Hello. There are three ways to provide combustion air for gas appliances. 1) All air from outside. 2) All air from inside. 3) Combination of air from inside and outside.

Direct vent furnaces will use option #1, via a pipe from the appliance to an appropriate location outside. Older furnaces (and most gas fired water heaters) will generally use either #2 or #3. If the appliance is installed in an unenclosed space (see definition - deals with permanent openings to the exterior), generally you would need about 50 cubic feet of air per 1,000 BTU of gas appliance to qualify as an Unenclosed space. Usually this is not the case, however most basements even have at least one window that would qualify as a permanent opening to the exterior. The attic would also generally qualify as an area that has a permanent opening to the exterior. If the gas appliance is installed in a room adjacent to the space (attic or basement), and the adjacent space has permanent openings to the exterior, then you would generally need two openings provided for the room where the appliances are installed. In your case there is a duct (opening) into the attic. This would be an acceptable way to provide make up air for combustion, however you need a second opening (one within 12 inches of the floor and one within 12 inches of the ceiling). Each would need to be a minimum of 100 square inches. In your case you would need an opening that extends from the attic penetration down to within 12 inches of the floor.

This is my take on your situation, hope it helps. Please refer to Chapter 17 of the IRC for requirements.

Regards, Bob