I have two fans in my workshop: a desktop one with a 6" carbon activated filter, and a box fan with a MERV 13 HVAC filter clamped to the back. Between them they probably do a reasonable job at keeping flux particulates low in the air (though, I haven't measured).

However, there's a few negatives in the setup:
- with both fans running, it's noisy
- the desktop fan takes up space on my work table, and has to be pretty close to the soldering to draw the flux smoke
- the box fan has to be on a stool to keep air at tabletop height moving, and therefore is clutter

First world problems, for sure, but the more time I spend at the soldering the bench the more I've been thinking about installing an exhaust duct venting air to the outside to replace them. If the fan was on the outside, the noise would likely be lower. The duct could drop down over the table, freeing up table space.

Has anybody done this? What does your setup look like if so? Aside from duct size, type of fan, etc I'm also wondering how to prevent cold air/bugs making their way back through the duct when I'm not running it - maybe there's some way to automatically close it when not in use. Any ideas appreciated.

Try a bathroom ceiling vent fan. You could duct it to a 4" line and let it hang over your solder station. The fan would not be outside, but you could duct it to the nearest outside wall and most of them have a one way flapper door to prevent backdrafts and critters from getting in. If not, you could install a dryer termination on the outside of the wall and just tie into that. Just my .02 cents.

Oh right, dryer terminations would totally work. I'll dig around and see if I can find one that wouldn't lead to too much thermal loss. Thanks