504.3.4 Vent connector manifold.
FGC § 504.3.4
Where the vent connectors are combined prior to entering the vertical portion of the common vent to form a common vent manifold, the size of the common vent manifold and the common vent shall be determined by applying a 10 percent reduction (0.90 × maximum common vent capacity) to the common vent capacity part of the common vent tables. The length of the common vent connector manifold (L M ) shall not exceed 1 1/2 feet for each inch (18 mm for each mm) of common vent connector manifold diameter (D) (see Figure 504.3.4). Example: Manifolded Common Vent Connector LM shall be no greater than 18 times the common vent connector manifold inside diameter; i.e., a 4-inch (102 mm) inside diameter common vent connector manifold shall not exceed 72 inches (1829 mm) in length (see Section 504.3.4). Note: This is an illustration of a typical manifolded vent connector. Different appliance, vent connector, or common vent types are possible. Consult Section 502.3. Figure 504.3.4Use of a Manifold Common Vent Connector
