Determining how much intake is needed for a grow cab

Learninglots420

Learninglots420

Well-Known Member
I’ve been looking for threads or posts on finding out methods of testing how much intake I need for my stealth grow cab. I don’t want to weaken my exhaust fan because it is overworking to pull air in. One thing I have learned is that it is very difficult in determining it mathematically, especially with considerations such as: exhaust CFM, (including CFMs lost with a carbon scrubber and ducting length), intake fan CFM, small amounts of passive intake gained from cracks, intentional passive intake spots etc.

Just for the record my cab is about 4 feet long, 1 and 2/3rds feet wide, and 5 and 2/3rds feet tall. My exhaust fan is rated 250 CFM (inline duct fan) and my intake is only one 120mm PC fan rated like 85 CFM. My light is a 250 w MH that will be switched to a HPS for flowering. My grow will be four 5 gallon DWC buckets.

I am sure I need to install a passive intake to make up for the negative pressure debt, but I’m not sure how much I need or how to test if it is enough/too much (aside from waiting for a grow to start stinking and “fixing the problem”.) Even when the door to the cab is almost shut I can feel it being “pulled” in.

Anybody care to throw a couple suggestions at how I can test how much more intake I need without overdoing it? Thanks for reading this!
 
Learninglots420

Learninglots420

Well-Known Member
No cooltube or air-cooled hood, and since it's bagseed im sure I will be culling a couple of the plants to make room for the ladies. My temperatures have stayed around 82-83 and will hopefully be dropping a bit more once I learn more about how much passive intake I need to cut open. If i need to add some CFLs for more light later on I will, but at the moment that would increase my heat levels too much more than likely.
 
Bonzi Lighthouse

Bonzi Lighthouse

Well-Known Member
Ok... Keep in mind I highly recommend running an air cooled hood/cool tube, you're getting rid of the heat before it dissipates into your cab.

My number 1 rule is suck don't blow. Turn that 85CFM around and move it to the top. You want the same surface area at the bottom for passive intake to start. Think about how you will trap light without restricting air flow.

What is the ambient temperature at the passive intakes (where your 85CFM fan is now)?
 
Learninglots420

Learninglots420

Well-Known Member
If I need more exhaust I would rather just buy another inline duct fan since the PC fan is so weak, and after attaching a carbon scrubber it would be even weaker. So do you have any idea how I can test if my level of passive intake is sufficient? Even if I made all the changes you suggest im still at a loss for how to test if I have enough/too much.
 
Learninglots420

Learninglots420

Well-Known Member
Do you mean the ambient room temperature to know how warm the air being brought in is? Or do you want me to move my cab thermometer to where my intake fan is currently at to know what the internal cab temp is at the bottom of the cab?

Anybody else with some experience able to weigh in?
 
Learninglots420

Learninglots420

Well-Known Member
Hmmm, I wasn't saying you weren't experienced. I wanted to answer the question you asked, which is why I asked you what I did. Thanks anyway though.
 
Bonzi Lighthouse

Bonzi Lighthouse

Well-Known Member
What is the ambient temperature at the passive intakes (where your 85CFM fan used to be)((the ambient temp of the air being brought into the cab))?
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Learninglots420

Learninglots420

Well-Known Member
Bumping for the night crew. Still have not been able to think of a workable solution to test whether or not I have enough. On the positive side if I make too many passive intake holes I can just seal the extras up. Anybody have links to journals/threads where this has been worked through in detail? I'm all about trial and error, but without any way to measure the negative/positive pressure I just don't know how to begin. Thanks again for reading this!
 
Learninglots420

Learninglots420

Well-Known Member
Last bump for the evening. Im guessing that there is probably no tried and tested methods for determining intake requirements. If there were I probably would have read about it by now :). One method may be to simply place my hand on the passive intake behind the cab (which is very difficult to get to in my case) and test for suction, although I would imagine that only a significant negative pressure would cause enough suction to detectable with bare hands. More than likely I will just take the plunge with a couple moderately sized passive intake holes (with light traps) and if smell comes out later do some troubleshooting to detect whether it's too much passive intake or my carbon scrubber not performing adequately.
 
I

IndianaHome

Member
Negative pressure is not bad. just be sure that you are not burning your fan up. Also a note, try to postion intake and outtake kiddy corner from each other if possible. Dont worry to much. As long as the fan is pulling fresh air across the plant you will be ok.
 
Learninglots420

Learninglots420

Well-Known Member
Thanks for the response Indiana! Currently my active intake and active outtake are kiddy corner, and I plan to position my passive intake towards the center (just so it isn't smack in front of one of my buckets, or right up next to my active intake.) I definately agree that negative pressure isn't bad, in fact a small amount will be required to prevent the passive intake from stinking up my apartment. I would just hate to shortchange myself if my ventilation could be much more effective, but as I am stating to see there is really no easy way of determining how much intake is needed.
 
Learninglots420

Learninglots420

Well-Known Member
Thanks for providing that chart, that's a good resource to have. My cab temperatures have dropped a couple degrees after I stopped running the fan inside of it during the light cycle (still want it on some to promote stem growth.) Quite an interesting nugget of information I mined from RIU the other day ^_^. So my temperatures are pretty good even with my current ventilation, so once I beef things up I may be able to throw in a couple supplemental CFLs for flowering.
 
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