To start this project you will first need to buy a 99.99% pleated HEPA filter, preferably designed
for a laminar flow hood. Decide what size filter you would like to work in front of, then source it
from an online vendor, eBay, or fungi.com.
After you have chosen a specific filter, you will want to find a blower that will match it.
Here is the basic formula to do so:
For laminar flow we want 100ft3/min coming out of the filter. To find this:
1) Multiply the length and height of your filter to find the area (in feet) of your HEPA.
Ex: 2' x 2' HEPA Filter = 4ft2
2) Multiply that number by 100ft3/min
Ex: 4ft2 x 100ft3/min = 400ft3/min
3) Add the static pressure (SP) your HEPA + .2" of SP for the pre-filter . The static pressure
of your filter will be listed on the filter itself or a spec sheet that comes with it,
and will read in inches of W.G. If you custom order a filter or order one from Stamets you can find
out the filter's static pressure from them. Add .2" for the pre-filter.
Ex: HEPA SP = 1" ; Pre-Filter SP = .2" ---> 1"+.2" = 1.2" SP
4) Find a blower that meets the required airflow @ X" of SP (where X = total from #3 above).
A great place to start looking is Grainger because most of their products have spec sheets
online that will tell you what each fan's output is at many different levels of static pressure.
I always recommend overshooting the size of the blower a bit because it can always be toned
down by choking the intake with extra pre-filters, or you can regulate the motor
with a speed controller.
Example:
For my filter, 2ft x 2ft = 4ft2, then multiplied by 100ft3/min, equals 400ft3/min.
Then by adding the static pressures of the filters (found on the filters themselves),
1" (for the HEPA) and .2" (for the pre-filter), the total static pressure comes to 1.2".
So in this case a blower that moves 400CFM @ a static pressure of 1.2" is what is required
to make laminar flow with my filter.
It is important to note that blowers are rated for different outputs at different levels of resistance,
or static pressure. As SP increases, the amount of air the blower can move will decrease. This is
why you find the sum of the static pressure of your filter & pre filter, so you can compensate for
the resistance of airflow created by the filters.
For this particular flowhood build, I did not get an exact blower to match my filter. I got a
free blower from a friend who works in HVAC and was able to make laminar flow by adding a
speed controller to regulate the amount of airflow coming from the blower.

BUILDING MATERIALS: TOOLS:
2 Sheets of 3/4" sanded ply wood - ($70) Drill(s)
2 Pieces of 2"x1" furring strip - ($2) Caulk gun
Box of star drive wood screws - ($7) Straight edge
Tube of silicone all purpose sealant - ($6) Drill bits
Variable speed controller - ($20) Measuring tape
Small spool of 12-18ga wire - ($5) Jig saw
Roll of duct tape - ($3) Skil saw
Few wire nuts - ($3) Pencil
3 Prong plug - ($3)
14"x14" Ultra Allergen Pre filter - ($14)
99.99% 24"x24"x6" HEPA filter - ($160 shipped)
Squirrel Cage Blower - (FREE - $400)
TOTAL FOR THIS BUILD $300 - $330

Get a friend to help out. Having an extra pair of hands throughout the whole process makes it easier.
Unless you consider yourself a craftsman,
have your wood pre-cut at Home Depot or Lowe's
when you go to get it. They will be able to cut it for you in just a few minutes for free. Be sure
to account for the width of the 3/4" plywood in your measurements. The 5 sides of your box
will not have the same measurement!
Pre-drill all of the holes for your box.This will keep the plywood from splitting when you
run the screws into it. Use 2 drills to keep from having to change bits every few seconds.
Also use star drive screws (most of the screwboxes will come with the bit); they grab much better
and won't strip out on you while trying to put them in.
The biggest thing that will make this project easier is
using a speed controller.

The filter I ordered was a
24"x24"x6" wood framed 99.99% HEPA from
filtera-b2b.com.
I had to
call the company to place the order, which took about 25 days to reach my
doorstep, including the time it took to manufacture (they are made to order). The main
reason I picked the 24"x24" filter is because I wanted to be able to G2G into spawn bags.

I learned from reading
EvilMushroom666's build that he and
Total were able to wing it on
their blowers by calling and asking local HVAC companies if they had any old squirrel cage
furnace blowers sitting around that they might be willing to part with. As it turns out,
most of the time they will have a few, and be willing to let you have one for relatively next
to nothing. I ended up knowing somebody who knew somebody who worked in HVAC, and
all I had to say was that I wanted a blower to move air out of my screened in porch
during the summer.
I chose to shoot for a bigger blower than a smaller one because it is always possible to choke
a bigger blower down, whereas a smaller one would need to have a bigger motor
installed, defeating the purpose of getting it for free. In the end, the blower I got
moved so much air that I wasn't able to choke it down enough by blocking the intake, so I
went to Harbor Freight & bought a
speed controller that the motor plugs into.
If you also choose to use a used blower for this project it will be a good idea to take it apart and
clean it before installing it to keep a bunch of nasty shit from immediately getting stuck to
the back of your new HEPA filter. Take the motor off & remove the wheel, then clean the parts
(minus the motor) with brake cleaner.

I decided to go with a 2 foot cube for my box. I'm sure you could go with a smaller plenum if
your blower is smaller. My advice is to build the box around the filter, that way you can be
sure that it fits properly. Doing it this way will also help you to put the 4 sides together,
(the top goes on last after the blower is mounted) and helps keep all of the angles right.

After the 4 sides are together, measure the depth of the filter and mark it inside the box. This is
for the furring strip, which is used to butt the filter up against & hold it into place. Cut your furring
strips at the length of the box's panels (2 of the strips will need to be 1" shorter as they will run
into the adjacent strips).

For the top part of the box trace the shape of the outlet on your blower & cut it out with a
jig saw. I used some small nuts & bolts to attach it to the box but screws will work too.
Make sure to silicone around the edges where the fan's outlet meets the wood.
Next use the caulk gun & run a bead of silicone around every joint inside the box, including
around the furring strip on the inside. After that put on a glove and smooth it all out with your
finger. This will ensure that the silicone gets pushed up into the cracks & makes a proper seal.

Now the box should fit around the filter like a glove. You may have to use a rubber mallet and a bit
of strip to convince it to slide back into place. After, attach furring strips around the outside face to
make a trim that will sandwich the filter in place.

I also chose to build a top box on my flowhood. Doing this makes it quieter and also provides the
best way to mount a pre filter. Make sure to drill a small hole on one side of the top box for your
wires to come out of. If you build a top box, be sure to put silicone all around where the two boxes
come together, as well as around the hole where the wires come out.

*** Do not mount the pre-filter inside the top box as I did in the pictures above! It made it difficult to
change & couldn't put a new one back in that way anyways. Mount the pre-filter outside of the top box! ***
Most of the time blowers have a few different speed settings you can try by changing
which color wire you give power to. This one had 4 different speeds on it, each corresponding
to a different colored wire going to the motor. The
white wire coming off of the motor will be
the constant. Connect that to the
silver screw in the 3 prong plug. Test out each of the colored
wires and figure out which one you think is going to work best for you. Whichever
colored wire you use, (I used yellow) connect to the
gold screw. It took about 30 minutes of messing with
the wiring on this thing to get it right. It is simple enough & there are plenty of videos on
youtube explaining how to wire a blower if you can't to figure it out.

Once the plug is all wired up, plug it into the bottom of the speed controller & play around
with it. You want the flow coming out of the filter to be just strong enough to
bend a bic lighter 60-90° without blowing it out.