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Where should I put my intake fan?

Discussion in 'Grow Room Design & Setup' started by completeretard, Jun 12, 2013.


    completeretard Member

    I'm growing in a tent in an attic that gets very warm in the summer.

    The fresh air comes from a vent outside, and then there's 9 feet of ducting to the tent. The intake and exhaust fans are both rated 235cfm.

    Where should I place the intake fan along the 9 feet of ducting to get optimum performance?


    contraptionated New Member

    What are the duct diameters for the intake and exhaust? What accessories do you have inline with the exhaust trunk? It will be easier for the intake fan to push through the 9' of duct so you would do best to put the intake fan as close to the source of fresh air as the 9' run would allow , but be careful. You wouldn't want the sound of that intake fan to be detected by others.

    I don't use intake fans myself but there are always exceptions to my "rules". If you have limited openings for fresh air and if they are remote then you could balance an intake/ exhaust configuration. It would be wise for you to get an airflow meter so that you can make sure you do not unintentionally create positive pressure that would leak odors and heat to the surrounding ambient of the tent.

    MYOB Well-Known Member

    Always pull air when possible. IT is much more efficient.

    I doubt you need an intake fan at all. The exhaust will pull air through the intake passively.

    You also want to have more power int he exhaust so that negative pressure is created which is how fresh air gets drawn in. Put the exhaust at the top of the tent and the intake on the opposite side on the bottom. Intake should be twice the size of the exhaust.

    twistedj420 Active Member

    ^^What MYOB said, it is always best to exhaust at the top of the tent because warm air rises and that is what you want to get out, and if I were you i might want to upgrade to a 6" intake, they will move about 440 cfm. 9 feet just seems like alot of travel for a 4" intake.

    contraptionated New Member

    I agree with you 100% MYOB. But I think his problem is that he has not too many openings to take the air from. He stated that the air intake opening is from a vent outside. If he could it would be great to make more openings so that he could take it in passively. But he's only working with one opening (I'm assuming its too small to do passive intake until he tells us the duct diameters he's working with) , the vent register coming from outside of the attic, as he implied.

    I'm definitely not arguing with you MYOB. I'm a proponent of the passive intake / constant exhaust scenario. I've actually agreed with you quite a few in times in other threads. But if he only has the one opening for intake air, then he will not be able to do passive intake unless his duct can be sized accordingly. Which might be possible.

    P.S. if he has no choice but to do a forced air intake (because a duct for the intake cannot be upsized for passive) then the fan is more efficient pushing air through the beginning of the 9' duct with the influent side of the intake fan unrestricted by a ducted connection.
    completeretard likes this.

    ginnzy Active Member

    for myself, I have 2 800 cfm fans venting my main room, But I have a rheostat on my intake, so i can turn it down a wee bit, do I can create some negative pressure. works great.

    completeretard Member

    Thank you for all the replies. As has been pointed out I have limited options for intake. I'm using 4" ducting, and the exhaust fan is at the top of the tent pulling from a carbon filter and into a cool tube.

    It seems I should place the intake fan very close to the opening. I may need a fan speed controller to set it up to maintain negative pressure.

    contraptionated New Member

    That is definitely a great way to use a rheostat (speed controller). I concur!

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