Swimming pools are harsh environments. The swimming pool ventilation system’s design must provide gentle air movement to avoid unpleasant draughts. Wet bodies are much more sensitive to air movement. Too much velocity across the swimming pool surface causes evaporation.
Traditional rigid duct systems for indoor swimming pool ventilation are quite large due to the air volumes needed to heat swimming pools. They also need to be protected and painted to avoid corrosion, a job that may need doing more than once over the life of the ducting.
Thankfully, all these issues can be avoided by using fabric ducts.