5 Steps To The 'Just Right' Extraction System

5 Steps To The 'Just Right' Extraction System

Fan speeds, CFM, m3/hr, rpm....blah blah blah it's a headache waiting to happen. Great if you know what they all mean and how they apply to your grow room, I pity those lot who are new to the game and then they've got to get their heads round complex calculations before they can whip out the dosh for any fan.  It's important to get this right though, so it's better to sit down with a cup of tea and get your pencil and paper out (you can get your colouring pens out too if you want to jazz it up) to do some mathematical problem solving now rather than having to deal with a bunch of diseased droopy plants that stink of shit. With this calculation, you'll also be able to work out what filter you'll need for your grow room too, so it's not all bad...two birds, one stone and all that.

Why is it so important? Surely you just need a fan to pull out/push in air from the tent or room? You'd think wouldn't you? But what the fans are actually doing is managing and maintaining the delicate balance of Oxygen, CO2 and pressure in your enclosed growing environment. Think of it like the walls of your grow room or tent sheets are like the Earth's Ozone layer and your grow space is the world, but all you want your world to do is grow some incredible plants, not just any incredible plants but your incredible plants. You have to effectively create your own atmosphere inside your own little Ozone layer. Make sense? You'll need fresh air feeding into your room and an exhaust pulling out depleted air reserves and heat. Push in too much air with your intake and not only will your grow tent inflate, your space will be massively hot, with uneven distribution of air and it'll smell stale because your exhaust cant cope with that amount of air. If your pulling out too much air, yep, your tent will suck itself in and it'll be pulling out the good, clean fresh air your plants need before they've had a chance to do anything with it. So, like our own atmosphere you need to create a BALANCE.



If you've got a tent, this is a piece of piss to work out, it's your tent size and it'll written on the box and you'll have probably picked it in a shop or online, so you'll definitely know what size you've got. To work out the volume see the calculation further down, otherwise, tent dwellers can move onto number 2.

If you've got a whole room dedicated to your grow project, first, congratulations, you are officially the envy of about 80% of growers right now, instantly making you look like a champion...no pressure! In your room you've got to work out your actual growing area, this is the area that the lights cover, if you've got them all over the room, you can use the room height x width x depth to work this out. If youve got a portion of your room without lights, measure the space from the light spread. To make this easier we've got a few measurements that might help to get you started.

Based on a standard Dutch Hammertone Reflector and HPS Bulb

  • 250 Watt Light 75cm x 75cm = 0.75m2
  • 400 Watt Light 100cm x 100cm = 1m2
  • 600 Watt Light 120cm x 120cm = 1.2m2
  • 1000 Watt Light 150cm x 150cm = 1.5m2


Say if you've got three 600 Watt lights your square meterage of ACTUAL growing space is 3.6m2. BUT, we need to know the how many cubic meters your grow space is, the volume, not the square meterage. To do this take a height measurement, then multiply the height x width x depth in cm/mm/m whichever you prefer using.


 So let's stick with the three 600 Watt lights in this pretend room we've got and our room height is 2.4m (240cm) our calculation would be 2.4 x 1.2 x 3.6 = 10.36m3. 



Fans are measured in in m3/hr thats how many m3 of air they can shift an hour, every hour. Our first volume calculation is for the STATIC volume, we need to now know how to clear this amount of air every minute, if we work it out at every minute this will tell us our MAXIMUM amount of air exchange allowed in our grow space before we start pulling out useful stuff, dropping the temperature and fucking up the humidity. As there's 60 minutes in an hour we multiply our m3 calculation by 60 to get our maximum m3/hr suitable for our room.

HEIGHT X WIDTH X DEPTH = m3 x 60 = m3/hr

Back in our three 600 Watt light grow room with a volume of 10.36m3 we need to multiply this number by 60 so 10.36 x 60 = 621.6m3/hr is our maximum exchange rate for our exhaust fan.

WAIT! before you go off and have a look at our amazingly priced and high quality fans to find one that matches your calculation, there's a few other important things we need to work out.



Nope, not like blokes dressed as women type of drag. Adding a carbon filter and ducting on your exhaust fan creates drag and reduces the efficiency of your fan quite a lot, as we've got to have these things attached to our extraction system we need to account for them in what seems like today's maths lesson. Don't fret though, its easy to work out. We take our m3/hr calculation for our room and multiply it by 1.33, this will account for loss of fan performance by attaching a carbon filter, ducting and accessories.

GROW ROOM m3/hr  x 1.33 = ACTUAL m3/hr

So, with our 3 light grow room with three 600 Watt lights burning our maximum air exchange rate was 621.6m3/hr. If we multiply this by 1.33 we get 621.6 x 1.33 = 826.7m3/hr. This is our actual MAXIMUM air exchange rate for our room with all of the extra bits added onto the system.



As were making our grow room achieve the most perfect and ideal atmosphere for specimen crops to thrive, it's not always as straight forward as a numbers game. In most cases the equation we've just been through will work for you, but there are some situations where it'll be off the mark, just a little bit, not much, definitely enough to make a difference though. What are they? and what can we do about them?

  • If your grow space is in a hot attic you'll want to increase your m3/hr by 20%
  • In a cold basement? Take 15% off your m3/hr
  • Add 20% for extra long ducting or if your ducting has loads of bends
  • Air cooled lights in your set up? Minus 25-30% from your m3/hr



The intake fan size is loads easier to work out, mainly because you've already done all of the problem solving already. Remember earlier at the beginning of the post we spoke about the balance of the intake and exhaust and the importance of getting it just right, like baby bear's porridge from the three bears...'just right'. The fancy term for this is negative pressure, you can google it to find out more about the science, but we're going to keep on referring to it as baby bear's porridge.

For your intake, you want a fan about 15-20% less powerful than your extraction fan, as long as it fits in this range your plants will start to think that you're in charge of the world and they'll do anything you command. As long as it's grow some high quality crops, they'll follow orders.

If we look at our pretend grow room to work this out, it'll make more sense. So in our grow room, we have three 600 Watt lights blazing away with a calculated maximum air exchange rate of 826.7m3/hr with carbon filter and ducting attached. We've put in an 8" RVK A1 Fan pulling out air at 796m3/hr. Now we want to take 17.5% (its 17.5% because thats the middle of 15-20% and we can use this number better to find a more suitable intake fan) off the extractor fan's rated speed 796 - 17.5% = 656.7m3/hr, this is the fan power we're looking for to use as our intake fan.

If all this has given you a headache and it's still not clear what you need for your grow room, you can come in store (or give us a ring) and have a chat with one of the team who can sort all this out for you and point you in the right direction. 

If you fancy adding a fan controller to your extraction system (highly recommended) I'd go for fans rated slightly higher than your maximum exchange rate so you've not got them running at maximum capacity as the room heats up, they'll be able to cope with the changes in the external temperatures outside your grow room that affect the internal ambient temp.


That's it! Well done for getting this far, it's a long winded description that's far easier when you actually sit down and do it. Following these guideline calculations will save you so much hassle in the long run when you realise your extraction system isn't balanced and isn't doing it's job properly, more than that, working this out now will save you an absolute fortune. Bad air exchange can seriously fuck up an entire grow.


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