How deep are ducts usually buried below final grade for regular installations?

The correct depth for burying ducts in regular installations is typically 24 inches below the final grade. This depth is established to ensure that the ducts are adequately protected from surface activities, such as landscaping or construction. It also helps to prevent potential damage from frost in colder climates and minimizes the risk of interference from surface-level disturbances.

By adhering to this standard, it promotes safety and functionality, ensuring that the duct system operates effectively without the risk of being exposed to damage or environmental factors that could lead to malfunctions. The other options suggest depths that may not provide sufficient protection and security as defined by commonly accepted installation guidelines.