7631 Ductwork (2) Ductwork for mechanical systems fabricated generally from sheet metal, including related ancillary items 7632 Ductwork insulation (2) Ductwork acoustic and thermal insulation, and sheathing of ductwork 7633 Attenuators and acoustic louvres (9) Duct mounted sound attenuators and wall mounted acoustic louvres 7634 Air grilles (11) Fixed and adjustable grilles and diffusers used in air conditioning and ventilation systems 7635 Variable air volume terminals (0) Variable air volume (VAV) terminals (VAV boxes) including; series and parallel fan assisted types and non-fan, pressure independent boxes