Ducts are not allowed to be located within how many inches of the earth?

Ducts must be installed at least 4 inches above the earth's surface to prevent issues related to moisture and damage. When ducts are placed closer to the ground, they can be susceptible to flooding or moisture accumulation, which may lead to deterioration of the duct materials and potential mold growth. By maintaining a clearance of at least 4 inches, the installation ensures better airflow, reduces the risk of corrosion, and helps protect the ductwork from environmental elements that could compromise its integrity and performance.

This specific clearance is crucial for both safety and efficiency in mechanical systems, ensuring that ducts function as intended without the risk of these issues. Therefore, the correct answer emphasizes the importance of proper duct placement in relation to the earth to maintain system reliability and durability.