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These instructions are here to help you get into a quality Flow Hood at an economical price. If you've done any checking into hoods, you know most places charge an arm and a leg for them. These plans are for a 12"x24" Flow Hood like the one pictured below.  Depending on supply availability, you could build it for around $350-$425 US. 

Wait for the filter to arrive to get the exact measurement of it's size (they may vary slightly).

The two largest costs are the fan (blower) and the HEPA filter. If you can get a hold of a used squirrel cage style blower that would be perfect. These are used in furnaces and industrial applications mostly. If you know anyone in HVAC they can get you one for practically nothing. Otherwise, a good place is HERE. You will need at least a 495 cfm blower to power a 12"x24" HEPA filter figuring .8" static pressure.

If you are looking for a good HEPA Filter click HERE

Other Supplies Needed for the Job

  • Skill saw or table saw (jig saw even, anything to make a straight cut)

  • tape measure

  • drill with small drill bits

  • 1 1/2" wood screws (small box 150pcs.) ($5)

  • screw driver (Phillips)

  • power strip w/breaker on/off switch ($8)

  • 4'x4' pc of 3/4" red oak plywood ($30) (3/4" particle board will work, but it's not near as nice)

  • 8' of 1"x1" furring strip ($3-5)

  • 8' of 1" edge trim ($5)

  • tube of all purpose silicone sealer ($3)

  • pair of 3 or 4" steel handles ($3)

If you have a Lowe's, Menard, Home Depot or any other large home store, check there. Most of these places have pre-cut wood in their lumber sections that work perfect for this project with very minimal cutting. If you have access to a saw, then it's way cheaper to buy a large sheet and cut it yourself.

Getting Started

To get started, you need to cut all the pieces for the box itself. You'll need a back (12"x24"), top & bottom(16"x24"), and both side pcs.(13 1/2"x16"). When you have these pcs. ready, you'll need to start constructing the box. It helps to have someone to hold pcs. together for you in the beginning. Start by lining up a side with the top/bottom as shown in diagrams below.

Drill pilot holes for all screws so as not to split the wood. Put together sides, top and bottom first. Don't tighten the screws all the way tight, you may need some room to slide the back in. When the sides, top and bottom are together, slide the back in (either end, it doesn't matter yet). Before you screw the back in, tighten the rest of the screws holding the box together. Make sure it's lined up flush and screw it in place.




1"x1" furring strips around inside of box 6" from front edge

The purpose of the 1x1's is to create a flange 6" deep to push the filter against so a tight seal is created.

Once the box is complete and the flange is in, it's time to cut a hole in the top for the blower to push air through. Measure the hole in your blower, and draw it out on the top of your hood about 1" from the back (a bit more if your blower needs). Center it from side to side. Then cut this out.

Once the hole for the blower is cut, you need to clean out any sawdust real well.

Next, you need to seal all the seams on the inside of the box. Use your silicone sealant to run a bead on every seam. Then smooth it with your finger.

Once the box is sealed, you can put on your blower. Put a small bead of silicone around the edge of the hole for the blower. Place the blower over the hole and screw it in place (it helps to center blower before silicone, then mark where the holes need to be and predrilled).

After the blower is mounted, you can plug it in and it'll blow out any stuff (dust, etc.) out of the hood. A cheap way to set this up, is to get a six plug power strip, and mount it in front of the blower on top of the hood. You can then plug the blower into this (has on/off switch) and also plug in any other stuff you might use while running the hood (bag sealer, etc.).

Lastly, you need to cut your trim pieces so that you have 4 pcs. that go on the very front edge of the hood to hold the filter in place. 45 degree angles look good, but straight cuts are fine. Once the pieces are cut, line each piece up where it's going to fit (leave about 1/4 to 1/2" overlapping the edge in front of the filter). Drill pilot holes through your trim pcs. into the front edge of the hood.

Once again, clean any dust, wipe down the inside lightly with a antibacterial cleaner and then insert the filter into the front opening. It can be a snug fit. Push it in evenly (it's a bitch to try and pull out) all the way against the flange. Screw your front trim pcs. into place and your ready to go.


If your looking for a bigger hood, or can't get a 6" deep filter, just adjust for filter depth with your flange, and plenum depth so you have room for your blower behind the flange.


Here's a pic of the little hood above in front of a 2'x4' hood. The large hood uses a reconditioned furnace blower (1750 cfm) that I got for free. I used 1/8"x1" aluminum strips for the front trim on both of these. Square cuts on the big one, metered cuts on the small one. The large hood uses the power strip on top for the on/off, where as the small one I wired with a heavy toggle switch from Lowe's home store.

There you have it, I hope this helps people get some idea's, and maybe even build one of these for themselves. It's not really that hard if you have all the materials gathered, and takes a couple hrs. to put together. And they are super sweet to use. Makes life way easier.