In my research I found that I could get the highest volume of air moving with squirrel cage fans. These are really efficient, moving an incredible amount of air. The drawback of these turns out to be significant noise. I decided the trade off wasn't worthwhile. I bought a Doran darkroom fan on ePay. I just waited for it to become available. It draws out to a basement window (removed the window, replaced with sheet of 3/4" plywood with a 6" hole and a flange for dryer vent). This fan is above the sink at one end, connected to the outside via a 6" flexible duct. At the other end of the sink is a HEPA filter made for furnace returns, in a factory-made frame for that size filter. Bought both at a furnace supply place for under $75.
I tested the fan with a piece of paper over the dark-vents. It really didn't seem to "suck" with the kind of force I wanted or expected. It would clear a room of fumes, but it seemed to take a while.
I called Doran and they told me to try removing some of the grille vents. I opened the fan up and discovered that there's really overkill on the baffles in their grille. Like, there were I think 4 layers of baffle. I was venting into a dark area, so I just removed half of them. The result was a significant increase in airflow, and the air flow is now sufficient to suck and hold a piece of paper against the grille. Scientific? No decidedly not, but to me it indicates fumes are going the right way.
My darkroom is in a naturally dusty environment. Outside the door of my darkroom is an oil burning furnace and a gas clothing dryer. This arrangement of fan and HEPA filter has resulted in the cleanest and most enjoyable darkroom I've ever had. I guess if I was working regularly with really dangerous chemicals I might consider a beefier ventilation system, but for me (working with PtPd, pyro, basic b/w printing chems) this was very successful.
Good luck.
N
