Home workshop paint fumes: how to spec a DIY vent system?

(Topic ID: 235515)

Home workshop paint fumes: how to spec a DIY vent system?


By goingincirclez

13 days ago



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  • 5 posts
  • 3 Pinsiders participating
  • Latest reply 13 days ago by goingincirclez
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    #1 13 days ago

    I mostly spray paint, clear, etc in a basement workshop. The setup is almost ideal but has no direct outside ventilation. However, the walkout is 14 feet away though an adjoining room and doorway. Whenever possible I open this for ventilation... and/or spray when family is gone and/or just before leaving the house... but that's neither purely ideal nor always possible. More often, I can't even vent the door because of weather (temp / humidity), so I have to spray indoors with it shut. This is not good and I don't like doing it for all the known hazardous reasons... but the other option is waiting for weather to cooperate with schedules, meaning nothing would EVER get done.

    The other day I was following utility and plumbing runs in the workshop's unfinished ceiling and realized, DUH! There's a wall above that should be hollow (need to confirm, obviously) all the way the attic. In theory I could drop a pipe from the attic - hidden in that wall - down to the workshop ceiling, and put something like a bathroom fan there to suck fumes out of the workshop and blow them through the pipe and into the attic (and preferably on from there through a roof vent). It seems so stupid simple and even affordable (under $100): TOO easy. I'm an idiot for not seeing it sooner. So there must be something I am missing.

    The constraint will be the size of the pipe I can fit in the wall; TBD but expect typical framing clearance, so let's say only 3 inches diameter. Is that even enough? How should I spec a fan to push X volume through a run of say 10 feet? Or 16+ all the way to the roof?

    Are standard bathroom fans rated for volatile fumes (don't want to cause a fire), cuz it seems like most "hazmat" fans I've seen are larger than a ceiling vent.

    I'm sure there are builder's formulas, but those usually deal with water vapor or conditioned air. When it comes to workshop fumes which may be denser and/or flammable, "houses" don't spec for that, but I'm not scaling to industrial gear either. Surely some of you can lend some advice. Last thing I want to do is rig up a DIY solution like I should be proud for thinking of it, only to discover it doesn't work well or is somehow more dangerous than what I was trying to solve.

    #2 13 days ago
    Quoted from goingincirclez:

    For the last couple years I've been (barely) getting away with spraying paint, clear, etc in a basement workshop. The room itself is nearly as ideal as any inherited random basement room turned workshop could be, except it has no direct outside ventilation. However, the basement walkout is 14 feet away though an adjoining room and doorway. Whenever possible I open this for ventilation... and/or do my spraying when family is gone and/or just before leaving the house... etc... but that's not purely ideal nor even always possible. I don't like doing it for all the known hazardous reasons. Yet more often than I'd like I can't even vent the door because of weather temp or humidity, and have to spray indoors with it shut. This is not good! Yet my only other option would be waiting for weather / outdoors to cooperate with too many other schedules, meaning nothing would EVER get done.
    The other day I was considering utility and plumbing runs in the workshop's unfinished ceiling and realized, DUH! There's a wall directly above that should be mostly hollow (need to confirm, obviously) that runs to the attic. So in theory I could drop a pipe from the attic - hidden in that wall - down into into my workshop ceiling. Put something like a bathroom fan there, to suck fumes out of the workshop and blow them through the pipe and into the attic (and preferably from there on through a roof vent).
    No demo, no goofy curves.... it almost seems so stupid simple, and even affordable (I could do this for under $100): TOO easy. I'm an idiot for not seeing it sooner. So there must be something I am missing.
    I know the constraint will be the size of the pipe I can hide in the wall: TBD but I expect typical framing and other clearance, so let's say only 3 inches diameter. Is that even enough? How should I spec a fan to push X volume through a run of say 10 feet? Or 16+ all the way to the roof? Are standard bathroom fans rated for volatile fumes (don't want to cause a fire)... cuz it seems like most "hazmat" vent fans I've seen are larger than a ceiling vent.
    I'm sure there are simple builder's formulas, but those are usually dealing with simple water vapor or conditioned air. When it comes to workshop fumes, "houses" don't spec for that, but I'm not scaling to conventional industrial gear either.
    Surely some of you can lend some advice. Last thing I want to do is rig up some DIY solution that looks like I should be proud for thinking of it, only to discover it doesn't work well or is somehow more dangerous than what I was trying to solve.

    Not sure what scale you are looking for but there are Several low cost Mini paint booths on Amazon (or other) that work rather well for small projects. Most are portable and have integrated lighting and ventilation to some extent. Have a look you may like them.

    #3 13 days ago

    How "big" is "mini"? I'm familiar with mini paint booths for model cars and trains and the like... but by the time you scale to pinball playfields and cabinets, you're out of a "large booth" and into a "mini room".

    Plus those little booths need to vent into something. I know they have filters and such but again: that seems to work for small items. A filtered booth for a pinball cabinet... seems a bit unwieldy, may as well try to spec the room the "booth" is in if possible?

    #4 13 days ago

    Depending on the length of the run of pipe, you may reach a point where regardless of what you do, the size of the pipe will limit the amount of air that can be moved. That amount of air may be less than what is required to adequately ventilate the room safely. I doubt you can get much more than a two inch piece of pipe from the basement up to through the roof, so some math will be in order.

    You need to figure out how much volume of air you need to replace, and at what pressure will be required to get that much air through the size of pipe you are using. Don't forget to calculate in the pressure drops due to elbows, bends, etc. I know for bathrooms, you need to exchange the air eight times per hour, or about 1CFM per square foot or area. I am not sure of the number of times you would need to exchange the air while painting.

    To me, the bottom line is it will help, but you will likely find your suggested method inadequate.

    #5 13 days ago

    Thanks for the insight. I wondered how CFM translated to actual SQFT in practical use, but comparing specs it seems like a rough 1 to 1 relationship is rule of thumb, like you said. The room itself is about 12' by 22', but some semi-permanent fixtures and shelving reduce that somewhat. So ~250 SQFT is the total room size - well beyond a typical kitchen/bath vent, biggest I saw was ~140 CFM.

    However, I think I still have a few factors in my favor to make a solution:

    1- I inspected the void wall above the room and by pure rare coincidental good fortune, there is indeed a straight, unobstructed run (void) direct to the attic (I can see the attic floor from the basement) that would easily accept a 3" pipe!

    2 - That location is within a few feet of my typical working area - almost directly overhead to offset 4 feet.

    3 - The run would be direct: I would only need one elbow immediately after the fan outlet (side), to the vertical vent, all the way to the attic or roof. So no major flow loss.

    So if I understand what "CFM" means - the rate of air removed from a space measured in SQFT is not really 1-to-1. Because a 100 sqft room with (say) 8 ft ceilings is actually 800 CUFT. So a theoretical 100CFM fan would take 8 minutes to replace all the air in the room. Most bath fans are around 70.

    My room is ~250 SQFT. 8 ft from floor to "ceiling" (subfloor above) makes it ~2000 CUFT airspace. Nicest fan I found locally runs 110 CFM through a 3" outlet, so it would take 18 minutes to completely evacuate the room. Not quite 4 times per hour.

    HOWEVER, a "full replacement" scenario in my head concerns *an entire room of spray fog* which never happens or I wouldn't be alive to type this. That reality of spraying is also nowhere even close to a fully steamed-up bathroom scenario either. So, I'm assuming the localized, incidental spray fog - again almost directly beneath the fan - would be evacuated at a reasonable rate. It should certainly clear out in a few minutes (~18 max worst impossible case) versus the... couple hours or more (depending on other factors) I have to dance around now! A marked improvement. Even though that fan is more expensive, it's sure to be worth the added comfort, convenience, and safety.

    The wildcard in my proposal then is how volatile solvents might be in a bath fan. Putting a filter ahead of it would probably restrict flow too much. I would probably put it on a GFCI just for added insurance.

    The other factor is density of vapor: can that 110CFM fan push it all up a 10' or 16' pipe. I am assuming it could if only because CFM ratings take into account vent runs that are probably at least that much if not more, and mine is nice and direct. Though I haven't figured out how the volume of that pipe (about 3" @ 10ft = .5 CUFT) affects the CFM of a fan...

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