in a brake or press brake. Today, they are made with roll forming machines that form the flange on one of the pieces and the pocket on the other piece. Once the one piece is placed into the pocket, the lock is closed by hammering the tail over.

Recently, the innovative Button Punch Snap Lock has been introduced. Previously, to permit shipping nesting, the continuous snap lock was applied to light gauge furnace and stove pipe. Then, the sections of pipe were snapped together.

The buttons are spaced by the button punch on about two-inch centers across the flange that is to be inserted in the pocket. The pocket has a continuous sharp fold that allows the button flange to be snapped into the pocket.

With a machine that matches the gauge of the metal being formed, the flange and the pocket may be formed. If this is not done, a loose pocket will result and air tightness and stiffness will be reduced. Aluminum is not recommended for the button punch snap.

These commercial and residential low pressure fittings are ideal for securing duct pieces together and prevent leaking under pressure.

Low Pressure Longitudinal Seams

Low Pressure Longitudinal Seams

Duct Reinforcement

HVAC applications that use HVAC duct usually require duct reinforcement – and this is particularly true for a rectangular duct system that is integrally reinforced.

Larger and longer sections of HVAC duct need properly spaced transverse reinforcement along their lengths.

There are multiple factors that certain reinforcement requirements depend upon, including system air pressure, joint type, finished duct size, and gauge of material – these are established by building codes and industry standards.

One example is the manual published by the Sheet Metal and Air Conditioning Contractors’ National Association, Inc. (SMACNA) – HVAC Duct Construction Standards, Metal & Flexible. These standards require systematic dust reinforcement. When this type of reinforcement is required, it must align with SMACNA or other applicable standards.

Sometimes duct reinforcement is required at intermediate locations spaced along the length of a given duct section or at the ends of a duct section with connecting joints. Along with the increase in the quantity of inherently-reinforced joints, if short duct sections are also used, there may not be a need for as much or any additional intermediate reinforcement.

There are various types of intermediate duct reinforcement used today. The most commonly used forms of reinforcement are channel iron, zee, and conventional angle. This reinforcement is pre-fabricated into shapes or brackets and placed transversely to the direction of flow around the duct.

The sub-sections of duct – for example, the L-shaped duct parts – may be nested and transported to the job location for final assembly and installation, reducing shipping damage and shipping costs.

For more information about the low pressure longitudinal seams we offer for residential and light commercial HVAC requirements, give us a call today at 763.755.7677 or email us at info@spiralmfg.com.

Low Pressure Longitudinal Seams