Duct booster fans are used to carry the air through ducting from the extractor fan. This is when the ducting may need to be stretched over a long distance and the extractor fan is not strong enough to push the air all the way out. The ducting booster fan is then installed approximately half way to help carry the air further and out. Duct fans can also be used as extractors and inlet fans for smaller grows to bring air in and out of your grow space. This is not recommended for bigger grows as they will not be strong enough.