Do you plug these fans into receptacles or are they hard wired? Box fans? What do you mean? The best way to control any motor loads is with a motor starter. You can get very good quality and very inexpensive motor starters. Try
www.wegelectric.com Search for "CW" miniature contactors. Under $50.00 for one.
Just wire the sensor to engage the starter. The starter is just like a switch. If you go with a 3 phase starter you will have three NO contacts assigned for the motor and one NO relay you can also use for a motor under 10 amp. This way you only need one. They are designed to start 3 phase motors, but you can use each individual pole for each individual fan. However, all the fans will come on at one time. If you need independent control, you need a starter for each motor. The neutral does not need to be switched. Someone here with drawing software can post a drawing for you, how to hook it up.
These starters/contactors are just as cheap as relays. Relays work the same way and can be used if you like.