Have you been avoiding the growing layer of dust that's developing on your ceiling vents? HVAC registers placed in the ceiling aren't exactly out of sight, but you may try to put them out of your mind ...

We have an en-suite shower room with no window. We have an extractor fan that lifts vertically through about 2m of a non-insulated ducting through the attic up to a roof-tile vent. There's no water ...