7691 Ancillary items (3) 7692 Energy recovery systems (0) Systems to recover energy from air, for reuse in the installation within which the system occurs or for transfer to other installations 7695 Mechanical commissioning (0) Balancing, commissioning and performance testing of mechanical systems 7697 Cleaning, repairing and renovating ventilation and air-conditioning installations (0) Cleaning, repairing and renovating ventilation and air-conditioning installations 7698 Alteration work; removing and reconstructing ventilation and air-conditioning installations (0) Removing and reconstructing ventilation and air-conditioning installations as part of general demolition, reconstruction, alterations and additions