I have a small 13'x23' shop where I make small crafty items that require a lot of sanding. I have a small sanding station with a 4" connection to my 2HP dust collector that is in a different building. I upgraded the 4" to 6" to match my 6" main duct. and saw a good improvement to my dust collection at this location.
Now that it is over 100° outside my air conditioners are working to keep me comfortable in my shop. Exhausting all this air warms up my shop because the air is leaking into the shop at the doors and windows.
Today I installed a 6" make up air duct that comes from outside my shop to this sanding station. Now when I run the dust collector for this operation the air exhausted by is is made up by the new make-up air duct. Sawdust is drawn away just fine but this system is not warming my shop up as much now because there is less inward leakage around my doors and windows. It worked well, so I put in a second 6" make-up air duct at my stationary belt sander. I have blast gates on all these ducts so I can control the airflow as needed.
I penetrated my outside wall with the 6" metal duct and I made two 6" "dryer vents" from sheet metal that are mounted on the outside of my shop. (I am a retired sheet metal worker so this was simple for me)