How high must ducts be above dirt unless protective features are in place?

Ducts must be installed at least 4 inches above dirt unless protective features are in place due to concerns related to moisture and the potential for soil contaminants. When ducts are too close to the ground, they can be subject to dirt and debris accumulation, which may result in blockages and damage over time. Elevating the ducts helps to ensure proper airflow and reduces the risk of deterioration from exposure to moisture and organic materials. This code requirement forms part of good building practice, emphasizing the importance of durability and functionality in HVAC system installations. The other height options do not provide the minimum required clearance specified in building codes for such conditions.