Ducts must not be installed less than how far above the earth?

Ducts must be installed at least 4 inches above the ground to ensure they are protected from moisture, debris, and physical damage that can occur when they are too close to the earth. This clearance helps to prevent issues related to flooding or ground overflow, which could lead to rusting or deterioration of the ductwork. Additionally, having ducting elevated at this height allows for better airflow and prevents blockages that could be caused by soil or vegetation. Adequate clearance also makes maintenance and inspection easier, ensuring that the HVAC system operates efficiently and effectively.