Originally Posted by dambo29
You definitely want an exhaust fan. A common approach for positive net pressure is to use two fans, one for exhaust and one for intake. The exhaust fan should move less air per minute than the intake fan. Likewise, you will want to make sure that the intake fan has a replaceable (or cleanable) filter on the air inlet, to prevent it blowing dust into the darkroom. For my part, I'm working for a period of time without the intake fan. I'll install one in a later upgrade. I'll need to be diligent about cleaning the darkroom, and may get a HEPA in-room air filter.
As for the fans, I have a Panasonic WhisperWall fan in the darkroom and upgraded a bathroom to a WhisperLite. These fans are fantastic -- they move air well yet are very quiet when in use.