Underground metallic duct without approved coating should be encased in how many inches of concrete?

The requirement for encasing underground metallic duct in concrete is primarily for the protection of the ductwork against external pressures and corrosion. Encasing the duct in a minimum of two inches of concrete provides adequate shielding, ensuring that both the integrity of the duct and the efficiency of the HVAC system are maintained over time.

Concrete serves as a robust physical barrier that protects the metal from moisture, soil chemicals, and physical damage from external loads. Insufficient encapsulation, such as one inch, may not provide the necessary structural support or moisture barrier needed for longevity, while greater thicknesses, like three or four inches, might not be practical or required based on standard regulations. Thus, two inches strikes a balance between adequate protection and reasonable construction practices, aligning with industry standards for such applications.