What Would Make A Shop Go Boom?

I've often heard that using a bathroom exhaust fan to exhaust finishing odors is bad idea, but I'm sure there are certain conditions that would have to be present for it to really be a danger.
Consider this room:
9' x 12' x 7' South West corner: A door that opens to the main part of the basement North West corner: A door that opens to the great outdoors (screened)
Consider this idea:
Mount a 140 CFM bathroom exhaust fan in the North East corner (opposite the 2 doors). If my calculations are right, that's about 40% more than the minimum CFM for a bathroom of that size.
(Amazon.com product link shortened)
Planned uses: Occasionally exhaust odors from primer, paint, mineral spirits, oil based stains, oil based poly, farting, etc. No spraying. Brush or wipe-on only. Small projects or small parts of large projects. e.g. stain a headboard, paint a dresser, etc. Not to be used when the air is dusty, although adding a holder for a furnace filter is a possibility.
What would it take for this to go boom?
FWIW, the fan shown below, placed on a stool by the door to the great outdoors hasn't blown the place up yet. Of course, it's only been about 35 years, so there's still time.
http://i.imgur.com/QzieJU1.jpg
That thing is a beast. I don't know the CFM rating, but it'll dry a pair of jeans in about an hour.
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On Mon, 19 Jun 2017 19:23:32 -0700 (PDT), DerbyDad03

No spark means no boom - so ground the fan to prevent static buildup, and locate the switch in fresh air or use a sealed / electronic switch. Being a shaded pole or split phase motor with no starter switch the motor should never spark. Forget any motor with brushes or mechanical / centrigugal starting switches.
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On Mon, 19 Jun 2017 19:23:32 -0700 (PDT), DerbyDad03

It seems that you are using bathroom fan specifications - and applying them to paint-room ... ? < 40 % more CFM's > Perhaps bathroom fan specifications are related to removing moist warm air - rather than fumes from solvents ? Some sort of recommended CFM for a paint room might be a start. John T.
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On Jun 19, 2017, DerbyDad03 wrote

. Does it run on DC? It looks like it should. May be an old Vortex unit.
. War story: I lived and worked in Sweden for a year, around 1974. I was doing a lot of soldering, and the rosin smoke was giving me headaches, so I asked my business partner for a low-speed fan to blow the smaoke away. He went down to the sub-basement of the large condo block he lived in, and came up with this beautiful old enameled cast iron with brass table fan, which I cleaned up and re-oiled, and then plugged in. It sat there humming slightly and moving slowly. Lightbulb! Loked at the nameplate - Gleichstrom! (DC) That fan had been in the basement since the 1920s, when Sweden converted to alternating current, because it was too good (and too expensive?) to just throw away. So I added a silicon rectifier bridge to the fan, and it worked like new. It will outlast us all.
Joe Gwinn
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On 6/19/2017 10:23 PM, DerbyDad03 wrote:

Read up on LEL or Lower Explosion Limits. The ability to explode depends on the concentration of the gas. Most flammable gases have to be in the 1.5% to 5% by volume to explode. Given the list of things you are using it is doubtful you will ever reach that concentration. https://www.mathesongas.com/pdfs/products/Lower -(LEL)-&-Upper-(UEL)-Explosive-Limits-.pdf
Spraying lacquer may be a concern but I doubt that a wiping stain would ever get near the LEL. If you spend a day at the bean festival farting may be a concern and your wife will probably confine you to the shop that night too.
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On 6/19/2017 9:23 PM, DerbyDad03 wrote:

I am no expert on this but IMHO any "not rated for explosive fumes" fan can cause an explosion if the air to fuel mixture is right. One step to insuring that the air fuel mixture is not right is to have the fan on BEFORE you mix the fuel into the air. Read that as keeping the fumes dispersed before they have time to accumulate. IF the fuel concentration is to low to begin with there is less chance of the fan or it's switch creating the spark in the explosive environment.
Personally I would not draw fresh air in to exhaust out through a small fan. If the fan does not exhaust directly out side, or if the duct has a leak in the attic, you might be causing a dangerous situation in your attic, or where ever the duct work runs.
I would pressurize the room to vent fumes out the large doors. Run the exhaust fan backwards. Or put a fan on the south door blowing in and a fan on the north door blowing out. AND another fan in the room to keep the air moving.
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On Tuesday, June 20, 2017 at 9:55:48 AM UTC-4, Leon wrote:

The fan will exhaust to the exterior via a 4" rigid duct, through the rim joist, just like a dryer vent would. The fan will be less than 4' from the exterior vent with no turns that will obstruct air flow.
The goal is to use the basement air as well as the outside air as makeup air so that all fumes are drawn toward the fan and ultimately out of the house. While the "fan through the open door" has worked in the past, I have to keep the screen door open which lets bugs in at night. It also makes it inconvenient to use that door since the fan/stool blocks the entrance.
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On 6/20/2017 10:35 AM, DerbyDad03 wrote:

And that scenario would scare me to death, you have a carburetor concentrating the fumes into a small exit hole and those exhaust ducts are designed to deliver non combustibles so if they leak no danger. In this case if they leak into your attic.......

I think most explosions happen because some one does not vent properly. IIRC paint booths are pressurized so that the vapors exit through the walls and ceilings to prevent the accumulate into a concentrated area.
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On Tuesday, June 20, 2017 at 12:11:07 PM UTC-4, Leon wrote:

I don't understand your attic concern.
Perhaps I should explain. This room is in the basement and has walk-out access to the back yard. The 4' duct will run in the exposed ceiling joist bay. Fan in joist bay, duct in same joist bay, out through the rim joist and into the back yard. Any leaks will simply return right back into the room after passing through the fan. Nothing goes up to the attic. The duct is all within the same room right up until it leaves the house.
Any leaks would simply mean that not all of the air is being turned over. I'm sure the "fan by the door" method I've been using for 35 years isn't turning over the air efficiently considering it is on the same wall as the interior door to the basement. At least the new fan will be on the opposite side of the room from the 2 doors, drawing make up air in and across the room.
Am I missing something regarding your "leak into the attic" concern?
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On Tue, 20 Jun 2017 09:58:03 -0700 (PDT), DerbyDad03

Soimthing missing in HIS "attic"??
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On 6/20/2017 11:58 AM, DerbyDad03 wrote:

No, I was missing where you live. LOL We have no floor joists except between floors, no basement. I thought the joists you were talking about would be in the attic or between floors. In other words all enclosed.
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On Tuesday, June 20, 2017 at 7:24:40 PM UTC-4, Leon wrote:

Ah! It all makes sense now. ;-)
I take it you've never had the pleasure of drilling a 4" hole for an exhaust vent in a 2x rim joist above the cinder block foundation wall. It's pure joy!
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A what in a where? LOL. NO, I have not.
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On Wednesday, June 21, 2017 at 8:43:22 AM UTC-4, Leon wrote:

The hardest part can be locating the hole so it lands where you want it in the joist space above the foundation, taking into consideration the siding and other factors on the exterior.
One trick that I have used is to chuck a piece of a wire hanger into a drill, locate where I want the center of the 4" hole to be on the inside and use the wire to drill a small hole through rim joist, siding, etc.
Then I go outside, find the end of the wire and adjust the "center" if required/possible. Now I can drill the big hole from the outside where I typically have more room as opposed to trying to drill the hole up in the joist space.
That's the plan for tonight.
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On 6/21/2017 12:33 PM, DerbyDad03 wrote:

We will need progress pictures. ;~)
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On Wednesday, June 21, 2017 at 7:07:10 PM UTC-4, Leon wrote:

Sorry, the best I can offer are completed pictures. Pretty boring.
Interior:
http://i.imgur.com/CCgtZqk.jpg
Exterior:
http://i.imgur.com/Joa2zFv.jpg
The fan exhausts under my deck, whose surface is about 6.5' off the ground. (Remember, it's a walk out basement)
Drilling the hole was an adventure. I had to go through the deck's ledger board, the cedar shake siding, then the rim joist. Roughly 5" in total depth, but with 1.5" gap between the 2x ledger board and 2x rim joist. The biggest issue was that the bottom of the ledger board is flush with the bottom of the sill plate, not the bottom of the rim joist. I had to find a vent cover that I could place high enough on the ledger board that the hole would clear the sill plate. I wanted to use one with flaps, but the housing on those didn't allow me to mount it high enough.
I drilled a series of holes with a 1 3/8" spade bit and then rounded it off (sort of) with a reciprocating saw. The hole is far from perfect, but caulk behind the vent cover and spray foam around the duct sealed it nicely.
I tested it by hanging a piece of toilet paper on the screen door opposite the fan. The toilet paper hangs straight down until I turn the fan on. Within 2 seconds the bottom of the toilet paper moves about 2" away from the screen, so I know I am getting good air flow all the way across the room.
I'm ready to paint the repurposed hutch-kitchen island, so we'll see how well it keeps the paint fumes from smelling up the house.
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On 6/24/2017 7:49 AM, DerbyDad03 wrote:

Did I see a glowing ember in the shadows? ;!)
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On Saturday, June 24, 2017 at 1:11:21 PM UTC-4, Leon wrote:

That's not as funny as it may appear to be. I seriously need some new reciprocating saw blades. I had to turn on my old exhaust fan to clear the smoke I created while installing the new one.
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On Tue, 20 Jun 2017 17:55:07 -0700 (PDT), DerbyDad03

A lot less fun than drilling it through the brick outside the rim joist - - -
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