What is the minimum slope for horizontal duct runs greater than 75 feet?

What is the minimum slope for horizontal duct runs greater than 75 feet?

• A. 8 percent
• B. 2 percent
• C. 0 percent
• D. 4 percent

Answer: (A)

The concept being tested is providing gravity drainage for condensate in long horizontal duct runs. When a duct runs a long distance, condensate and grease-laden liquids can tend to pool if the slope isn’t enough to pull them toward the discharge point. For horizontal duct runs longer than about 75 feet, this drainage requirement becomes more critical, so a steeper minimum slope is specified to ensure liquids travel by gravity rather than stopping and accumulating inside the duct. An 8 percent grade means the duct drops about 0.96 inches per foot of run, which creates a reliable path for drainage along the entire length and helps prevent standing liquid, which can cause corrosion, odors, or fire hazards. Gentler slopes (0, 2, or 4 percent) may not provide sufficient drainage over such a long distance, while the 8 percent grade meets the need for long runs to stay clear of liquid buildup.

The concept being tested is providing gravity drainage for condensate in long horizontal duct runs. When a duct runs a long distance, condensate and grease-laden liquids can tend to pool if the slope isn’t enough to pull them toward the discharge point. For horizontal duct runs longer than about 75 feet, this drainage requirement becomes more critical, so a steeper minimum slope is specified to ensure liquids travel by gravity rather than stopping and accumulating inside the duct. An 8 percent grade means the duct drops about 0.96 inches per foot of run, which creates a reliable path for drainage along the entire length and helps prevent standing liquid, which can cause corrosion, odors, or fire hazards. Gentler slopes (0, 2, or 4 percent) may not provide sufficient drainage over such a long distance, while the 8 percent grade meets the need for long runs to stay clear of liquid buildup.