How To Choose a Replacement Bathroom Fan
Bathroom ventilation fans serve a much more important function than just removing unpleasant odors. A bathroom develops a lot of moisture and if it isn't removed, it can lead to peeling paint, mold and wood rot. The bathroom fan's purpose is to remove all that excess moisture.
It is very important to have a fan that is properly sized for the bathroom. A minimum number of complete air exchanges is eight per hour, many prefer twelve per hour. An air exchange is the complete venting of all the air in a room and replacing it with make up air. The way to select the proper size fan is to first measure the volume of the bathroom. This is the width times the length times the height. So a six foot by nine foot bathroom with eight foot ceilings would be (6x9x8=432) 432 cubic feet. If you want eight complete air exchanges, the fan must move (432 cubic feet x 8 exchanges) 3,456 cubic of air every hour. Fans are rated for cubic feet per minute (CFM) so divide the total by 60 minutes for a CFM rating of 57.6. Select a fan with a rating no lower than this number. Higher volumes are fine, that just gives you more air exchanges per hour.
Old bathroom fans are notorious for being noisy. However, there are many models now that are much quieter. If you turn off a noisy fan, then the bathroom moisture will never be properly vented - so pick a fan you can put up with. Select a fan with a low noise rating. Noise ratings are based on "sones". The lower the sone number, the quieter the fan. An exhaust fan with a sone rating of 1 is comparable to the sound of a refrigerator running.
When selecting a bathroom fan, consider selecting a timer switch as well. This will allow you to leave the fan running after you leave the bath. The fan will operate for a preset time before turning off. This will help to ensure that the moist air is properly vented.
Finally, many professionals, as well as DIYers have been known to simply vent the moist air into the attic. That is a big no-no. Moist air builds up in the attic, accumulates in insulation, gathers on wood framing and leads to expensive damage down the road. A bathroom ventilation fan must be vented to the outdoors. Typically rigid ducting, like used for venting a clothes dryer, can be connected to a duct on the side of a home or through a roof vent.