Poor Man's Exhaust Hood

DESCRIPTION

I wanted to rebuild the carburetor on my 1978 Fiat Spider during the winter, and I needed to clean the carburetor indoors (it was 10°F outside).  I needed some way to exhaust the solvent fumes outside, so I built this exhaust hood.  You can use it to exhaust most any kind of fumes, including paint fumes.

I built my hood from a cardboard box, mainly because I wanted something quick and easy.  Feel free to build yours out of wood, Plexiglas, sheet metal, etc.  The easiest materials would probably be either plywood or wall paneling.  Wall paneling is not particularly strong, so if you use it then I would recommend reinforcing the corners with some 1x2 or 2x2 lumber (or 1x1 if there is such a thing). 

If you use a cardboard box and the parts listed below, your exhaust hood shouldn't cost more than about $50 to $60.  About the only way to get one cheaper is to steal one.  If you do steal one, please grab one for me too  :-)

Please be sure to read the notes at the end of this document.

DISCLAIMER

OK folks, I don't want anybody suing me.  So build this at your own risk.  I make no warranties or guarantees as to the suitability for a particular purpose or as to the safety of this exhaust hood.  The applications I described (exhausting solvent fumes and paint fumes) are merely suggested applications.  I have not done any extended testing.  Please do not blame me if your house fills up with toxic fumes or you blow yourself up!  If you are too worried about it, then consider purchasing a commercial exhaust hood.

PARTS LIST

                        Fan

Most parts can be found at your local hardware store.  Get the type of exhaust vent that uses louvers as shown in the picture.  The louvers open up when air is flowing.  Also be sure to get aluminum ducting.  Do not get plastic duct or that stuff that looks like aluminum foil.  Make sure it is solid aluminum (the flexible type). 

The 12V fan can be purchased from a local computer store.  The power supply can be a cheap 12V wall adapter from Radio Shack, etc. with an output of at least 300mA (0.3A).  If you run 2 fans, then the output should be at least 600mA (0.6A).  Ideally it would be nice to have an adjustable power supply so that you can vary the exhaust rate.  An adjustable voltage range between 9V to 15V would be nice.  Running the power supply up to 15V will give some extra exhaust flow, and shouldn't hurt the fan.

An outfit called All Electronics sells inexpensive power supplies.  Their part number PS-1524 is $12.50
http://www.allelectronics.com/cgi-bin/item/PS-1524/480/15_VDC,_2.4_AMP_SWITCHING_SUPPLY_.html

They also sell a fan for only $3.75
http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=CF-197&type=store

Use a fan with a brushless motor.  Motors with brushes will produce little sparks inside the motor, which could possibly create an explosion hazard (the sparks could ignite volatile fumes).  A brushless motor should be safer.  "Muffin fans" like the ones for computers (pictured above) are typically of the brushless type.  I don't recall ever seeing one that was not brushless.  Many of them will say right on the fan that it is brushless.

CONSTRUCTION

The next step is to cut a hole in the plywood for the dryer vent.  This was easy for me because the dryer vent I purchased came with a trim ring that I used as a template to draw the cutout.  After drawing a circle, drill a hole near the inside edge of the circle and then use a jigsaw to cut it out.  When done then attach the exhaust vent to the plywood.  Here is a picture of my window, the panel after cutting, and the vent assembly mounted to the panel:

For some weather-proofing, put a bead of silicone sealer around where the tube meets the panel.  One of the pictures below provides a better look at the silicone.

Actually I did not use a piece of plywood, I used some wall paneling that I happened to have.  Since my window was small, the paneling will be strong enough.  If you have a large window then either use plywood or use a double thickness of paneling (two pieces stacked together for double-thickness).

The paneling (or plywood) most likely will be slightly warped.  It's impossible to get one that is perfectly flat.  This means that it will not perfectly seal around the window opening.  When cutting the plywood/paneling, set it up so that after installation it will bow outwards (not inwards).  You can then drill a hole through the center and use a cross brace as pictured here:

I used a long screw with a big washer, which was then passed through the paneling and cross brace.  Use a really big washer (I think they are called "fender washers").  On the inside I used a wing nut.  By tightening the wing nut it will "pull" the paneling snug up against the window opening.  This works best if you do not put the vent through the center.  You can see that my vent is mounted to one side, so that my cross brace can be in the middle.

Lastly I completely covered the outside of the panel with duct tape to "weatherproof" it. 

The aluminum tube on the exhaust vent was rather long, so first I cut it to a shorter length.  To cut it, first draw a line around the tube.  This can easily be done by wrapping an 11x17" sheet of paper around the tube and taping it.  This will let you draw a perfectly square line around the tube:

I then used a Dremel tool to cut the tube:

Now you need to prepare the fan.  If there is a connector on the end of the power leads, then cut it off.  Next, hold the fan up to the tube.  You will need to cut the corners off the fan so that it fits inside the tube.  On mine I had to cut the corners off right through the middle of the mounting holes. 

After eyeballing it, use a Dremel tool to cut off the corners.  Here is my fan after cutting:

Next, choose a location for the fan (how deep into the tube you want to mount the fan).  This is strictly by your preference and may depend on how you are mounting the exhaust vent to your window.  I recommend mounting it deep enough so that you can install a 2nd fan if desired.  From the following pictures you can see where I mounted mine.  After choosing a location, mark the front edge by drawing a line around the tube using an 11x17 sheet of paper.

Next, hold the fan up against the line on the tube and then mark the width of the fan on the tube.  Now use a ruler to make a second mark that is 1/8 inch farther.  Use the 11x17 paper to draw a second line through this mark.  You should now have 2 lines, and the distance between the lines should be the width of the fan plus 1/8 inch (for example if your fan is 6/8 inch wide then the lines should be 7/8 inch apart).  The extra 1/8 inch is to allow for some screws that will hold the fan in position.  Here's a picture:

(In the above picture you can also see some screws which are described in the next step)

Now slide the fan into the tube into the approximate position and eyeball the corners.  Draw 4 lines on the tube to mark the approximate location of the corners.  Also mark an approximate location for the wires to exit the tube.

To mount the fan, drill 4 holes on each line (one at each corner) for a total of 8 holes.  Also drill a hole for the wires to come through.  Now screw a small sheet metal screw into the holes of the line that is closest to the louvers (the exit side).  CAREFUL!  Do not tighten the screws too much, the tube is thin aluminum and it is very easy to strip the holes.  Lightly tighten the screws.  Here is a picture looking inside the tube:

Next put some electrical tape around the wires of the motor where they will be exiting the tube.  This will protect the wires from the sharp edge of the tube.  Now start the wires through the hole and then slide the fan into the tube until it hits the screws.  The screws will keep the fan from sliding any farther.  Then install screws into the remaining holes.  These will keep the fan from sliding back out.  Here is a picture:

There should now be 4 screws on each side of the fan, which will keep it firmly in position.  Wrap some electrical tape around the tube to cover the screws.  That will keep them from backing out. 

(In the above picture it looks like the tube is bent and distorted at the screw locations, but it is an optical illusion caused by reflections.  The tube is actually perfectly round.)

See those big gaps between the sides of the fan and the tube?  Seal those off with duct tape as shown here:

OK, below is the completed assembly (except I don't have electrical tape covering the screws).  You can see the clear silicone sealer I applied where the tube meets the panel.

Set the fan assembly aside.  It's now time to build the hood.  The following pictures show how I built the hood out of a cardboard box.  If you plan on building a more permanent hood, you still might want to start by making it out of a cardboard box.  If you screw it up then just get another cardboard box.  When you get it right, you can then use the cardboard box as a template to build your permanent hood.

Begin by cutting the top panel.  Make it out of thin plywood or wall paneling.  Mine is wall paneling.  Here's a picture:

I laid the top panel on the box so you can see that I made it smaller than the box.  In the picture you can see that I already drew the cutout for the exhaust tube.  I will give you dimensions of the top panel as a reference, but your box will probably be a different size than mine. 

The top of my box is 23 inches long by 16 inches wide.  The top panel is 13.5 inches long by 6.75 inches wide.  I didn't exactly use rocket science to determine the dimensions of the top panel (nor any kind of science for that matter).  I randomly chose the width of the top panel to be the same as the width of the panel that fits on my window.  It just seemed to be a good size.  I then chose the length by angling the side flaps of the box to what looked like a good angle.  You will see some pictures later.

With the top panel smaller than the box, the top of the hood will have angled sides.  This will help channel fumes towards the exhaust tube.  The size is not critical, just make it smaller than the box.  Make sure that the top panel is wide enough to mount the exhaust tube into it.  The 6.75 inch dimension of my top panel is close to the minimum width.

Next, temporarily tape the top panel into position as shown below.  Make sure the top panel is centered on the side flaps. 

Now mark the corners of the top panel as shown here:

And now draw lines from the marks to the bottom corners as show below.  In the picture you can see a bar clamp I used to hold things together while I was working.

You can now cut on the lines, after which the flap should look like this:

Now assemble the top panel to the box as shown below.  Just hold it together by hand.  Then draw lines on the other flap (the long flap) as shown.

Cut along the lines to trim the corners off the flap.  Your box should now look like this:

Now is a good time to cut the hole in the top panel for the exhaust tube.  Prepare the exhaust tube from the other dryer vent as shown here:

Notice that I have removed the louvers.  I also cut the tube so that it is about 4.5 inches long.  The length is not critical, I just didn't want it sticking up so far.  Mount the vent onto the top panel.

Here's the whole assembly:

We are almost done with the box.  The only thing left is to cut a window into the side of the box and then tape all the corners with duct tape.  For strength and good sealing I recommend taping on both the inside and the outside.  Here's a look at my box with a window cut into the side and with the corners duct-taped:

See how my window does not extend all the way to the edges?  I did this for the following reasons:

• Provides a bit more strength to the box.

• If the fumes are heavier than air, the bottom lip will help hold fumes inside the box until they get exhausted.

• If the fumes are lighter than air, the top lip will help keep the fumes inside the top of the hood until they get exhausted.

OK, your box is now ready!  The only things left are the following:

• Mount the exhaust fan assembly into the window.

• Connect a power supply to the fan.

• Connect the aluminum dryer duct between the box and the exhaust fan.

Keep the box close to the exhaust fan so that the duct line can be kept as short as possible.  Also, keep the hood lower than the exhaust fan.  The dryer duct can be stretched as needed for the proper length.  You can use either large hose clamps (sold near the dryer ducts) or duct tape on each end of the dryer duct to secure it in place.  Actually I used neither -- my duct stayed nicely in place without securing the ends.

Congratulations, you are finished!

NOTES

If you are using an adjustable power supply, do not run the fan too slow.  You need enough air flow so that the louvers open up and the fumes can exit.  I run the fan at top speed while I am working, and then run it at a slower speed for a while to exhaust any residual fumes.  Besides, in the winter I don't want to keep blasting all my warm air outside.  My gas bill last month was over three hundred f---ing dollars!!!  (Thank you Mr. Bush)

I used my One-Shot Timer (from one of my other construction articles) to run the fan for a while after I was done working.  Click HERE for info on the One-Shot Timer.

If you need more air flow, try mounting a 2nd fan in the exhaust tube.  Just mark a 3rd line, slide in the 2nd fan, then install 4 more screws to hold it in place.  If that is not good enough then run a whole separate exhaust line (2 exhaust lines mounted to one hood).

The hole I cut into the side of the box is rather large.  There was not much of an air flow because it is such a large opening.  In fact it was hard to feel any air flowing at all.  So I installed a 2nd fan inside the exhaust tube as described above.  When I installed the 2nd fan I had to remove the duct tape that was sealing around the 1st fan, so that the 2nd fan would slide all the way in.  Before sliding in the 2nd fan I sealed the large gaps around the 1st fan with 100% silicone sealer (only use 100% silicone).  I then also sealed around the 2nd fan with silicone.

I then also taped some Plexiglas across the front to reduce the size of the hole.  It worked great.  Using Plexiglas across the front lets you still see inside.  Here are pictures:

In the left-hand picture there is one piece of Plexiglas across the top of the opening.  This worked very well for me.  You can also use an additional vertical piece down the middle, shown in the right-hand picture.  This makes for 2 hand holes, one at each lower corner.  I just duct-taped the Plexiglas to the box along the top, which then acts like a hinge.

You will find Plexiglas at the local hardware store in the window department.  It might be called Lucite, etc.   Then buy yourself a special knife blade for scoring Plexiglas.  The blade installs on a regular utility knife.  Using a straightedge (I used a level), score the Plexiglas about 5-10 times.  Then position the score line along the edge of a table and bend until it cracks along the score line.  It's a lot easier than trying to cut Plexiglas with a saw.  You get a nice, clean, straight cut.

ADDITIONAL NOTES

When I was preparing to clean my carburetor, it occurred to me that I would be spraying carburetor cleaner inside the hood.  The carburetor cleaner will be splattering all over inside.  I didn't want the cardboard hood to be soaking up lots of solvent, so I wanted to cover the inside.  After considering several options I decided to use vinyl floor tiles.  Advantages of vinyl floor tiles:

• They are resistant to solvents (unfazed by my carburetor cleaner).

• They are self-adhesive, just peel off the backing and press into place.

• They are very easy to cut.

• They're cheap!

If you get floor tiles, buy cheap flimsy ones.  I got mine from Menards for 50 cents per tile.  It took 13 tiles, and I bought a couple more to have as extras.  Menards had a cheaper tile for 40 cents, but the one I chose was thinner and so it was more flexible.  Try to get the thinnest one, which will then be most flexible.  This will help the tile conform to the shape of the box, since the sides of the box will not be perfectly flat.

If you have used vinyl floor tiles before, then this is a no-brainer.  If you have not used vinyl floor tiles before, here are some hints:

• The adhesive is very strong, so be careful when placing a tile.  You don't want to try and lift it once it is in place, so make sure the tile is in the proper position before setting it down. 

• You can use scissors to cut the tiles, but it is better and easier to use a utility knife and a straightedge.  I used a level as a straightedge.  Just make one pass along the TOP of the tile with the utility knife.  Then pick up the tile and bend it at the score line.  It will suddenly crack perfectly.  VERY easy to do.

• The stores usually have little pamphlets that describe how to install vinyl tiles.  Grab one for further hints.

Here are a couple pictures:

                                           First Tile                                                                                                      Done!

When done installing tiles I duct-taped all the seams, including the corners (you can see that I covered the corners in the picture below).   This will keep solvent (or whatever you are spraying) from seeping into the seams.  I did not try installing tiles on the angled top sections of the hood.  I didn't want them to come loose and fall down.  Instead I covered the top section with duct tape.  I also covered the inside lip around the opening with duct tape too.  Here is a picture where you can see that the top section is covered with duct tape:

That's it! 

When I used my hood to clean the carburetor, it worked very nicely.  Very little solvent fumes inside the house.  The only bad thing was that when the carburetor cleaner got on the duct tape, it started loosening the adhesive and the duct tape started lifting.  This happened only at some of the seams that I had taped.  Some of the seams had gotten pretty well soaked with carburetor cleaner.  The top of the hood was unaffected.  What I ended up doing was removing the duct tape and caulking all the seams with silicone sealer.  If you do this, be sure to use only 100% silicone sealer.  It will be very resistant to solvents.  But if you plan to do only spray painting, then the duct tape should be fine.

Best of all, even with all those flammable fumes from the carburetor cleaner everything was fine.  I didn't burn the house down.  ;-)

Good luck!

P.S.  If you actually build one of these hoods, let me know how it turns out.  It would be nice to know that someone actually built one so that I don't feel I wasted all this time posting the info!

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