When replacing or installing a new bathroom ventilation fan system, not only do you have different styles but you also have different sizes that accommodate certain square footage. Other considerations are noise levels (decibels) and air movement capacity. Without proper bathroom ventilation, mold can grow, humidity levels remain high, and odors can remain and build.
Your first step is to decide exactly what features you want in your ventilation fan. Not all are created equal. There is your basic fan which serves its purpose well. However, you can also select a fan that has a light source built in, a fan that has a heat lamp built in, or one that has both. Generally, these will all be ceiling mounted fans. However, there are fans that are made to be wall mounted. The important factor to consider with any fan is that it takes the hot, humid air from the bathroom and forces it outside.
There are two types of fans: axial flow fans and centrifugal extractor fans. Axial fans use blades or propellers to move a large volume of air. The centrifugal extractor fan does not use a propeller but uses a drum-type blade. They are more appropriate for ducted installations.
When you see the capacity of a fan it will be measured in L/s (which is liters per second) and cfm (this is cubic feet per minute). The larger the number, the more powerful the fan. Ventilation Fans are rated in "sones". The lower the sone rating, the quieter the fan. One sone is equal to the sound of a quiet refrigerator. A quiet ventilation fan is rated at 1.5 sones or less.
You need to know the area that is to be serviced by the ventilation system. Obviously, the larger the bathroom, the larger fan you need. This can be determined by finding the cubed footage of your bathroom if you work on standard measures of 12-inches to a foot. Cubing takes the height x the width x the length. However, may manufacturers make their numbers in meters so you may need to convert your feet into meters.
There are industry standards on how many times the air is exchanged per minute. For bathrooms, the standard is 10-15 air changes per minute. You can stay at the recommended levels or you can select a unit the goes above and beyond the recommendation. A larger room will need a larger fan that can exchange a greater amount of air in the recommended time.
Just as there are new high-tech solutions to mobile living, there are high-tech solutions to bathroom ventilation. There are models available that do not have to be turned on manually or with a timer. These units are sensitive to humidity and automatically turn on when the correct level is sensed and turn off when the humidity drops to a certain level. These units do save on electricity.