Building a flow hood
Selecting the right size filter for your job- When thinking about this you need to think bigger picture
o How far am I taking this
- If you are thinking commercial think wide and tall
o What work am I going to do in front of it
- If you are working bags think tall
o How many people will work in front
- More than one think wide
o Work space
- You cant fit an elephant into a mouse cage
- For me my thought process was :
o I want to try commercial in the future but on a small scale and will upscale with the business
o I will be doing bag work which means tall
o I will be working alone so it doesnt need to be too wide
o In the 6 Meter x 3 Meter grow area I built the lab room in the corner of that is only 5ft by 4ft plus I wanted space for lab equipment so.
- In the end with all this in mind I decided on a 24’’ x 24 ‘’ filter and so to the task of sourcing the right filter.
The right filter for the job- Hepa filter 99.99% filtration you want H13-H14 - HEPA H14: pass up 0.005% of 0.1 micron particles per liter of air.
o It doesn’t matter where you get it from or cost but in the UK you are looking at £150- 300 for a decent filter
Matching your filter to your Fan- This is the most complicated part of the process and I will try to simplify it for you
o Each fan has a pressure drop-This means the resistance the filter gives you when you blow air through it to get a proper flow
o You need to match that with the speed of the fan so it gives you a speed of 100 feet per second or 400 m3/h
Lets be honest if you get sterile air blowing continuously at your work at any speed it will keep most mould spores and air bound bacteria at bay. However what you really want is a laminar flow
* Laminar airflow is defined as air moving at the same speed and in the same direction, with no or minimal cross-over of air streams (or “lamina”). By contrast, turbulent flow creates swirls and eddies that deposit particles on surfaces randomly and unpredictably.
- My example will be the fan I bought for my hood
The filter I bought was this MP78-2424 from Json filter
https://www.jfilters.com/
and the pre filter Hi Cap Pleated Panel 12x12x1
It is a H14 meaning it filter 99.995% of all particles 0.1 microns.
You will notice that it is a new school thinner filter and there were some interesting debates on the threads at the end* on whether or not its right. It Is!!!!!!
On this chart you will see the 2424 filter data and all you need to know is pressure drop verse the flow rate.

For the blind and lazy its here
As you can see the pressure drop on this is 100 PA and the flow rate is 603M3/hr. In the US they measure differently. Being in the UK the go to Dayton blowers aren’t affordable or readily available and so what I did was go round all the uk mushroom growing sites and got a consensus. Which lead me to the path of the RVK range. You can find them on the internet and I suggest you buy local if you can.

Each blower comes with a chart that allows you to find its compatibility with any pressure drop you face in any job it needs to do. In this case its this one
What this info is telling you is the Pressure drop verses the m3/h. Keeping in mind im using a prefilter and a controller -looking on the chart I decided to get a RVK250E2-L1. As you can see it comes in at just over 900 m3/h. Bearing in mind the prefilter has a pressure drop and using using the controller, 100% compatible with the fan unit(you need to find this out, important!)I can get the desired airflow by dialing it down. Again the reasoning behind this was that in the future as the filter gunks up I can up the flow rate accordingly giving me extra life out of it.
https://www.ebay.com/itm/Fan-Speed-Controller-3-amp-Triac-High-Quality-ME1-3/292264269477?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649CALCULATIONS AND DEBATE ARE ALSO IN THE LINKS PROVIDED.*

and

Well there is all the boring shit out of the way. It’s a bit like making a spliff when you are tripping so hard you cant see or remember what you are doing. Its fucking hard but as long as you stick the rizzla's the right way round, totally worth it. All you need now it the material you choose to build it out of. I chose some nice looking ply.
The buildFirst off you
need to build a box for the flow hood and
can if you want build a box for the fan, I did.

For sizes you want to work out a length where the fan sits on top but doesn’t overhang as it would be a pain trying to slide it up against a wall and deep enough so it misses the filter casing and wooden flange. This is 2 lengths in my case at 18’’ x 24’’ (457mm x 610mm) and 2 length at 18’’ plus 24 (457mm x [610mm + 18mm x 2] = 646mm) plus the thickness of the wood x 2. The back panel is 24'' x 24'' 610mm x 610mm

I have a merchants who will cut your wood for if you buy it there so get your measurements right and save time. My guy fucked it up, I was too lazy to go back………and I had to work round it.
For every screw drill a small pilot hole and you wont get splitting wood.
Attach the bottom, sides and back together with 4-5 screws on each join

Add the top but don’t screw in
Place the filter in the build and mark how far it goes in by pencilling in a line round the filter.
Remove the filter and make a seat for the filter to buffer up too. This is done by cutting a 45% angle on the 4 -2 inch by 1/2inch pieces and fitting them to the inside of the box.

Add the top but don’t screw in.

Check to make sure the filter fits, we are looking for as snug as fuk

Place the fan/blower on top of the hood making sure the back doesn’t overhang and the sides are centred by measuring the distance.




Pencil round the fan to mark the area you need to cut to sit the fan/blower in.

Drill some holes in that area to allow the jig to do its work
Important infoSet the fan/blower in the hole and wire the controller up. Its usually plug to Fan/blower live neutral Earth to live neutral Earth lug on the controller, then Earth lug to earth, UZ to live and U to neutral on the fan.

measure round it to work out the size of the top box

Build the box separately and add a hole for the top filter dont worry about glue here suction will keep it in place












Cut a small hole for the lead to go through

Cut 4 lengths of wood with a 45% angle to hold the filter in place. I took the bottom one out in the end as I like some work up as close to the filter as possible for some work.

Take everything apart glue all the joins, screw together again and silicon all the edges. Place the filter in the hood and screw in place using the batons without gluing


Its then ready to test with and anemometer.

This one is shit and not sensitive enough but to be honest the lighter test is sufficient. As long as you have a 90-ish degree angle when you hold a lighter near to the filter and it doesn’t go out that is sufficient.
Your hood is now ready to use.
Find yourself or build yourself somewhere to work

And get on with it.



One small piece of advice. Treat your wood with some anti mould substance. I didn’t and grew a little black mould……genius, so I painted it


And built some shelves for luck

and I bought a 2nd hand fridge for cultures

I love working with mushroom in any shape or form and having a hood allows you to concentrate on all the good stuff confidently. Be good to each other and don’t ever forget

*Here are my research thread with some interesting debates especially from Alien and Jbg. Thanks for everyone who helped me get my head round what at the time seemed super complicated. I loved the journey and hope this helps others. What a banging cool community this place has.
https://www.shroomery.org/forums/showflat.php/Number/26168622/fpart/4/vc/1#26168622https://www.shroomery.org/forums/showflat.php/Number/26092698 ********************************
***UPDATE*************************
It is very important to seal up the casing around the edges with something like wood filler or silicon.. You need to check on the monthly too