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Low Humidity Issues in Grow Tent 400W MH 3x3x6 PLEASE Help!

Discussion in 'Grow Room Design & Setup' started by george22879, Feb 18, 2009.


    george22879 Active Member


    I have a new mylar grow tent that I am working on. This tent is for VEG only with light on 18/6 schedule. Length 3ft x Width 3ft x Height 6 ft.
    Thats 9 square feet with 54 cubic feet.
    I have a 400 W MH Bulb that gets quite hot in this mylar enclosed space drying up some moisture.
    Right now I have a 170 CFM bathroom fan as an outtake on the top with 2 passive intakes at the bottom of the tent. The 2 passive intake holes have light traps and are about 6" each. This creates a negative pressure inside the tent once its all zipped up. The room is exchanging about 3 times per minute.
    This gets my temperatures down to about 80 degrees F with an oscilating fan when the light is on. The humidity however is a mere 20% tops. I feel that the outtake fan is sucking all the moisture out of the tent and bringing in the ambient humidity which is only about 20% this time of year.
    I added a humidifier to both the tent and the ambient room and still 20-25% humidity tops.

    When the light and outtake fan is off at night the temperature gets to about 72-75 degrees F and 60-70% humidity which IMO is perfect!

    As soon as the light and outtake fan are on BAM humidity goes down to 20% and temperature to 80-82 degrees F.

    Is 20% humidity for VEG OK ???
    Is the drastic humidity change a major stress to the plant? 20% day and 70% night humidity?

    I use Domes for seedling/clones so thats not a problem. My mother plant might not like the 20% humidity though...

    Trying to tweak as best as I can. I fix the temperature issue but at the humidity's expense.

    Please help!
    LAX Skunky BwS

    LAX Skunky BwS Well-Known Member

    maybe u should look into getting a humidifier

    mikecar312 Active Member

    if you have some money, buy a temp/humid controller and let it take care of temp/humid all year round. ime, as long as your ambient temps aren't too high, you shouldn't have a problem with humidity. i bought one for my tent, the cheapest is about $200. with a 400w, your temps won't rise that much and you can set humidity to 60%-70% which is optimal for veg.

    dusty26 Well-Known Member

    ok here's my imput on this post... lol if you added a humidifier to both the tent and the outside room and your only at 20 some percent then the humidifiers you bought are shit...
    you should be hitting 50+ very very easily especially with a humidifier in the room itself... look

    go to walmart they have a nice holmes 1 gallon humidifier for like 24 bucks buy it... and put it inside your tent on high ... gurantee it will boost your humidity to 50+ percent...

    MaDz Well-Known Member

    Invert the fan to create a positive pressure and put a humidifier connected to the tap water nearby if you can... This will help a lot! :)

    Good luck

    BongJuice Well-Known Member

    What part of the house is your grow tent in?
    If you have a passive intake then it's going to be sucking the air outside the tent faster than what the humidifier is going to put off.
    You can try placing the humidifier right outside the intake opening or you might want to get a large humidifier for the room that your tent is sitting in.
    Another option would be to purchase a motor speed controller for your blower. Then dial it down to a lower setting, so the humidity can fill up the tent.

    gardenman Well-Known Member

    maybe you should set your fan to exchange the air once every five minutes instead of 3 times a minute.

    george22879 Active Member

    Right now I have 1 humidifier in the ambient room and 1 inside the tent.
    That managed to get the humidity up to 30%.

    I put my outtake fan on a 15 min ON/OFF timer. This helped the humidity a lot but while the fan is off the tent soars to 82-85 degrees.

    So I think I need to rethink my ventilation.

    I ordered a kit that will allow me to run the duct through my hydrofarm hood/reflector. I also purchased a better fan which is 740 CFM that will be split with another tent so this tent will be receiving 360 CFM. This should help alot with the temperature and should allow me to control the humidity better since the fan will be drawing air from the hood/reflector instead of the whole tent.

    If I have 360 CFM as the outtake fan, what type CFM fan should I use as an intake?


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