In no case may the vent connector, for its entire length, from a gas designed appliance be smaller than the outlet vent collar provided by the manufacturer, unless otherwise recommended by the appliance, chimney, or vent manufacturer. Refer to ASHRAE Handbook – HVAC Systems and Equipment. (NFPA 54, A.12.11.3).
Two or more vent connectors shall not be joined unless the common connector, the manifold and the vents are sized properly to serve the appliances connected thereto and adequate draft is available to remove all products of combustion to the outdoors, or otherwise prohibited by the Administrative Authority. (NFPA 54, 126.96.36.199).
Each vent connector of a multiple venting system shall have the greatest possible rise consistent with the headroom available between the draft hood outlet or the vent collar and the point of interconnection to a manifold or to a common vent. (NFPA 54, 188.8.131.52).