Exhaust/intake, pressure in box

jbleez · Jul 2, 2013

I've seen so many different answers to this question online..

What is the size ratio or power ratio of intake to exhaust supposed to be? In the Marijuana Bible I believe it says to have twice as big of an exhaust as an intake... well I have an almost 20 cf box and I have a bigger/stronger exhaust than intake but when I close the door to the box it seems like the exhaust has to work much harder, and is pumping less air out. Does that mean I need more intake? Thhanks

Chiefems · Jul 2, 2013

Yeah you will need to increase your intake. I've had the same problem before.

BygonEra · Jul 2, 2013

You can add a passive intake. The reason it seems like that is because the exhaust is struggling to pull air through the intake which is moving air at a slower rate... when you open the door, it's able to move more air.

MorroN · Jul 3, 2013

jbleez said:
I've seen so many different answers to this question online..

What is the size ratio or power ratio of intake to exhaust supposed to be? In the Marijuana Bible I believe it says to have twice as big of an exhaust as an intake... well I have an almost 20 cf box and I have a bigger/stronger exhaust than intake but when I close the door to the box it seems like the exhaust has to work much harder, and is pumping less air out. Does that mean I need more intake? Thhanks

Passive intake hole will solve your problems, no need for more fans.

jbleez · Jul 3, 2013

Awesome thanks guys. But isnt it good to have some negative pressure?

Also, i have an 8 inch fan, not sure about cfm but this thing has some power. How much passive intake should i have?

MorroN · Jul 3, 2013

jbleez said:
Awesome thanks guys. But isnt it good to have some negative pressure?

Also, i have an 8 inch fan, not sure about cfm but this thing has some power. How much passive intake should i have?

Your exhaust fan is struggling because the pressure in the grow room is too low (its sucking and there is nothing to suck), if you cut/ drill a few small holes into the bottom of the cab you can even out the balance and you should still have a nice amount of negative pressure. Just experiment, start small and see when the fan stops struggling

notballin · Jul 3, 2013

I'm in agreement with what everyone else said (I'm using a passive intake myself). A hole at the bottom that's half the size of your exhaust will do the trick. No fans needed for the intake as the exhaust creates enough negative pressure to force air in from the intake.

Exhaust/intake, pressure in box

jbleez

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Chiefems

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BygonEra

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MorroN

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jbleez

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MorroN

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notballin

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