My Glowforge sets about 6" to 8" away from the outside 4" exhaust vent in a straight line. I’m thinking about using one of the AC Infinity fans instead of the internal Glowforge fan. Since there isn’t a lot of distance or bends in the vent hose, can I get away with using the 4" AC Infinity fan? Any input appreciated.
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I know several people here are using the 4" AC Infinity fan. And your setup sounds ideal for it.
I had the 4" fan and returned it to get the 6" one. But I have two 90 degree turns in my setup and I just didn’t feel like the 190 CFM was enough to pull out the smoke to my liking. The 6" fan was rated 390 CFM and I continued with the 6" exhaust out the window.
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Not in my experience. A 4" fan is not sufficient for use alone if you still have your exhaust fan installed.
You need more airflow (CFM) than the built-in fan if it has not been removed, as it presents quite an obstruction.
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I use a 4" inline fan, and my exhaust fan has not been removed, and it’s working great for me; no smoke in the room when I cut, and very minimal smell. My setup is similar to yours; I had to put two 90º bends in the hose to fit the inline fan behind the GF, but it doesn’t seem to be a problem.
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@elfyguy, I use a 4" fan, and this is the first I have heard suggesting removal of the internal fan. Are you saying the internal fan running at low speed presents an obstruction? Or did you remove yours so you don’t have to run it at low speed? Just curious as mine works fine as is so I won’t likely make the removal.
There is no speed control for the exhaust fan.
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I ran a 4" fan for two years, in addition to - not instead of - the built-in fan. When the built-in fan died, I immediately noticed significant accumulation of debris inside the machine. I then removed the broken fan, and also cut out the grill, to allow better airflow, and that helped, but there was still more debris accumulating than had been before.
I switched to a 6" / 390CFM fan, and the machine now stays clean like it used to.
So, in my experience, the 4" alone is not sufficient. Your mileage may vary. I would never encourage anyone to remove the built-in fan, broke or not, because support may refuse to repair the machine down the road when necessary.
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Thanks for the followup, elfyguy. Very interesting. The external fan is new for me, so I’ll be sure to keep an eye out for buildup.
@beerfaced, to reduce the speed of the internal fan, click the More icon (three stacked dots) and choose “Glowforge air filter attached.”
That turns the fan off. I don’t think it reduces its speed.
If it is still moving it is only because the external fan is pull air past it.
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@bill.m.davis, I’m not saying you are mistaken (because I have not verified one way or the other) but here is what the manual says happens when you select “Glowforge air filter attached”:
“The fans will now run at a lower speed during every print on this machine, to match the Compact Filter’s internal fans.”
That might be true. I was just told that turning on that option disabled the internal fan because the Filter has its own fan with its own speed control and they didn’t want the two fans to be fighting one another.
So since the control on the filter cannot talk to the 
it would make sense for it to disengage so the filter can control the fan speed independently.
4" working fine here with 2 bends in a 5’ run.
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Just to clarify. I don’t intend to remove the internal fan or grate on the back as this is a new machine. I was just wondering if the 4" fan would be good for the short and straight distance I will have to vent.
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