Was wondering about negative pressure inside tent. I have a 6ft tall x 8 ft long x 4 ft wide tent. I was running 2 4 inch exhaust fans. Was told that that wouldn’t be enough exhaust for that size tent. I switched to a relatively cheap 8 inch fan and was curious if it is too much . Not sure you can see it but in pictures it sucks the tent in like crazy. For the most part i dont even need to run my intake fan. If i open up vents it creates more then enough intake in my opinion. Any thoughts or ideas as to if this is good or bad. , only thing i can thing of is that the tent wont last as long .
Yea that was at half speed , i keep it at the min at dark time and maybe a 1/4 during light. Trying to keep heat down. I been using the mars hydro sp 6500 light and my tent was getting 86 degrees with the 2 4 inch fans , but with this 8 inch i can keep it around 79 to 80 .
Read the tent sucking in as the exhaust fan trying to flow more, but being restricted (not enough intake area). When you open the vents, you are reducing the restriction & allowing the exhaust fan to flow more. If you want to rough it in to see if the 8" fan is really too much flow, give it enough passive intake area to where it just starts to build some negative pressure with the exhaust fan at 100% power, & see if that is too much flow. If so, then you could reduce the intake area & burn power but reduce flow, until you get a speed controller. If you can turn down the speed, you’d probably want to re-dial the intake area again, since having too much intake area slows down the air at the intakes for no gain in cfm. No need for the intake fan. Basically you’re using it to make up for the lack of passive intake area. Here is a drawing that shows what happens by adusting passive intake area. Exhaust fan power stays the same. The yellow line is static pressure or when the tent stops sucking in once it has enough intake area. The sweet spot in terms of intake area is a little to the left in the red area, so you give up a little cfm from max, to keep the velocity of the air being sucked in higher (green line) at the intakes.
I would keep it simple! Just open a zipper about 4 inches, and hold a bic lighter up to the opening. If the flame pulls towards the tent there is negative pressure. If the flame blows away from the tent there is a positive pressure. And of course if it the flame stands straight up the pressure is equal. Just adjust the intakes, exhaust, or vents to go from equal to a slight negative pressure. This will give you a good starting point, and you can adjust intakes and exhaust from there to obtain the desired temperature.
I used an 8" when had my 4x8 up. Just lowered speed. Never any issues. Exhaust up top. Intake bottom. No intake fans ever. Using a 6" in my 5x5 but a 4 would do it. All the tent needs is a slight bowing showing negative. Full recycle of interior air every couple minutes. Airflow for plants is achieved with oscillating fan inside. Just my experience with this. Have 3x3/4x4/4x8/5x5/32"x32". Not all up right now but i change when whim suits.
Ok, so there are a few 8" Vevor brand fans. Do you know your model number? The one I’m guessing you might have VV-GDFJ-AC8 says it can move around 750 cfm. That would be with no restrictions on either side. So if you have some restrictive exhaust tubing into a restrictive carbon filter, you would lose some flow there. Even still, that 8" fan is probably oversized for your tent. But since you have the speed control, you can at least dial in the passive intake area for the exhaust fan at its lowest speed setting, see if that is enough flow, & if not, start increasing the speed setting & dialing in the intake area to match, until you have enough flow. You should be able to do that all the way to the max flow for the exhaust fan (minus whatever you are losing on the exhaust side), if you have enough passive intake area. But for the size tent you have, you’ll probably be able to find enough flow at a lower power setting & keep a little negative pressure on the tent, as long as you dial in the intake area for the power level.
Try to dial in your intake area to hit the sweet spot in the red box, for any given fan power level:
Do you have enough passive intake venting to open up to drop most or all of your negative pressure when the exhaust fan is at its lowest setting? (This would be with the intake fan & intake ducting disconnected & out of the way.)
When you have the vents open & the exhaust fan at the lowest speed, & the intake fan & ducting unhooked & out of the way, is there enough flow through the tent to cool the tent, but is it still creating too much negative pressure?
One thing I was wondering about is if any of the intakes that you opened are up against the wall(s). If they are, then you’d want to pull the tent away from the wall(s) blocking the intake(s) by a few inches, maybe 4" to 6", & you’d pick up some flow & lose some negative pressure that way now.
