PC lighthood fan operation

I have a 2 bank lighthood. The fan operates when the forward bank of lights are energized. I am thinking I would like to operate the rear bank as primary 10 hour bank and the forward bank as secondary 2 hour midday boost. This would mean that the fan would only be running during the 2 hour midday boost. Is it advisable for the fan to be operating during the entire photoperiod or could I save a little electricity and only run the fan while both banks are energized? I certainly do not want to cause premature ballast issues because I let them run too hot. But it seems like the manufacture would not allow the fan to be off when a single bank is lit if one bank of lighting could overheat the hood. Any thoughts/experience on/with this? Thanks...