Need to add vVentilation fan

[adam18]

Good afternoon...

I have the blower unit from a 3T residential a/c system, it appears to be a straight forward synchronous motor with three speed taps...

We have a laser cutter/engraver (not a Glowforge, this is much larger and more powerful), which can often have smoke or fumes. Currently, I'm using several axial fans extracting from a crude manifold (a wooden box), then there's a 12" metal duct running about 25' from the fan box to the laser. This just exhausts it out at the front of the open garage door. I would like to build a manifold box for the 3T blower and locate it in the attic, mounting the fan's enclosure to the trusses right at the roof. Then I plan on reworking two metal gooseneck style roof vents to essentially enlarge each of their intakes and make them have one common input plenum on the attic side, with two **normal looking** vents outside. I realize this will be a pretty substantial restriction on the output side, but I'm hoping it will work, especially if I use larger duct on the inlet side to help minimize loss. I'm looking at maybe 24" or 30" spring-coiled uninsulated ventilation hose, such as this...

https://www.amazon.com/gp/product/B07Y7YRM8H/ref=ox_sc_saved_image_9?smid=A287553B1S88XD&th=1

So am I on the right track??? Would I be better suited to install the fan at the laser and have the duct at a positive pressure? Big thing to keep in mind is this is a very long run, from the far corner of the garage, to the back far corner of the upstairs. I have not measured up in the attic with a string, but I'm guessing I'll need 50', hopefully no more.

I have larger fans, I have two blowers from 5T units, one is ECM that I'd rather not have to mess with trying to power, and the other one sounds like it has a failing/failed bearing. I could probably get my hands on a third 5T blower pulled from a working unit if you guys think the one from the 3T unit on high isn't enough for 50' ducting at suction.

I know there are flammability requirements that should be followed as well... I'll worry about that, but this would only operate while someone is using the laser.

 

[dbill74]

We really need more information to give sound advice here. Fan manufacturer and model number for starters.
That fan will be overkill. It is designed to deliver 900-1200 CFM of air when you need maybe 200-300 CFM.

It sounds like and I"m assuming here, that your gooseneck roof vents have are not connected to anything in the house/attic. If this is the case

Do NOT, I repeat, do NOT touch them or connect to them!!

These vents are critical in maintaining air flow and temperature in the attic and your house. Connecting to these will cause your attic temperature to increase significantly in the summer or decrease in winter.

Adding a new vent through the roof (not recommended) or wall (recommended) is the better option.

To use a spring-coiled hose on the inlet side of a fan is an "Oh Hell NO!" Those hoses are not designed to handle the negative pressures with being on the suction side of a fan and will implode. Use it on the discharge/positive side of the fan is fine.

Flammability is only an issue if the ducting is going to be transporting any kind of flammable fume such as grease from a commercial kitchen hood.

You are kind of on the right track, but your plans needs work. If you really think you need an industrial sized system, hire a PE to design you a system.

Amazon has some reasonably priced fume extraction kits which includes fans and in some cases hoods.

 

[dbill74]

If you really want to DIY it; go to your local Home Depot or Lowes, get a toilet exhaust fan and mount it close to your laser engraver. Then some 4" dryer exhaust duct and go to a dryer wall louver with a backdraft damper.