Introduction: Building a Small Fume Hood for Stinky Projects

My wife Jennifer makes cast-resin jewelry1. She uses a polyester resin, which works a lot like fiberglass resin (mix the resin with a hardener/catalyst, pour), but unfortunately it smells about as bad too. Because of the toxic fumes, good ventilation is a must. In the summer months, it's no problem to do the casting outside, but doing it outdoors in the Canadian winter is not really an option. We needed some sort of indoor ventilation system to protect her from the fumes (like some sort of fume hood or something!).

What emerged is a fume hood based around a range hood, with a sealed wooden enclosure and a clear plastic window that forms a fixed sash. It is built on a cart, making the whole setup portable; all that's needed is an electrical outlet and somewhere to vent the fumes. In practice however, it's essentially a fixed installation that we can move out of the way if necessary.

I guess I should do the disclaimer thing and point out a few things. This was a quick-and-dirty design and build -- we were basically interested in moving the stink outside, not setting up a chemistry lab. This hood is NOT meant for chemical storage or use with highly flammable or explosive substances (do your homework on the chemicals you're using before building one of these). Also, this project involves basic wiring of a 120V circuit as well as various sharp and pointy tools. Builder discretion is advised.

1<plug> Check out more of her work at her ETSY page. </plug>

Step 1: Design and Sizing

I found more info than I needed on fume hood design here. The first image below is also from that page.

The key piece of information needed is the face velocity, the speed at which air is sucked through the open face of the hood. According to the link above (and others) 60 - 100 ft/min is good for most applications. I chose 100 ft/min as the design point and made a guess of 2 ft2 for the opening, which would mean that I need a fan capable of 100 ft / min * 2 ft2 = 200 ft3 / min (cfm).

I started looking around for exhaust fans, thinking I would build it around a bathroom fan. The obvious choice of a range hood didn't occur to me until it popped up in my searches. It turns out a range hood suits the project nicely since it has an appropriate-sized fan, a light, and a shroud already built-in. I found a used-but-new range hood on a local classifieds website that has a 180 cfm fan and 30" shroud.

Without changing the face velocity, 180 cfm means the opening can only be 1.8 ft2. The front of the hood tapers inward, ending at about 2 ft at the front. Leaving the entire width open means that the height of the opening should be no more than 1.8 ft2 / 2 ft = 0.9 ft * 12 in / ft = 10.8 inches. I'm a fan of round, conservative numbers so I went with 10". Don't forget to mock it up again at that height to make sure the opening would be big enough to work through comfortably. Once you're satisfied with the geometry it's time to get building.

Step 2: Get Your Materials

Since I'm working on a grad-student budget, I try to use as much free/found/recycled material as possible. The result is probably less pretty than it could have been, but I'm a form-follows-function kind of guy. YMMV.

What you'll need:
- A range hood
- A cart (optional), stand, or other place to put the thing
- Some material (plywood/chipboard/sheetmetal?) to form the enclosure
- Some plexiglass or other transparent window material for the front face
- Assorted fasteners to hold things together
- A power cord with plug (two-prong is fine), or just some wire if you plan to hardwire it.
- Some wire nuts to connect the wires
- Some medium-sized cable ties
- A 4" (100mm) dryer (or similar) duct and a boot for the exhaust
- Duct tape (I used about half a roll)

Check your local classifieds site (eg here in the Ottawa region, craigslist, kijiji, etc), there are usually bargains to be had. I got my range hood in new condition for $10 from a guy who bought a new place and replaced the hood to match his appliances. I had a pile of scrap chipboard and a sheet of thin plexiglass (or similar) lying around, and I used the power cord from a VCR I took apart a while ago. Check to make sure your cord can handle the nameplate current draw of your unit (wire gauge charts can be found on the interwebs). The only other things I had to buy were the duct boot and the duct, making the grand total for this project about $20 (well, $30 after having to buy a second duct, details later).

Oh, and tools you'll need:
- Drill & bits
- Saw of some sort (guides are nice)
- Utility knife
- Screwdrivers or hammer, depending on your fasteners
- Small round file
- Wire strippers (or whatever works for you)

Step 3: Creating the Enclosure

Before cutting anything, decide where the fume hood will live, since the height of that surface will affect the height of your enclosure. Mock up the enclosure by holding the range hood aloft and pretend to use it (an extra set of hands here is helpful). Figure out how high you want the hood to be so that you can work comfortably and see what you're doing. We went with 16".

Measure the dimensions of your range hood and build an enclosure that follows its contours and is the height you just decided. I built mine from some scrap tongue-and-groove chipboard; I left the tongue on the rear panel since it provided a handy shoulder to hold the hood up and prevent it from sliding forward.

The front section of my range hood makes a 20-degree angle with the sides. Those panels were cut at that angle and attached with flat brackets bent to the proper angle in a vice. The front window was then cut out and fastened to the enclosure. Taa-daa!

The enclosure doesn't have to be perfect, but it'll look much nicer if you take your time here. A radial arm saw, table saw, etc (anything with a guide) would have been a big help... my jigsaw and circular saw skills aren't great, so I had to spend some time with a rasp to line things up properly and there are still some small gaps when I was done (I cut to the wrong side of the line, so I'm missing about a blade-thickness in a couple places). Never fear, duct tape fixes all.

Once you have the enclosure built, slap on the range hood to make sure everything fits and make adjustments if necessary. Next we'll wire everything up.

Step 4: Wiring It Up

Wiring up a range hood is rather easy. But first, the obligatory warnings: 120V is enough to kick your ass if you do something stupid (like plugging in the cord before you start wiring it). As long as you use your head and maintain a healthy respect for the danger electricity can present, you should be fine.

Right, on with the show. Find or build a cord with one male end, long enough to reach the nearest outlet. The cord I used was salvaged from a VCR, so it already had a male end to plug into the wall. If you have no such cord, you can find bulk wire and end connectors at your local hardware store. My range hood had a single live (black) wire that ran to the switches on the front panel, and two neutral (white) ones returning from the fan and light. Pass the cord through the opening in the back, strip the ends of the wires, twist the corresponding ends together, and secure them with a wire nut. Make sure the connection is secure by holding the nut and gently tugging at each wire. If any come out, remove the nut and try again.

A sealed strain-relief bulkhead should be used to pass the cord through the shroud, but I'm cheap so I made my own. The appliance end of my cord had a square strain relief where it passed through the VCR casing, but unfortunately my range hood has a round hole. I could have cut out a square hole to use instead, but I found a rubber grommet (I think it was a motor mount from a burnt-up washing machine motor) that was about the right size to use as a seal. I had to cut it open to get the cord inside it, and closed it back up with a cable tie, squeezing it snug against the cord.. To prevent it from pulling out I wrapped a second cable tie around the cord on the inside of the grommet, tight enough so that it won't easily slip but not tight enough to crush the cord. Wedge that baby in and we're just about done.

Check one last time to make sure there are no loose connections, etc. Throw in a light bulb and plug it in to test the light and fan. If everything works, go ahead and install the wiring cover (if you don't have a cover, just tuck/tie/tape them out of the way).

Step 5: Attaching the Two Halves

I used cable ties to attach the hood to the enclosure, since nothing better was handy. To do this, drill corresponding holes (big enough to pass a cable tie) in both the enclosure and the hood, then loop a cable tie between them. Don't forget to file away any burrs or sharp edges that could damage the tie before you attach it. On mine, the back and sides have two ties each and the angled parts have one, for a grand total of eight. To keep the outside looking clean, I arranged the ties so that the heads are inside the hood, to be hidden under the duct tape that will seal things up. Install them all loosely at first and gradually tighten them all evenly; tightening one side first may pull the hood out of alignment and you may need to cut it off and start over.

Step 6: Seal Up the Enclosure

The enclosure should be sealed as thoroughly as possible so that the front face is the only place where air can enter. Leaks in the enclosure where air can get in will result in a lower face velocity. Gaps on the exhaust side of the hood are even worse, since that means fumes will escape into the room. If nothing else, make sure that the exhaust side is sealed well.

Range hoods have a thousand and one holes to seal. As I found out later when I tested it, all these little holes are plenty big enough to let some stink escape. Seal everything you see! If you drilled extra holes to mount the hood, make sure those holes get sealed too.

I used duct tape for everything. If you are more concerned with appearances than I was, you can use less ugly methods like caulk or some other more discreet sealant, or even just prettier-coloured duct tape. I did as much of the sealing from the inside as I could to keep the outside looking as clean as possible.

Edit: To be honest, I was just being lazy. I had duct tape handy. Other methods of sealing would have worked as well or better, and probably looked better too. The duct tape did end up "un-sticking" a bit, but just needed to be pressed back in place before using.

Step 7: Install the Duct Boot

The exhaust needs somewhere to go once it leaves the hood. I used a "universal" duct boot, even though it was a bit too big, to mate the rectangular hood exhaust to a circular flexible duct. For some reason, the so-called hardware store I went to had boots for 4" or 5" ducts, but only sold 3" and 6" flexible ducts... go figure. Rather than try somewhere else, I got the 4" boot and 3" duct and planned on forcing it to work, dammit. Wrong. I ended up having to replace it in the end. Get a 4" (100mm) duct, minimum.

Tape the boot to the hood around the entire perimeter, then double it all to make sure there are no leaks. If you're worried about it coming loose, you could punch a hole through both flange and boot and secure them with sheet metal screws first. I left the duct tape as both sealant and structural member.

Step 8: Set Up the Exhaust Vent

The last thing needed is somewhere to exhaust the fumes to. Ideally the fumes should go up a stack and exhaust high up so that no-one on ground level is exposed to it. The choices you have here depend on your particular location. In our house we have a couple of options: steal the dryer vent, tap into our chimney via an access door, tap into the furnace exhaust, or just have it blow out a window. It would probably be possible to tap into the sewer vent as well, but that would require a lot more work. We decided to go up the chimney through the access door.

To mate the round duct with the square access door, I made a bulkhead out of sheet plastic. The piece needs to be at least the size of the opening, and it's a good idea to leave a bit of overlap. Trace a circle around the duct and cut it out just a bit smaller with an utility knife. Carefully guide the end of the duct through the hole and seal/hold it together with tape. Stretch the duct some now before attaching it to the wall, so that you won't need to pull on the connection later. Tape the bulkhead to the opening in the chimney and make sure it's good and sealed. In my case the tape did not stick well to the painted concrete of my chimney, so I had to wrap it around a corner.

Once that's done, secure and seal the other end of the duct to the boot on the hood. Testing time!

Step 9: Testing, Testing

Turn it on, hear the whirr and watch the tissue paper blow in the breeze... so far, so good. Open a container of something stinky inside the enclosure, then get out the duct tape and get ready to play 'find that smell'. As I mentioned earlier, there were many, many small holes in the shroud of the range hood that was squirting out fumes. We sniffed out and patched dozens of leaks, basically covering the top and back of the shroud with tape. Don't forget to test all the way along the duct and whatever exhaust connection you chose. When you're done, you should not be able to smell any fumes at all.

Time for a proper test. Do a test batch of whatever stinky project you built the hood for. This will produce much more stink than just an open container, so go around and check for leaks one last time. Once you are satisfied that it's sealed, you're done! Enjoy being able to do your stinky projects indoors!

(Aside: As I mentioned earlier, the small duct we had installed at first could not handle the full flow rate so the face velocity was too low to keep everything in. It still worked reasonably well, but the smell of the resin was perceptible.

The volume of air that a duct can carry scales with cross-sectional area; our duct is circular so the area scales with the square of the diameter. That means that using a 3" duct rather than 4" cuts the diameter by 25%, which cuts the duct capacity down by about 44%! Oops, an embarrassing oversight... I ended up having to go back out and find a 4" duct after all. Once I replaced the small duct with one of the proper size, we tested it again... success! We could no longer smell the fumes!)