Laser Exhaust Ducting Questions

[Mayo Pardo]

Hello everyone.
I'm new to laser cutting/engraving and will be receiving my laser next week, or the week after depending on the trucking schedule.

I intend to attach metal ducting in order to blow the smoke and fumes outside.
The laser will be installed in my heated garage and I have two options for the location of the ducting.
The natural gas garage heater hangs from the ceiling and is ducted through the roof with double wall round ductwork.

My first idea was to install a Y in that duct and attach the laser ducting to it.
My brother in law who is a home builder advised against this.
I can see the potential for fumes getting blown into the garage through the heater if the laser is on and the heater is not on, even if using a Y connection. But would that really happen?

My second option is to drill through the wall next to the back door of the garage and run the duct to and through that hole. The drawback to this is the location happens to be within a couple feet of my kitchen window, which may be open at times. This would allow fumes into the house through the window. It is also located within 6 feet of our central air conditioner. If the air conditioner draws the fumes through the cooling fins, could this have a deteriorating effect on the air conditioner fins or coils?

The materials I intend to engrave or cut would be acrylic, paper, cardboard, matte board, plywood, coated engraving stock, leather.

Ideally the outside end of the duct should be routed above the roof however there are eaves which project about two feet from the wall where the duct would exit the garage wall. In order to get the duct higher, it would require a 90 degree elbow. I didn't want to put the duct straight up and make another hole in the garage roof.

If this is all unclear I can probably post a photo of the exterior of the area in question.

Thanks in advance for any suggestions!

Mayo

[Rich Harman]

Bad idea sharing the duct with the gas heater. Almost certainly against code.

Make sure that in addition to ducting the laser exhaust outside that you have a way for the replacement air to get into your garage. If not it will be sucking the heater's exhaust backwards into your garage.

[Mayo Pardo]

Oh thank you - that's another good point I hadn't considered. That the laser may draw in the heater exhaust.
And the planned location of the laser is close to the heater so I may have to rethink where I planned to put it.

[Gary Hair]

I'm with Rich, a Y in the heater is a bad idea and probably against code. Your second option doesn't seem much better. Is there any other place to put the exhaust that won't allow it to be drawn back in the house? One thing I have to point out - "blowing" the exhaust out vs "sucking" it out is a big deal - you want the exhaust fan/blower at the farthest point away from the laser, preferably the end of the run, so that it sucks the exhaust out. If you have it blowing then any leak in the ducting will leak the fumes into your work area - if it is sucking out the exhaust then a leak will simply introduce air into the exhaust.

[Mayo Pardo]

Thank you for the responses - placing the blower at the farthest point is another thing I hadn't considered.
I have another possible location if I move some stuff around. This is at the opposite wall and I could duct it through a panel I place under the window. This has the disadvantage of pointing at the neighboring (residential) property however that is the garage side of their house.GarageLayout.pdf

[Bill George]

The suction side of the fan should have the shorter run of duct. My fan is hooked to the machine with flex duct because you will want to move it around from time to time. The the duct should have 3 sheet metal screws in each connection and wrapped with metal foil tape. The crimped end of the pipe should be going away from the fan.
I'd either put the fan in the attic or up next to the ceiling, flex up to it, hard pipe across the ceiling and out under the overhang.

[Joe Pelonio]

You have to really think this one through, and if necessary, re-arrange the location of the laser and the exhaust or use a stronger fan and longer run. In a commercial building I had several complaints from neighbors involving visits from the fire departments due to the aromas emitted from cutting plastics. Hera t my home, I exhaust to the side facing only a street (corner lot) and the nearest neighbor's home has the garage at that end, so no complaints in 5 years. I do also go outside at tomes while running on windy days, because it can blow the fumes another direction. While the fire department was on my side in the leased space, they may not be as cooperative here. You definitely don't want it to exhaust near any window that may be open. If you have suitable location to vent, you may have to consider an air scrubbing system.

[Dave Sheldrake]

+1 for Rich and Gary, sharing a gas heater duct is a criminal offence here in the UK so that will give you an idea of how dangerous it could be.

Double venting works, if you have say a 6" pipe on the exhaust then two 4 inch pipes from the outside to the front of the laser can provide input air without sucking all the heat out of your workspace, it also creates a good crossflow and keeps the fumes heading one direction.

cheers

Dave

[Gary Hair]

The suction side of the fan should have the shorter run of duct. My fan is hooked to the machine with flex duct because you will want to move it around from time to time. The the duct should have 3 sheet metal screws in each connection and wrapped with metal foil tape. The crimped end of the pipe should be going away from the fan.
I'd either put the fan in the attic or up next to the ceiling, flex up to it, hard pipe across the ceiling and out under the overhang.

Bill - what happens when (not if, when) your duct leaks? The exhaust is forced through the leak into whatever area the leak happens to be located. If you have a slight leak you may not smell it but there may be enough noxious fumes to cause harm. However, if you have the fan at the farthest point then any leaks will simply draw in air and reduce the efficiency of the fan - much rather that than force exhaust into my work area.

[Bill George]

Bill - what happens when (not if, when) your duct leaks? The exhaust is forced through the leak into whatever area the leak happens to be located. If you have a slight leak you may not smell it but there may be enough noxious fumes to cause harm. However, if you have the fan at the farthest point then any leaks will simply draw in air and reduce the efficiency of the fan - much rather that than force exhaust into my work area.

A properly installed duct system has No leaks. Crimped end of pipe pointed in the direction the air is flowing, 3 screws in each connection or fitting, commercial jobs the sheet metal installer used something called hard cast to seal the jioints if spec'd by the engineer . Residential people used the foil tape for sealing. Inducted draft water heaters and some furnaces and boilers use a positive venting system.
Both fans and pumps are more efficient on the pressure side, than trying to pull air through a long duct system. Sure it can be done, but the losses on the low side add up quickly.

I would assume and I may be wrong since your not supposed to be cutting PVC or the like in a laser, the fumes you do exhaust are just off gassing from plastics and other materials . If they will kill someone perhaps something other than directly venting outside should be considered.

[Keith Colson]

Hi Mayo my laser goes right up against the wall in the garage like your drawing, to save space my pipes are not flexible and my laser cutter is just a few inches from the wall (for air flow) I put a 1.1kw centrifugal blower outside with a VSD on it. It all works really well. I have more ducting to fit so I can put my outlet anywhere. Here is a picture of my set up. Make sure the pipes that go through the wall go down hill so any outside moisture is not pulled in to the laser cutter, I still need to add a rain shield but its holding up well in winter weather. Having the VSD is great because after a job I can lower the speed and leave it on to keep the smells out of the garage.

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[AL Ursich]

I bought direct vent heaters for the wood shop and laser shop. My theory was that I could back draft not only the exhaust but suck the gas inside causing a flame out and big BOOM problems. Sometimes blow the wood shop out into a collector outside and the laser engraver out too, that is why the Direct Vent LP Gas. Not so worried about the drop in temp.

So is your space heater a direct vent? Double pipe?

AL

[Gary Hair]

A properly installed duct system has No leaks.

that's the problem...

[Bill George]

And frankly on a small job like this about any method would work just fine.

[Mayo Pardo]

In further thinking about the vent, the option to exhaust fumes out the window at the side of the garage, towards the neighbors house has potential for complaints when winds are blowing east. The vent would be only 4 feet from the ground.

I have discovered a 3rd option to put the vent through the ceiling of the garage but instead of cutting a hole in the roof, I can cut the hole in the vertical side of the gable. This would still be facing the neighbors home however being up that high I think would help dissipate the fumes somewhat before they could reach the neighbors windows.

This will require locating the laser more towards the side to side center of the garage, since that's in a direct line
vertically where the end of the gable is. I'll just have to find a new place for a bunch of saved junk which I should have thrown away 5 years ago! There is electric in the ceiling in this area from former lighting, so a new circuit shouldn't be a problem in the existing conduit.

Thanks again for everyone's thoughts and comments. Hopefully next week I will have the laser.

Al - my heater is a natural gas heater which vents through a double wall pipe going through the ceiling and roof.
It's not a direct vent heater.