Forays into flowhoods

Forays into flowhoods

DIY Flowhood build for the Myco-lab

May 2023, by Melinda Dunnet


What is a flow hood?

A laminar flow hood is essentially:

  • A box
  • containing a suitably sized fan
  • blowing a steady stream of air though a very fine air filter (medical-grade HEPA filter).

This generates an almost sterile stream of air over a workbench. 

Working in this constant air flow with the proper technique allows you open and inoculate things like petri dishes, grain jars or sterilised substrate with a very low risk of contamination.

* In case you were wondering: commercial flow hoods have an actual hood (a glass or metal box in front of the HEPA filter, helping to create an enclosed, clean workspace). What we have built here, and what most mycologists use, is a slightly simpler version without the hood – more accurately called a laminar flow box.

Why do you need one?

When we grow mushrooms, we want to give our fungi friends an edge over all the other organisms (usually bacteria and moulds) competing for their food (substrate). This is especially important when we are inoculating sterilised substrate containing added nutrients (e.g. sawdust with added bran or soy – delicious for all sorts of things). 

We also need a way of maintaining ‘mother’ cultures (e.g. agar plates and grain jars) so that we are sure they contain nothing else except for the type of mushroom we are trying to grow. To do this, we need a very clean space to work in: a spot where the air and surfaces are as free of contaminants as possible. You can have a reasonably high rate of success with a Still Air Box (SAB) or glove box, but if you are serious about cultivating, you will eventually want a laminar flow hood. They are much more convenient to use and you have a much higher chance of clean cultures.

The design

Our new Afrifungi satellite lab needed a flow hood. We wanted it to perform, but also adhere to rule No. 1: look good ;) So we set out to build the sleekest, simplest and most awesome laminar flow box that we could. After some time pricing filters and squinting at fan curves, we decided on an H14 610x610mm HEPA and a HIT315B fan. This kind of inline fan meant that we could mount it directly behind the HEPA and the whole flow hood could be a single box. We decided on a pre-filter the same size as the HEPA, so that the entire box could be a cube. 

We designed it with a central vertical panel dividing the box into two compartments – separating the ‘inlet’ side of the box from the ‘outlet’ side, with the fan blowing through a circular hole in this panel. This prevents the fan from just pulling air round and round inside the box – it needs to pull air in through the pre-filter and push it out through the HEPA filter. The panel also creates a ‘plenum’ or space behind the HEPA filter – this must be large enough to create laminar flow.

Sven designed the box in Autocad, we ordered all the parts and had the wood (18mm MDF) cut to size by the supplier.

The build

We put the box together using wood glue and screws, using a counterbore and plug cutter to hide the screw heads. A small jig helped us get the screws in the right place and the box square. We sealed all the interior joints with extra wood glue (you don’t want any air leaks, anywhere!) and then all the wood with sanding sealer. The outside of the box was finished with white spray paint. We mounted the fan to the bottom of the box with four bolts and fitted it through the dividing panel, sealed with lots of weather strip. The fan was wired up (after a short headache – electrical diagrams are not our strong point!) to a 2-speed switch mounted on the side of the box. 

We used strips of wood inside the box to hold the HEPA and the pre-filter in place (note to self: double-check that the filters fit BEFORE glueing any wood in place). The pre-filter is held in place with an aluminium frame screwed to the wood and clips (buy these with your pre-filter). To hold the HEPA in place, we used aluminium flat bar along all four front edges, attached with stainless steel cap screws and threaded inserts – this not only looks cool, but is more sterile and easier to clean. Lastly we fitted some Teflon feet to the underside.

The details

Choosing a filter:

HEPA stands for High Efficiency Particulate Air filter. HEPA filters are made of very finely intertwined threads that trap particles (dust, pollen, mold, bacteria, etc), cleaning the air blowing through them. Grades H13 and H14 are considered ‘true’, medical-grade HEPA’s. The higher the number the more particles they remove e.g. H14 removes 99.99% (10 particles out of every million get through) while H13 removes 99.97% (30 particles out of every million get through) – also pretty good! You could probably get away with an H13. Don’t get anything less. H14 is better. 

You will also need a pre-filter. This removes larger particles like dust from the air before it hits the fan and the HEPA. Without a pre-filter your HEPA will clog very quickly. 

Choosing a fan

Because the threads in a HEPA filter material are very tightly packed together, you need to have a strong fan to be able to blow enough air through it to create ‘laminar’ flow. Laminar means smooth flow without any turbulence. Turbulence or eddies in front of the HEPA would blow dirty air back over your workspace. The speed of the air flow determines how far away from your filter you can work and still be in the sterile air flow. Take-home message: don’t skimp on the fan! 

The calcs

To find the right size fan for your filter:

  1. calculate the air flow that your HEPA filter needs for laminar flow
  2. calculate the combined pressure drop of your filters
  3. use the pressure drop to read the air flow delivered by a fan off the fan curve

Step 1

To achieve laminar flow, air needs to be moving through the HEPA at an air velocity of between 0.3 and 0.5 m/s. How much air you need to blow to reach this speed depends on the size of the HEPA. To find the volumetric air flow rate, multiply the area of the filter (width x height, in meters) by the air velocity.


For example, for a 1220x610mm filter at 0.5 m/s: 

W x H x air velocity = 1.22m x 0.61m x 0.5m/s = 0.372 m3/s

If you want to convert this to m3/h, multiply by 3600 (the number of seconds in an hour) 

0.372 m3/s x 3600 s/h = 1340 m3/h

For the same filter at 0.3 m/s the answer is 804m3/h. 

So, for this size HEPA, your fan needs to be able to blow between 804 and 1340m3/h, given the resistance of the filters. It’s always best to overspec a little. You can always add speed control if it’s blowing too much, but if it blows too little there’s not much you can do except buy another fan.

Step 2

The pressure drop of a filter is a measurement of how much it resists the flow of air through it. This should be given by the filter manufacturer on the spec sheet. There should be an initial and final value. Initial is when the filter is new and final is when it is becoming clogged with particles and needs to be replaced.


For example:

HEPA initial = 120 Pa

HEPA final = 600 Pa


Pre-filter initial = 70 Pa

Pre-filter final = 250 Pa


Add them together to get the total 

Initial = 190 Pa

Final = 850 Pa


Step 3

Now, find the fan curves for the fan you are looking at. This is a set of graphs showing how the pressure, efficiency and power of a fan change with the pressure drop (the resistance against it). You are looking for the graph showing how the volume of air it puts out changes with the pressure. On the left-hand pressure axis, find the total pressure drop you calculated in the previous step. Draw a straight line across to the curve. Then draw a straight line down to the volume curve to find the air flow rate that the fan will give at that pressure.


For example:

This graph shows the three speed settings of the HIT315B fan: low (green), medium (blue) and high (orange). Looking only at the high speed setting (orange curve) for the initial pressure drop:

  • Find 190 Pa on the left-hand axis and draw a straight line across to the orange curve (red arrow)
  • Drop a line straight down to the bottom axis (orange arrow)
  • Read off the volume (approx. 0.38 m3/s)
  • To convert to m3/h, multiply by 3600 s/h

= 1368 m3/h


So, this fan on its highest setting would work for a 1220 x 610mm HEPA at the initial pressure drop. However, there is not much room for a decrease in flow rate as the pressure drop increases (as the filters get old and blocked with particles). The graph does not even go up to the final pressure drop of 850 Pa. Once the pressure drop exceeds about 350 Pa, the volume would drop below 800m3/h (0.22m3/s) and the flow would no longer be laminar. But the HIT315B should be well big enough for a 610 x 610 HEPA.


Shopping list:

  • HEPA filter H14 610 x 610
  • Pre-filter with holding frame and clips 610 x 610
  • HIT315B fan with mounting bracket (315mm inline tube fan)
  • 2-speed switch for fan
  • 18mm MDF cut as follows:
    • Top/bottom    2 off 550 x 613
    • Sides              2 off 550 x 649
    • Separator       1 off 613 x 613
    • Frames long   4 off 613 x 18
    • Frames short  4 off 577 x18
  • Aluminium flat bar 38mm x 3mm
  • M4 threaded inserts 20 off
  • M4 x 25mm stainless steel cap screws 20 off
  • Small screws for attached pre-filter frame
  • Weather strip 15mm x 5mm x 10m
  • Wood glue
  • Sandpaper
  • Aluminium polish
  • Wood filler
  • Sanding sealer indoor
  • 5mm x 35mm chipboard screws
  • Paint for exterior
  • Teflon feet
  • 4m electrical cable
  • Couple electrical connectors
  • Plug




Counterbore and plug cutter (optional but cool)

Flat work surface, preferably with a backstop that you can push against





Saw for aluminium

Wire stripper

Happy Building!

If you have any questions please don't hesitate to give a shout at

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