Fans for gas extractors; Fans for ventilating; Ventilating fans; Air filter installations; Air filtering installations; Apparatus for filtering breathing air; Filtering installations for air; Filters for use with apparatus for ventilating; Apparatus for exhausting gases (ventilation); Extractors (ventilation or air conditioning); Ventilation (air conditioning) installations for buildings; Heat exchangers for the removal of exhaust gases; Ventilators for heat exchangers