Reduce to 3-inch exhaust?

My Glowforge is in a basement room with no windows, so I use a filter. I have been trying to come up with a way to vent outdoors. Today a possible solution presented itself. Our water heater died, and we’re replacing it with a tankless, which will leave an unused 3-inch vent pipe to the roof (20’ long, maybe). Can I safely reduce to a 3-inch exhaust pipe? The Glowforge room is adjacent to the utility room, and I would, of course, use an in-line fan. Anyone have experience with such a setup?


Likely not enough. 4" is pushing it.

If you did go with 3", you would need a very powerful fan designed to operate with that much restriction.


Yeah, I was afraid of that.

Perhaps it can still be a solution, though, if I can replace the existing vent pipe with a larger one.


That would be a better choice, I think. A fan powerful enough to vent at least 400cfm thru a 3" duct would be pretty loud.

The window I vent thru requires 35’ of ducting. The first 25’ is 4", then I use a 6" fan and it goes out the window from there. Very efficient, and virtually silent.


After dealing with the noise of the filter for 2.5 years, “virtually silent” sounds really nice.


It makes one heck of a difference, for sure.


I agree 3" is pushing it, but failure is our teacher. I would give it a try and see how well the evacuation performs. That’s just me.
We replaced our furnace with a 97% efficient unit which abandoned the original furnace flue, so I employed it for my GF exhaust. A good 35 feet of 4" vent pipe, with a booster near the machine, and one at the flue going up. I would do what I had to, or to put it differently - ‘whatever it takes is what will happen’…


The big factor here is the larger the fan or impeller, the slower it can turn to move the same CFM.
That stock 4" exhaust fan has to scream at 13,000 RPM to achieve sufficient flow. The reduction in noise with a booster gives you an environment where you can speak and hear at normal levels - its a different world.



As it is, I use ear plugs.


Absolutely necessary. I was blown away by how quiet the machine is without that fan screaming.
Now I can hear the steppers singing.


i can actually be on phone calls right next to the GF while it’s running. i’ll keep my mic muted when i don’t talk, but i can hear everyone clearly. and i’m not using headphones, external speaker connected to my computer. don’t even have to turn it up. people can hear it in the background when i’m unmuted, but webex/zoom noice cancellation takes care of most of it. i just mute out of politeness.


There are 800CFM external blowers available. A 3" pipe will reduce the volume a lot but I would check out …



I’ll take a look at this. Thank you!

To move the same volume of air through the smaller vent, you will need to move air faster in the 3" portion of the pipe than in the 4" portion. One of the key factors you will have to deal with is the static pressure, i.e. as you try to push 4" of air into a 3" pipe the air will resist it. Inline blade fans are usually not great at handling higher static pressure. Look for a blower, one with a side-entry vent and more of a “squirrel cage” design. They generally handle the static pressure better.

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Okay, down I go, into a squirrel-cage-shaped rabbit hole.

Thank you for your help!


Spending a bit of time in the math-it hole I discovered that to move 300cfm through a 4" cylinder it has to go 40 mph but the same in a 3" cylinder has to go 70 mph. This does not account for any resistance or pressure difference but just the volume through the pipe.

The area difference is the 4" is 1.774 times the 3" opening


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