Bathroom Exhaust Fans need periodic cleaning to maintain efficiency. When the fan blades are as dirty as the fan pictured below it may spin and make noise but it moves very little air. This can lead to condensation damage to the walls and ceiling of the bathroom as well as condensation on the windows of the unit.
Dirty fans have also lead to ceiling stains caused by the moist bathroom air condensing in the duct due to the low air speed. This condensed water can then leak from the duct and through the concrete of the ceiling to cause a stain.
This risk can be avoided by keeping the fan blades clean and running the exhaust fan while showering and for 10-15 minutes after showering.
Cleaning the fan blades is best done with a vacuum, a small brush like an old toothbrush and a flat head screwdriver to scrape between the blades. The blades should be cleaned to like new condition to be most efficient.
Ceiling stains can also be caused by dirty dryer lint traps as they also slow the flow of moist air through the ducts. Check under the maintenance tab for information on cleaning them.