1. The air space above the dropped ceiling does not have free air communication with the space below.
2. Horizontal furnaces may be installed above dropped ceilings provided that:
a) The horizontal furnace is listed.
b) The horizontal furnace is installed in accordance with the manufacturer's instructions.
c) The space above the dropped ceilings is itself unconfined or directly communicates with a horizontally adjacent space through two permanent openings, such that the combined volume of the two spaces is unconfined. The two permanent openings shall be installed on the same wall. One opening shall be located within 12 inches of the top of the space above the dropped ceiling and the second opening shall be located within 12 inches of the bottom of the space above the dropped ceiling.
Note: The two openings shall not be installed in the dropped ceiling itself and the use of outdoor air to supply combustion air to spaces above dropped ceilings is not allowed.