You may hear it referred to in the following terms: Duct Fan,Inline Duct Fan,Air Booster Fan,heat duct fan, etc. It is used primarily to help move warm or cold air throughout your home utilizing existing or planned ductwork. What are CFM"s?CFM"s are not a measurement of power but simply a measurement of capacity. CFM stands for Cubic Feet Per Minute and in this application it is defined as a rate of airflow. Most CFM ratings are measured in a controlled environment persuant to specific industry standards. Higher CFM ratings do not always mean a stronger fan it simply means there is more space for the air to flow through. If you see a 240 CFM 6" Fan and compare it to a 1290 CFM 14" you think automatically the 14" is much more powerful unit when in fact it reacts to restriction in much the same way as a 6 inch unit does. The fact the 14" unit has 14 inches of free air space is the main reason it has a much higher CFM rating.