Fan unit and air treatment system including the same

A fan unit connected to a predetermined unit through a duct, the fan unit comprising:

• a fan with a variable rotation speed;
• a casing that has an intake port and a blow-out port and houses the fan;
• a controller that acquires a rotation speed of a fan motor that drives the fan;
• and
• a detector that acquires a volume of air or a wind speed of the fan, wherein
• the controller controls the rotation speed of the fan motor,
• the controller determines whether or not a backflow in which air in the casing flows from the blow-out port toward the intake port occurs on a basis of the rotation speed and the volume of air or the wind speed, or on the basis of front-rear differential pressure that is a difference in air pressure between the intake port and the blow-out port of the casing, and
• the controller calculates the front-rear differential pressure from the rotation speed and the volume of air or the wind speed on the basis of a relational expression that derives, from two values of the rotation speed, the volume of air or the wind speed, and the front-rear differential pressure, a remaining one value, and determines that the backflow occurs when the front-rear differential pressure has a negative value.