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How To Build A DIY Laminar Flowhood For Mushroom Growing

Laminar flowhoods are used in mushroom growing to stream a straight and steady current of highly filtered, super clean air in an uninterrupted flow, hence laminar flow! We use them to do sterile culture work, inoculate grain and substrate and many other things. Of course, this is all do-able in a SAB, but they are not as convenient, quick, or as efficient as a flowhood.

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Flowhoods come in many shapes, sizes, and orientations. Some blow from the back, some side and some from the top down. Some come as a cabinet style, where the sides back and front are encased, and the filter is blowing from the top or directly in front with an adjustable window to work behind.

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Some come as ‘FFUs’ or fan-filter-units where the fan, casing and filter are all together as a single unit. These FFU’s will soon be available in my store, you can see my post here where you can sign up to an email list and be notified when they become available! However, the majority in mushroom growing are homemade from HEPA filters, centrifugal fans, and wooden boxes.

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The Steps Of Making A Laminar Flowhood

Step 1 – The filter

First off you need to find a filter. In the UK, I recommend Jasun, I’ve used them many times and their quality and customer service are on point. There are also Add Filtration, another UK based company I’ve heard good things about.

The filter type we want is a H14 or H13 HEPA (High Efficiency Particulate Air) Filter. You can use lower efficiency filters but there is a negligible cost saving and a higher probability of contamination.

HEPAs are graded by how much they filter. H13’s filter out 99.95% of particles as small as 0.3 Microns. H14’s filter out 99.995% of particles as small as 0.3 Microns.  The difference in price between H14 and H13 isn’t vast in my experience.

The filter you choose, you want it to specifically reference Laminar flow. Filters too thin will blast turbulent air, that isn’t preferable as it can draw in contamination from outside the flow, so make sure they do reference Laminar flow.

In terms of physical size, I would select the biggest one you can afford, just bear in mind that you will need to budget for a fan and case to match. It’s no good spending your budget on a big filter only to find you can’t afford the rest! I would suggest going wider than taller, as you can always change the filter height by blocking underneath it, you can’t change the width!

Step 2 – The fan

The most commonly used fans are centrifugal, they can move a large volume of air within their compact designs, are relatively inexpensive and can be found almost anywhere. I have also seen them built with Axial flow fans, however, what matters most is the rating.

The biggest annoyance and confusion when trying to find your fan-filter combination is the mixture of units. For this guide, we’ll use metric. Don’t try to use a mixture of units to work out your combination, just pick one, it’s much easier.

The standard flow rate in clean rooms is around 0.51 m/s in air velocity. This is deemed sufficient for mycological use too. Don’t ask me why it’s this, but most Fan Filter Units used in clean rooms have a speed control of between 0.45 m/s and 0.55 m/s at the filter face, so anywhere in the middle is fine.

Fans move air in volume not velocity, so we need to work out what volume of air we want to move per unit of time in order to meet the velocity requirement, so, we need to do some maths to match our filter and fan.

Step 3 – The Box

If you can’t cut straight, like me, you have a few options. The first is to take the filter to a carpenter and ask them to build you one, giving clear instructions on the requirements. Another option is to sketch the box up and take the dimensions to somewhere that cuts wood, like B&Q. Option three is what I did, design your box on a 3D Modelling software like Fusion360 or Sketchup and have someone CNC machine the box out.

In the example below you’ll see the requirements of how I made the box, whatever style or design you choose to make, just be sure you build sufficient tolerances into the filter box. This is because wood expands and contracts, wood is never exactly the thickness it claims, the filters aren’t built 100% square and the person cutting the wood won’t be 100% accurate unless you’re a professional or use a CNC machine.

Step 4 – The Optional Prefilter

If you’re wanting to use a pre-filter, say you’re in a dusty environment, you’ll need to look at the spec sheet of that filter and add that to your pressure resistance and re-calculate again using the fans airflow curve!

Building A Laminar Flowhood
The Filter Calculation

I bought this filter, ULTIMA MP88 12X24, off of Jasun for about £180 with delivery.

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It took about 3 weeks to arrive. Looking at the spec sheet we can see the filter is approximately 610*305*88~mm, it has an internal resistance of 80pa for the H14 filter that it is. In order to begin searching for a fan, we must calculate the volume of air we need to move to generate the correct velocity, so, follow the steps below;

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Calculate Your Filter Face Area
Area = Width * Height – 61*30.5 = 1860.5 cm2

Convert it to cm2 to m2
Convert to m2 – 1 cm2 = 0.0001m2 so 0.18605 m2

Multiply it by your desired velocity
0.18605 * 0.51 = 0.0948855 m3/s

Multiply it by 3600 – The number of seconds in an hour
0.0948855 * 3600 = 341 m3/h

The Fan Calculation

With that quick maths, to achieve our desired velocity of 0.51 m/s, the filter requires 341 m3/h of air blowing through it. We now need to match the filters requirement with a fan, however, we can’t just find a fan that puts out 341 m3/h as we haven’t factored in the filter’s resistance, an optional pre-filters resistance, as well as an ageing margin. So, in order to achieve the velocity, the fan must be able to provide 341 m3/h with the 80Pa of resistance.

Now we need to hunt for a fan, I’d suggest eBay as a good place to start. I’ve used this fan for 2 of my flowhoods and they’re very good, however, you can use any fan as long as it provides the required flow at that resistance.

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Wherever you find your fan, locate the spec sheet that it comes with. There will be an Airflow Curve graph. On the Y Axis is the pressure and on the X axis the volumetric flowrate. We know we need 341 m3/h. Choosing the 700 m3/h fan line, we draw a straight line from the approximate location of your resistance until it hits the curved line, then draw straight down.

This tells us is at that pressure resistance of 80Pa the fan will put out approximately 600 m3/h, nearly double what we require. However, as your filter ages and fills up, the force required to push air through will create a high-pressure resistance, so having an overpowered fan is a good way of future proofing. If you are wanting to include a pre-filter, you would need to look at the spec sheet of that pre-filter to find the resistance and add that to the HEPA’s resistance and re-draw your line on the Airflow Curve chart.

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It’s worth noting that a fan that is too overpowered can damage your filter, imagine trying to push 600 cubic metres of water through your filter rather than the 340 cubic metres you require, it will wear far quicker and may damage the elements.  You can adjust the speed of your fan to put out the right velocity by buying an anemometer and a fan speed controller, if you just choose to buy a speed controller, you know running the fan at just over half speed will give you approximately the right velocity because you did the maths!

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However, the cheap anemometers are cheap for a reason. The bearings are so stiff that reading a low velocity of 0.51 m/s is quite hard. The more expensive ones, like this, are much better but reflected in the cost.

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Building The Box

You could also use a light switch dimmer, provided the power rating can handle the fan, and a lighter to measure flow, if the flame of a match or lighter bend back on itself at almost 90degrees you’ve got enough flow for mushroom work, you could also just block 1/3 of the fan’s intake off with some wood to restrict the intake and subsequently restrict the output, but if you’re building a high quality flowhood, you probably don’t want to do this as it looks terrible. So, with a speed controller like this, our fan will be more than adequate for our needs, especially since I’m not using a pre-filter.  Now we need to make a box.

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I have spent more time, money, and resources than I am going to admit on getting the box right. It was originally my intention to sell laminar flow flatpack kits, however, since I’m going to be selling purpose made FFU’s, I just can’t compete on value for money! Below are the prototypes… be gentle. 

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Essentially all you’re making is a box with a hole on top, a faceplate, and an internal backstop. The wood I chose was a 12mm high quality plywood. I’m showing most of this through my design software because I only took a few photos of the finished product before lending it to a friend!

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Start by measuring out the filter 2 or 3 times to make sure you’re cutting the wood to the right size. Then measure the internal size of the fan output and the outer lip of the fan.

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We know the filter is 88mm~ deep, however, it has a gasket on the front face which squishes down. Jfilters a style of gasket that is approximately 5mm thick. So that brings us up to 93mm from the internal backstop to the faceplate. I want the gasket to squish down and flare out a bit to create a nice seal, so I designed my box approximately 91.80mm from backplate to the faceplate.

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The way my box is designed is with dado/lap joints, this makes the box far more ridged than using but-joints or using plastic assembly joints. But-joints are fine too, especially if your material is thick enough. Originally, I tired an interlocking finger design with gave a great surface area, therefor greater rigidity, but was far more complicated to machine than dados. 

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From the front of my wood to the back stop is 97.8mm. 

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I designed the backstop to slot in as one continuous piece which creates an airtight seal. You don’t want air to escape around this backstop, you want all the air to go through the filter. Naturally, some will seep, that is why you have a gasket on the front, but a solid backstop is your first line of air-tightening up the box.

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From the back of the 12mm backstop to the backboard is approximately 145mm. This just allows the fan hole and securing points to sit comfortably in the middle without too much wasted space and material.

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You want to keep as much energy in the air as possible, if the box is too big, there is wasted space and wasted energy.

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Mirror the top and bottom just omit the fan hole. Then it’s just a case of building out the sides and front. For the faceplate and internal back stop I made them on continuous piece as I had CNC precision, in order to make it easier for you, your faceplate and internal stop can use single battens.

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When you make your joints, dado or but-joints, pre-drill the material with an undersized pilot hole before driving self-tapping screws in. This will avoid splitting and bulging the wood. Use a manual or ratcheting screwdriver or turn the clutch down on your drill to avoid over-driving the self-tapping screws in.

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You can seal with silicone or aluminium tape along all the internal joints and secure the fan by aligning the hole on-top with the fan outlet, marking up the 4 lip-holes and drilling. To include a pre-filter, make a little box that fits around your fan and leave one side open with a channel for your pre-filter to slot into.  Or just Velcro the filter over the intake of the fan, filter-cage side backwards to avoid turning your fan into a shredder.

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I made my own speed controller with an old dimmer switch and junction box I had. I’d suggest getting a speed controller for simplicity’s sake!

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The Cost & Source

ULTIMA MP88 12X24 HEPA Filter

£160 + Delivery – https://www.jfilters.com/i/UMP88141224/ultima-mp88-12×24

700 m3/h Centrifugal Fan

£70 + Delivery Included  – https://ebay.to/3jM0Htu

Wood & Cutting 

£60 + Delivery Included – A local CNC machinist 

Screws 3.5mm

£11 + Delivery Included – https://ebay.to/2TF3j1I

Speed Controller

£12 + Delivery Included – https://ebay.to/2TGjuvR

Silicone

£6 + Delivery Included – https://ebay.to/34I4HHg

Total –  £339

*Total excluding the cost of designing of the whole thing, the man hours spent doing so, the prototypes and mistakes which comes to about £450, 000. The cost of the sweat, blood and tears that were expended when prototype 3 didn’t work because of 1 silly error you forgot to check before confirming the cut… Priceless. 

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I hope I haven’t discouraged you from building your own laminar flow hood. If I have, feel free to join the mailing list on this blog post to be notified when the FFU’s will hit the store! Remember, there is a million ways to skin a cat. This is just the route I took which was going to lead to production models, however, with the FFU’s inbound, it just turned out to be a nice Flowhood that a friend is using!

As usual, any questions, feel free to email me or comment below! Cheers!

This Post Has One Comment
  1. Thanks for the guide. I’ve ordered the same fan and waiting to hear back from Jsun Filters, will purchase same filter. With a pre-filter what do I need to change? Apparently the fan controller shown is just a light dimmer in a different shell, but bought it anyway if its needed.

    How much wood do I need to buy exactly?

    Thanks. 🙂

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