Has anyone used a fake (not really attached to the gutter) gutter downspout as an exhaust run to get the laser exhaust to the roof line? My wife does not want to see a 4 inch PVC pipe run up the side of our house, so I am looking for other options.
I think the gotcha will be too much back pressure due to flow resistance through that long skinny tube.
It’s certainly worth trying, but get a wind speed meter to verify the before-and-after flow through the downspout before you nail it to your siding.
I measured the flow through a length of 6 inch duct:
You’d want a similar measurement with & without the downspout on the blower outlet.
I am hoping the updraft will help the flow, but I guess I won’t know till I try.
When I set up my chimney vent, I discovered downflow was a thing: install a blast gate to close off the pipe when you’re not using it!
Are you venting through a window and then up to the roof-line? Why not just vent straight through the window?
I have a dryer vent through the wall that I am using now.
I’m curious… why do you want to vent up when you have a perfectly good vent straight through the wall? Adding additional vent pipe to go up just adds flow restrictions to your ventilation.
If the existing dryer vent you use is ugly, there are lots of other modestly priced dryer vents, including some that could easily be painted to match the outside wall color. Your local “Big Box” hardware stores will have several different vents in stock, and many have online shopping that will provide even more choices.
Don’t make it more complicated than you need.
My vent looks perfect, it maybe one of the best looking dryer vents ever installed. The problem is the smell of burning wood. My vent is under our deck and the order drifts up to the deck and my wife wants the smell to go away. (And no, I am not going to spend a few thousand dollars on a HEPA air filter)
Easy solution to that.
Get one of them fake fire pits for the deck.
Or a real one would be better. 
I must agree… that is a good reason to extend the vent to a less troublesome location. If you are using custom rectangular duct, just make sure you have “equivalent” or better cross sections to minimize flow restrictions. There are several resources out there to help with this… here is one you might try out: Equivalent Diameter - Rectangular vs. Circular HVAC Ducts
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It ends with divorce,
then the smell goes away.