504.2.2 Minimum size.
FGC § 504.2.2
Where the vent size determined from the tables is smaller than the appliance draft hood outlet or flue collar, the smaller size shall be permitted to be used provided that all of the following requirements are met: 1. The total vent height (H) is not less than 10 feet (3048 mm).
2.Vents for appliance draft hood outlets or flue collars 12 inches (304.8 mm) in diameter or smaller are not reduced more than one table size.
3.Vents for appliance draft hood outlets or flue collars larger than 12 inches (304.8 mm) in diameter are not reduced more than two table sizes.
4.The maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10 percent (0.90 × maximum table capacity).
5.The draft hood outlet is greater than 4 inches (101.6 mm) in diameter. Do not connect a 3-inch-diameter (76.2 mm) vent to a 4-inch-diameter (101.6 mm) draft hood outlet. This provision shall not apply to fan-assisted appliances.
