What must be used when velocities exceed 4000 Feet Per Minute on all upstream edges of transverse joints to ensure that the liner does not delaminate from the duct?

When velocities exceed 4000 Feet Per Minute at the upstream edges of transverse joints, it is crucial to use nosing to secure the liner effectively and prevent delamination. Nosing serves as a reinforcing edge that helps to stabilize the lining material against the high air pressure and velocity forces. By providing a secure attachment point, nosing ensures that the liner is better able to withstand the dynamic forces experienced in high-velocity air systems.

This method is particularly effective because it offers enhanced coverage and protection to the edges of the duct joints, significantly reducing the likelihood of separation between the liner and ductwork. Nosing thus plays a vital role in maintaining the integrity of the duct system, especially under conditions of high airflow that would otherwise compromise the materials involved. The use of this technique is integral to ensuring the longevity and performance of the HVAC system.