Product review (1 stars): Not ready for your home...
So the product itself seems well built and up to the task.  Instructions weren't great - more appropriate to an electrician than a house-dad.  I found it difficult to determine how to wire the current sensor switch to the motor as the box didn't include an extra wire to bring the load from the wall, through the sensor, to the fan.  I called Tjenrnlund's customer support who were very responsive but scary-honest in the response I got.  Speaking to a tech there I described my problem and he said the M4 wasn't designed to move lint as a dryer duct booster!  OMG!  That is how the device is marketed!  When I asked about how to wire it, it basically came down to needing to cut the electrical cord from the fan and loop in a wire that runs from the load wire plugged into the wall to the sensor, and back to the load wire that runs to the fan.  Sorry... but this is a FAIL.  At the very least they should have suppled wiring that allows the switch to signal the motor to turn on.  Their design engineers need to talk to their tech support and marketing folks and come up with clear and safe way to set up the automatic turn-on function.