Which statement correctly describes the slope rules for horizontal duct runs?

• A. Up to 75 ft: 2%; Over 75 ft: 8%
• B. Up to 75 ft: 8%; Over 75 ft: 2%
• C. All runs: 2%
• D. Not specified

Answer: (A)

Gravity-driven drainage guides how we slope horizontal ducts. The idea is to keep liquids like condensate and grease moving downhill toward the exhaust, so nothing pools in the duct and causes buildup or blockages.

For shorter runs, a modest slope is enough to maintain flow without wasting space or creating excessive drain height. A 2% slope means about 0.24 inches of drop per foot, which reliably directs liquids toward the drain over a typical short run.

As runs get longer, gravity must do more work to push the liquid all the way to the outlet, so a steeper slope is required. An 8% slope provides a much stronger downward pull over longer distances, helping to prevent pooling in extended ducts.

So the rule described is that horizontal duct runs up to a certain length use a 2% slope, while longer runs (beyond that length) require an 8% slope to ensure proper drainage.