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| BTW, there are two narrow, parallel ducts in the upper left corner of the furnace closet which connect to nothing either in the furnace closet or the attic. Any idea what this pair of slim ducts was intended for? |
They are for combustion air and ventilation of the closet. They need to communicate with the great outdoors which means that the walls that make up this closet need to be insulated and the door needs to be weatherstripped.
As for the filter, the correct way would be to raise the furnace and install a filter rack between furnace and the floor. It is possible that you have a filter in the return air grille.