Taken right from Field Controls' website.
Q. Can a power venter be mounted under a deck?
A. Generally this practice will be accepted in most states except Massachusetts. Installer should check with local code officials before installation. The deck must be open on all three sides and the clearance from the top of the venter to the bottom of the deck should be at least 12 inches. When venting appliances that operate in the summer this may not be a good practice since people enjoying the deck may smell fumes.