What is the minimum slope for horizontal duct runs up to 75 feet?

• A. 2 percent
• B. 8 percent
• C. 0 percent
• D. 5 percent

Answer: (A)

When duct systems Carry moisture or grease, gravity drainage matters: you want to keep condensate from pooling in long horizontal runs by giving it a slight downhill slope so it can flow toward the drain or discharge point. A minimum slope of two percent means the duct drops 0.02 ft for every 1 ft of length. For a 75 ft run, that’s about 1.5 feet of drop, which provides enough gravitational force to move liquids out and prevent standing water while avoiding excessive pressure loss or installation complications. A flat run would allow pooling, while steeper slopes aren’t necessary and can complicate installation. So two percent is the practical, code-aligned choice for these horizontal runs.