Quote Originally Posted by Nicholas Lindan View Post
Positive pressure: the pressure in the darkroom is greater than the surroundings; air is pushed into the darkroom with a fan; it escapes through a free-flow exhaust and/or cracks and leaks; you need to place an air filter on the fan that blows air into the darkroom, dust exits the darkroom through the exhaust and the cracks and leaks and is blown from the room when you open the door - thus positive pressure is the way to go for a dust-free darkroom.

Negative pressure: the pressure in the darkroom is less than the surroundings; air is drawn from the darkroom with a fan; it enters through a free-flow vent and/or cracks and leaks in the room; you need to place air filters in the intake vent and the the cracks and leaks, however, you can never seal all the cracks and dust gets sucked into the darkroom every time you open the door - in general, negative pressure is a bad idea
I totally agree with what you're saying in that a positive pressure system is better, I just don't know how to create it. I'm currently designing my darkroom and the whole ventilation (and room temperature) system is the one I need most help with. I of course want to put an exhaust above and real close to the sink (thinking of using a Doran A1212 fan) and then using one or two louvers in the opposite wall so as not to create a vacuum. So I feel I'm creating a negative pressure environment which is what I don't want.

Any suggestions?