You should also consider the ACH, the air exchange per hour, that is: an air flow rate sufficient to remove the air volume in a given room at a specified number of times each hour. Your bathroom exhaust fan should be sized proper- ly to completely change out the air in the bathroom at the rate of 11- 15 changes in one hour. FAN LOCATION Locate your bathroom fan to ensure maximum moist air extraction. Steam rises, so the shower ceiling is always a good spot for a shower fan. If that location is not feasible then place the fan just outside the shower. In cathedral ceilings sometimes a sofit built over the shower can accommodate the fan or a high- mounted wall fan can be installed.
INSTALL THE FAN
PROPERLY Bath fan ducting can negatively affect fan performance. Un-insulated, undersized, or droopy flex ducting and excessive length or excessive use of elbows all can negatively affect the fan's rated airflow. In order to keep the fan effective, try to lay out your installation with minimal turns. The best duct pipe to use is solid smooth aluminum. Solid duct pipe
Connect duct elbows to the fan as needed to establish a straight duct run to the exterior of the home.