Over the weekend we had a leaking condensation pan in our furnace that left about 2" of water on the concrete floor under the furnace. The furnace and water heater are located in a closet in the garage just on the other side of our laundry room. The condensation pan was replaced about a year ago due to age/wear, and the drain line became plugged thus backing up the condensation pan causing it to overflow through the furnace and onto the concrete floor below which is the return plenum floor. The water eventually seeped into our laundry room thus causing us to investigate and we found a lot of moisture and some mold growing inside of the return air plenum. We removed a built-in cabinet that was above the plenum box and demo'ed the plenum box and placed a temporary filter at the inlet to the remaining return plenum at the base of the furnace. I would prefer not to rebuild the return plenum box, and would like to place a return register at the opening to the furnace and place two register grills at the original plenum opening in the opposite wall to ensure ample airflow, however I am unsure how close the opening of the return plenum can/should be to the furnace.