A modular air handler is adapted for installation in a racking assembly behind palletized product. The air handler operates to move air through the adjacent layers of palletized product upon placement of the palletized product in the adjacent space. The air handler can be deactivated in order to prevent unnecessary air flow when no palletized product is present in the adjacent space. A number of the modular air handlers may be provided for a racking assembly, such that individual pallet bays may be activated or deactivated as palletized product is deposited or withdrawn from the various pallet bays of the rack.