A furnace plenum supplied as a part of the air-conditioning appliance
shall be installed in accordance with the manufacturer's
installation instructions. Where a furnace plenum is not
supplied with the appliance, fabrication and installation
instructions provided by the manufacturer shall be followed.
The method of connecting supply and return ducts
shall facilitate circulation of air. Where the air conditioner
is installed within a c1oset, the air circulated by the appliance
shall be handled by ducts that are scaled to the casing
of the appliance and that separate the circulating air from the combustion and ventilation air. [NFPA 54:10.2.5]