The answer to everyone's ventilation questions?

HighOnTheHill · May 9, 2009

Hey guys. I spent a lot of time looking for the answer to the same question that's asked repeatedly on here - What CFM fan is necessary to cool my grow cabinet.

According to this, my space which is 24 cubic feet... only really needs 8 CFM? =/ Well I have a 21 CFM intake, my exhaust isn't as powerful the CFM is unknown but I've got air blowing out cracks rather than sucked in. Going to go buy a 50 CFM bathroom vent fan from Lowes, they're only like 15 bucks. That should keep a 24 cubic foot cabinet cool? The Exhaust would be 29 CFM larger.

Also, I figured in the venting pipes and stuff... I don't have any. It's a small box, ventilation is diretly on the top in the very middle, fan mounted directly to the hole. Intake is the same + cracks. I intend to fill the cracks somehow.

The ventilation system is one of the most critical aspects of growroom design. I'm going to shy away from discussion of AC use for now- most personal gardens can get by without, and those that can't will generally be served by the climate control system of the dwelling. For now, we'll assume that that primary cooling/ dehumidification is going to be through exhaust.

The ventilation system, at it's simplest, consists of three elements- the intake, the exhaust, and the blower/fan. Choices here are about equivalently important. Let's start with fan sizing, which is tied to room size (in cubic feet) and choice of lamp.

For now, I'm going to assume that people are using a single main exhaust blower and a passive ( unpowered) intake. (Active intakes can be very effective, but great care must be taken to insure that intake CFM does not exceed exhaust CFM. In this case, you'll achieve a positive pressure condition in the growroom. This excess pressure WILL disperse in an uncontrollable manner, bringing with it delectable but dangerous aromas.( See above primary principles.) I would advise always trying to run at as close to nominal pressure as possible, with any variation from nominal being negative.)

Calculating cubic footage is simple- length x width x height = cubic volume. You want a main exhaust fan which can exchange the air in your room in no more than five minutes. I try to budget for three minutes. Let's comprimise at four minutes. Therefore, a 5x5 room with 8' ceilings would require a fan capable of [ 5(l) x 5(w) x 8(h)] = 200 cf. 200 cf / 4 (minutes)= 50 CFM for your fan. Doesn't sound like much, huh?

Now let's get into efficiency factor multipliers. ( This is where it all goes to hell).

Take your unloaded CFM requirement, and add 10 % for each foot of flexible ductwork that you are exhausting thru.( ie- you need to clear a 4x4x6 room thru 10' of ductwork. That's {96 CF /4 (minutes)}= 24 CFM + {(10'x10%)=100%} 24 CFM+100% (of 24 CFM)= 48 CFM. ( Exhaust loaded CFM)

Now take your (E.L.)CFM and multiply it by 1.5 for each 90 degree bend in your exhaust ductwork, cumulitively. ( Ie- you have a loaded CFM of 48 cfm that makes two 90' bends in it's ten foot length. That would be (48 x 1.5)x1.5- or 108 cfm loaded w/ bend factor.)

OK- CFM requirements are adding up pretty quick, and we haven't even talked about odor control. I personally think that carbon filters are the best method of odor control- but I figure that we'll get a healthy debate about this too. I like to put my carbon filter inside the room, near the ceiling. I like to set up the filter before the fan, so that air is sucked from the space, through the filter, through the fan, and then out of the space. This way, all air being pressurized by the fan has already been de-odorized. You can blow through the filter if you mount it after the fan, but be aware that between the fan and the filter there will be a zone of pressurized, stinky air- any leaks in your ductwork moving air from fan to filter will create potential smell issues. ( See primary principles above.)

Take your EL CFM ( including bend factor) and multiply by 1.3 to allow for intake restriction of the carbon filter. Don't forget to allow for ductwork between filter and fan!

So, if we're running a filter that's 3' away from the fan- our total duct length ( in the above example) is now 13'. Let's adjust our math.

We have a 4x4x6 room. Our total duct length is 13'. We're using a filter. Our math now looks like-

4x4x6 room= 96 CF. Divided by 4 minutes is 24 CFM required.
24 CFM + 130% ( 10%x13')= 55.2 CFM ( I'm gonna round to whole CFM, to try to minimize decimal over-runs)

(55CFMx1.5)x1.5= 124 CFM â€“ to allow for our two 90 degree bends.

124 CFMx1.3 ( to allow for air velocity lost to the filter) = 161 CFM.

So we're looking at a 161 CFM fan.

But wait- we haven't even thought about how our light's going to effect this. We could go off into a discussion of determining system effeciency by measuring intake and exhaust temperatures so that we could calc differential temperatures, but I don't know how to make the little â€œ deltaâ€ symbol on my laptop keyboard, so I'm gonna skip that and assign yet another load factor...

For a 250HPS- multiply by .75
For a 400- multiply by 1
For a 600, multiply by 1.3
For a K, multiply by 1.6.

(Let's be reasonable here- I know that my math falls apart if you're running a K in a 4x4x6 space â€“ but is it reasonable to run a K in that space at all? In your very first room?)

So to put a 600 in that room, we'll take our base adjusted CFM and multiply by 1.3 .
161 CFM x 1.3 (lamp factor)= 209 CFM fan/blower to power the ventilation system.

I'm not going to blow out the math to establish what room intake sizes should be to prevent drag on the system- that get's WAY crazy... Instead, I'm going to propose that we use a rule of thumb stating that â€œ Intake area should be fan CFM x .5 square inchesâ€

Applying this rule, our 209 CFM fan would require an intake area of about 100 sq. inches- or 10â€ x10â€. This does'nt have to be monolithic- two 50 sqâ€ intakes will work as well as one 100 sq â€œ intake. You can check your intake sizing by just cracking open the door to the room and firing the fan- if the door moves at all, you need more intake.

When shopping for fans, round up- if you need a 209, and your choices are 180 or 240, grab the 240.
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The answer to everyone's ventilation questions?

HighOnTheHill Park your nonsense

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