504.3.21 Connector maximum and minimum size.
FGC § 504.3.21
Vent connectors shall not be increased in size more than two sizes greater than the listed appliance categorized vent diameter, flue collar diameter or draft hood outlet diameter. Vent connectors for draft hood-equipped appliances shall not be smaller than the draft hood outlet diameter. Where a vent connector size(s) determined from the tables for a fan-assisted appliance(s) is smaller than the flue collar diameter, the use of the smaller size(s) shall be permitted provided that the installation complies with all of the following conditions: 1. Vent connectors for fan-assisted appliance flue collars 12 inches (304.8 mm) in diameter or smaller are not reduced by more than one table size (e.g., 12 inches to 10 inches (304.8 mm to 254 mm) is a one-size reduction) and those larger than 12 inches (304.8 mm) in diameter are not reduced more than two table sizes (e.g., 24 inches to 20 inches (609.6 mm to 508 mm) is a two-size reduction).
2.The fan-assisted appliance(s) is common vented with a draft-hood-equipped appliance(s).
3.The vent connector has a smooth interior wall.
