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OptiClimate Plenumbox 3500 PRO3 and PRO4. The plenum box is an insulated case that can be installed at the back of the OptiClimate 3500 PRO3 or PRO4. With a plenum box it is possible to place the OptiClimate outside the room or to use the Dual Room System with a three-way-valve. The plenum box comes with four perforated holes that can be used to secure the air hose.