Technical Booklet K Ventilation·Page 47·3.19
Location of Extract Ventilators in Rooms
Extract ventilators should be positioned as high as possible, ideally within 400 mm of the ceiling, to effectively remove pollutants and water vapour from the occupant breathing zone. For PSV systems, extract terminals must be located in the ceiling.
Extract ventilators should be located as high as practicable and preferably within 400 mm of the ceiling. This will tend to remove pollutants from the breathing zone of the occupants as well as increase the effectiveness of extracting buoyant pollutants and water vapour. For PSV, extract terminals should be located in the ceiling of the room.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet K - Ventilation.pdf