The following article is courtesy of A Buyer's Choice Home Inspections. Many thanks to Reuben & Jeremy Meyer for the good information.
Bathroom and kitchen fans play an important part in your home's overall air quality. These fans remove moisture and odours from your home, which improves the quality of the air and also decreases the level of humidity. High humidity can cause damage to building materials and can be a factor in causing mould growth. Mould may have an affect on your family's health.
Common Fan & Exhaust Systems
The most common types of fan systems today are impeller and blower. Impeller fans move air with blades similar to airplane propellers. Blower fans have a resemblance to hamster wheels, and provide better air movement than impeller fans.
Exhaust systems consist of an exhaust fan, ducting and an exterior hood. Some houses may have a central exhaust system where one fan will draw moisture and odours from several rooms of the house using a network of ducts.
Kitchen exhaust systems usually have the fan and motor in the exhaust hood. Other systems may use an in-line fan located in an exhaust duct, or an exterior fan. In-line and exterior exhaust systems are generally quieter than systems with indoor fans.
A heat recovery ventilator (HRV) exhausts moisture and odours, and is a self-contained system. The HRV system provides balanced air intake and exhaust and can be connected to several rooms with ducting.
Fan exhaust capacity is rated in litres per second (L/s) or cubic feet per minute (cfm). A typical bathroom should have a fan that draws 25 L/s or 50 cfm. Larger bathrooms with spas require larger fans, and should be installed as close as possible to the source of the moisture.