Per the Mandatory Requirement of ASHRAE 90.1-2007 section 22.214.171.124.2 a gravity vent is required to be equipped with dampers that shut automatically.
Though the 90.1 does not have an exception if the system operates continuously. Of course this may be the interpretation since is states the damper shall close when the system is not in use.
Since the system is always in use, does it require the installation of gravity/motorized dampers?
The local energy code does spell out this exception.