I just ran into this on a 159 unit condo complex. There is a kit separate from the fan it is a pressure/air-flow switch (like the one zapped just mentioned) which controls the fan. It has a little vinyl tube that pokes into the exhaust duct and senses when the dryer is running a to turn the booster fan of and on.
This kit is generally furnished and installed by the Mechanical contractor and final 120V terminations are by the EC. Also note there should be an access panel and either a switch or plug for the booster fan for service.
[This message has been edited by ITO (edited 12-08-2006).]
