A burn-in oven is provided with a plurality of spaced, stacked burn-in-boards, each with a plurality of individual circuits being tested under heated conditions, and a plurality of valve trays positioned between two burn-in-boards to form a heat exchange compartment below the valve tray. Each valve tray has a plenum formed above it to provide a separate chamber that is a source of cooling air. Each valve tray has a plurality of valves, one over each of a number openings in the tray. Each opening overlies an integrated circuits or device under test on the burn-in-board below the valve tray. The valves control the flow of air for cooling the integrated circuits. The flow of air through the valves is the only path for cooling airflow to the integrated circuits on the burn-in-boards.