Underground approved duct should be encased and sloped to allow drainage.

• A. False
• B. Sometimes
• C. True
• D. Always

Answer: (C)

Moisture control and protection are essential when ductwork is placed underground. Approved underground ducts are designed to resist soil moisture, groundwater, and physical damage, so they’re encased in a protective enclosure to keep moisture out and to support the duct under earth loads. Sloping the trench so the duct has a downward grade allows any infiltrating water or condensate to drain away to a designated point rather than pool inside or around the duct, which helps prevent corrosion, mold, and leaks. This combination—encasement for protection and slope for drainage—is standard practice to ensure long-term reliability of underground duct systems.