The first thing you should consider is a timer instead of a switch. The success of your fan relies on a two-fold approach. The Home Ventilating Institute recommends that your fan be capable of achieving 8 air changes per hour, with the exhaust of air continuing for 20 minutes after use of the bathroom. Using a timer will help you achieve the extra run time without having to remember to come back and turn it off. Plus with the new quiet fans available it isn't uncommon to forget to turn a bathroom fan off now because we don't have that load roar to remind us it is on. To achieve the air exchanges we need to correctly size the fan.
1.) First find the volume of the bathroom. Volume = length x width x height
2.) Find the CFM (cubic feet per minute). Volume / 7.5
3.) This CFM is the minimum airflow required to achieve 8 air exchanges per hour.
Example: Volume = 10 x 6 x 8 = 480
CFM = 480/7.5 = 64
In this example you would only purchase a fan that has a CFM listed on the box greater than 64.
Next comes noise. Choose your fan based on how quiet you want it. Personally I want the quietest fan I can afford. Ideally you don't even want to know it is on. However, the price goes up as the noise levels go down, so you have to keep your budget in consideration.
Bathroom fans are measured by SONES. 4.0 Sones is the sound of normal television (Ridiculously loud for a fan) 3.0 Sones is office noise (Still very loud for a fan) 1.0 Sone is the sound of a refrigerator and 0.5 Sone is the sound of rustling leaves. A very quiet bathroom fan that will just make a gentle whoosh is a fan at 1.0 Sone or below.
