Build it yourself laminar flow hood for less than $100! The mycology community will LOVE this!

first published Feb 10, 2023

I just built something the mycology community is going to LOVE.  But let me just start off with this... I think building this is pretty easy and I've already built more than a dozen of them.  It takes a couple of hours and requires minimal tools, but if anyone reading this wants to make and sell them for other people feel free to drop a link to your listing in the discord.

For years it's always bothered me that laminar flow boxes were so expensive, out of reach for many young mycologists, but let's be real, they make everything so much easier and most serious mycologists have one.  They can also be quite heavy (the one I've been using for years weighs nearly 90 lbs) and by no means portable, or even easy to move around your house.  I came up with a design that is light weight, portable, has a powerful variable speed flowrate, and is PROVEN to work (unlike all the silly things out there like a furnace filter duct taped to a box fan haha, or a giant plastic bag taped to a small home air/hepa filter, nope that's not laminar flow, put your arms in there and your work is ruined).  This design passes the flame test as shown in the picture above, even when it's not turned all the way up to max flow rate.   It has a high quality fan and the HEPA filter is good for 2,000+ hours, which by my calculations means a lifetime of use if you were only using it for mycology work, but the filter can also be easily replaced if needed for as little as $18.

First, credit goes to an anonymous guy I'll call "Rob" who private messaged me about this on Instagram.  He described the design he came up with that was the inspiration for my own design which I will share with you.  Rob lives in Europe so I did my own research and found the parts that are easily obtainable in the U.S. This was easier said than done as I had to test numerous filters and fans before I found the legit ones.  It turns out there are LOTS of fake hepa filters out there that don't actually work.  I purchased an expensive Met One laser particle counter to test filters for this project, and PROVE that they were working as expected.  As far as I know, I am the first person to describe such a low cost laminar flow hood and also provide analytical testing showing that it works.

TIP: Mark the center of the circle first and drill a hole there with a normal drill bit, then attach your hole saw, using the predrilled hole, RUN IT IN REVERSE this will grind out the hole with no cracking of the tote, works beautifully (I learned the hard way, destroying 3 totes before I got it right). I'll show how to do it in the video if this isn't clear.

Flip your tote upside down and notice the 4 bolts on the fan housing, put the fan into the hole you made in the tote with most of the fan inside the tote and just a small portion (1 to 2cm or a half inch) of the fan sticking out the side of the tote (see below picture), with two of the bolts on the fan housing unit facing straight up, mark the location of these bolts with a marker on the tote.  Take the fan out, then drill holes only slightly larger than the diameter of your bolts where you marked.

Use  sand paper and/or a file to smooth all edges where you made cuts in the tote, you can use your fingers to break off any blobs of melted plastic that formed from the cutting.  Use sand paper to roughen the plastic around the holes that you cut.
Then clean the tote thoroughly with wet wipes or wet paper towels.

Put the fan unit back inside the upside down tote, with only a small bit sticking out the side.  Use the new bolts and washers to bolt the fan to the tote:

Then put silicone sealant all around the top lip of the tote and heavy where the slit is for the fan cord, then put the tote lid on and latch the handles down.  Let it all dry for 24 hours, if possible let it dry in a garage or shed or outside as the drying silicone is kind of stinky.

That's it, you are done!  I still recommend using good clean procedure when using your new flow hood.  In the very least, turn off your home heating/cooling system to prevent forced air from blowing into the room while you work, wash your arms and hands thoroughly with soap and water, wear an N95/KN95 face mask, do not talk, sing, cough, sneeze, or rap while you work, wear new nitrile/vinyl/latex gloves sprayed down with isopropyl alcohol, try to work more toward the center of the flow, that usually means using an elevated surface such as a wire cooling rack or paper holder as shown above.  Remember the air at the edges of the filter is turbulent and not completely filtered.  Keep your work as close to the filter as possible without EVER touching the filter itself.  Don't ever put your fingers/hands in-between the filter and your work piece.  Only keep agar dishes open as long as absolutely necessary (that said I kept some open for 5 whole minutes in front of the filter just as a test and got zero contamination).

Note: It seems the air becomes turbulent at about 7.5 inches away from the filter with this design.  No particles getting through at 7 inches from the filter, so that is your working distance with this unit.  I have done agar plates, agar to grain jar, and grain jar to substrate inoculations all in front of this flow hood with 100% success rate so I know the working distance is perfect for mycology work.   Also, turning the flow all the way up to max actually causes turbulent air too (particle meter clearly shows this) which indicates how important it is to have a variable speed fan.  At max output, you will not have laminar flow at any distance from the filter really.  Thankfully, you don't need an expensive particle counter to figure this out, you can do it all with a lighter!  I will show this in the video.  Using a lighter you can check that the flame bends at about 45 degrees or so, and remains very "steady" that is a great indicator of laminar flow, you will see the flame shaking in all turbulent zones, like at the edges of the filter or past 7.5 inches back from the filter.  Where the flame shakes, you can expect high particle counts, where it is steady, zero particle counts.

Also note that "more flow" is NOT better, you want a very gentle flow, if you hold your hand up near the filter you should barely feel the breeze.  You can fine tune the flow rate very easily with a lighter (preferably a long stem lighter), keep the flame in the general area you would do work from, you are looking for the exact point where the flame bends 45 degrees from the airflow and remains perfectly steady (no flickering or dancing). If you used all of the same components I did, this will be at the fan setting directly in the middle between "M" and "H".

I recommend keeping the flow hood covered when not in use, this is actually shown in the video (not yet published).  The cool thing about the tote design is that the top edge/lip extends OUT a little bit so anything you hang over the top is not likely to touch the actual filter.  You can use almost anything for this, a $1 plastic drop cloth (for painting) from Dollar Tree for example, or a towel or bed sheet, etc.  This will prevent dust blowing around your room from landing on the filter.  Always turn the flow hood on before removing the cover, and replace the cover while the flow hood is still on (turn off after cover is in place).  It's also a good idea to work in the smallest room you can find to work in, and let the flow hood run for at least 15 minutes in that room before you begin working, this will clear the air of all small particles in advance.

If any of the above is unclear, it will become clear when you see the video.  But I am happy to answer any questions and I will update this post for a while until it contains every detail needed.

UPDATE: You guys have already started sending me pictures of your homemade flow hoods and they look great.  Here is an example with the Phillips filter that is more readily available in Europe and Australia:

(This patron made a LOT of cool tweaks to the design, so many that I created a separate google doc for anyone interested:  Jim the Corndodger Flow Box tweaks )

Notes related to the  VIDEO version of this TEK:  If anyone is looking for the Mycocoil (foot operated induction sterilizer) shown in the video it is  here.  And here's an example of the  silicone tipped tongs that you can pressure cook and use for mixing grain spawn with substrate which is better than using your hands. If you want petri dishes that can be cleaned and reused, the  glass ones are an option but often break in shipping,  polypropylene ones are probably the better choice and they are less expensive.

Credit to: The Efficient Engineer (“Understanding Laminar and Turbulent Flow")

Learn more about  Reynolds number.

Q&A Is it easy to change the filter?  The manufacturer says it has a 4000 hour life, even if you cut that in half, it's probably still a lifetime of use for just mycology work for most people (remember you don't leave this thing on and running all the time).  But yes if you had to replace it, it should be very easy to do so, the silicone sealant is like rubber cement, you can just pull the old filter off and scrape away the leftover sealant and put the new one in.  Likewise the tote lid will still come off pretty easily if you pull on it hard enough. The whole replacement can be done in about 15 minutes.

Can or should I add a pre-filter?  You could add a pre-filter if you want, you could do something simple like stretch a cotton t-shirt over the fan intake and secure it with rubber bands or zip ties.  Or you could use the carbon filter that comes with the HEPA filter.  It's up to you.  The HEPA filter is supposed to be good for 4000 hours, if only used for mycology work, that could be a lifetime for many people.  But as noted above, the HEPA filter is not hard to replace if needed, it's basically a 10 minute job, can pull it off and scrape away any remaining silicone sealant, then put the new one on.

Can I use a bigger filter?  Yes, there are a couple of options for larger HEPA filters that may still work with the recommended fan, such as this one: https://amzn.to/4a0N7xl
There is also a much bigger one: https://amzn.to/3PP5JIX but I'm not sure if the recommended fan will work with that one. The truth is, you can do everything that is important with the recommended filter from the TEK, you can elevate the flow box for working with mycobags so the top of the bag is in the center of the laminar flow. Doing agar work, making grain spawn, and doing grain transfers is really all you need to do with laminar flow and you can easily do them all with a small laminar flow box. I recommend you keep it simple and stick to the TEK size and filters for proven reliability and simplicity.

I noticed some air coming OUT of the fan area, is that normal?  First please make sure that you installed the fan in the correct direction because if you did it backwards the flow box won't work at all. But that said, yes, It's actually normal and expected for air to be flowing both in and out of the fan like that, this means the pressure has reached an equilibrium which is exactly what you are looking for, nice even pressure inside the box producing laminar flow out the front.  You can very easily visually "see" the laminar flow by using a long stem lighter, you are looking for a nice smooth not flickering flame that is always pointed at a 45 degree angle no matter where you move it around the filtered air work area.

Can I install the fan in the back pointed directly at the filter?  No, you don't want the fan pointing directly at the filter, this would cause uneven air pressure on the filter, you want it on the side and you will just allow it to build up equilibrium pressure inside the box (I probably should have explained that in the vid).

Is it an improvement to add a top and/or sides to the front part of the flow box?  No, sides and tops are counterproductive, they create a vacuum and unfiltered air must then be sucked in to fill the vacuum, creating turbulence (the opposite of what you are looking for).  You don't want sides or tops but its even worse if you direct the output into a still air box which for some reason many people seem to want to do (this completely defeats the purpose of a laminar flow box, you will have nothing but turbulent air).

Can I use the 6" or 4" version of that fan instead?  The 6" model will work, the 4" will not.  The HEPA filter requires around 200CFM but a little more is preferred, keep in mind that as the filter ages, it slowly clogs, requiring more and more CFM over time to produce the same flow rate.

I can't get that fan in my country or it's too expensive here, is there an alternative?  Yes, you can use almost ANY fan you are looking for minimum of about 250CFM.  Some people have had success using cheap Dell computer fans but try to find one that has a listed CFM rate.  Worst case you might have to get two of them.  But you will need to add a variable speed control if it doesn't come with one for proper tuning.  It's a bit more work but not too hard. You can use a "dimmer switch" to do the variable speed but you would have to do the wiring yourself. Many computer fans require a power supply sold separately.

Does the design really produce laminar flow?  I've seen some critics saying the design doesn't produce laminar flow, but they are wrong. The design does in fact produce laminar flow when properly tuned, you can see this visually with a proper flame test (as shown in my vid at 18:53) and you can also see it with a smoke test, even though I didn't do an intentional smoke test in the vid, there IS actually an accidental one, if you blink you could miss it, but it happens at 25:48 as I'm sterilizing a scalpel that is smoking from the induction coil. As you will see, absolutely beautiful laminar flow! Compare this to other videos of smoke tests on laminar flow boxes. Most of the critics disappeared after hundreds (if not thousands) of people built these with absolutely fantastic results, there is no question that the design works well, many have called it a "game changer" for them. I think its funny that initially there were skeptics despite the fact that I show the design works by using an expensive laser particle counter, flame test, open agar plate test, and a complete start to finish grow with it. Producing laminar flow is not as hard as many once believed. But you do need a legit HEPA filter and capable variable speed fan.