Negative pressure advice..

Discussion in 'Growing Marijuana Indoors' started by Alphatronix, Jul 14, 2017.

  1. Hey, just curious, if I have a 15x6x8 room sealed as best as possible and I have a carbon filter system with a fan that sucks at 800+ cfm which is twice the recommended amount for my room size, do I need an inline fan sucking in air from the outside or will the filter fan create enough Negative pressure on its own to pull in all the air the room needs?
  2. Get a speed controller for your fan. And with a room that size I think a booster fan would probably be enough and costs about $20-25(?)
  3. Don't really get what you're saying.. You mean a speed control for the booster fan? If so, why? Either the filter fan is strong enough and I don't need a booster/inline fan or it isn't and I need a fan. If I need a fan, then it'd run on full.

    Or do you mean speed control over my filter fan? That I see no point of..
  4. You said your fan is twice the recommended size, surely it makes sense to run it on half power...

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  5. He meant that a booster is enough to push air in haha, and the controller on the big fan, turn her down a bit

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  6. Why would I turn it down. My question was simple. Would a filter fan twice the recommended size create enough Negative pressure not to install any extra fans..

    I'll find out regardless once it's installed, just wanted to know if anyone knew..
  7. Im done here, youre reducing the efficiency of that fan without some sort of assisted intake. You can run no intake but there goes a lot of performance

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  8. If you say so... Its not a helicopter that loses "efficiency" with altitude. Are you claiming the filter fan works harder, get hotter, increases wear-n-tear, if it doesn't have a supporting intake?

    I'll have 2 floor fans circulating air inside and the room isn't sealed so I don't see how that stresses the fan outside me regulating it.

    My concern is having enough CO2 and I've seen negative pressure suck in massive ammount of air without aaaistance inside tents so I was wondering if I can get same results from a modestly enclosed PVC tent 3 times bigger.
  9. It will attempt to such 800 cfm, but negative pressure is a restriction. The fan's inside wont do much but move the air around not induce more air.

    Run the fan with nothing, but as soon as it's flower time and you need to close all those ports to avoid light leaks. A booster fan with hose pointing away from light ( route mine behind my tent ) will be a huge help. Collapsing wall's and stretching fabrics from the pressure may cause pin like light holes if its a cheap brand.

    Also if you use co2 thatd be anothet reason to get a spees controller. That way you dont waste it with exhaust.

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  10. For grow tent purposes, air is an incompressible fluid, and whatever air gets exhausted must have been sucked in. Unless your exhaust fan is struggling to exhaust, you don't need a booster.
  11. Yea that much makes sense. I was just wondering if a fsn twice as powthe negative pressure was enough to pull in air at
    Why? The more air it sucks out, the more it sucks in clear air. The carbon filter helps clean the air it sucks but the faster it sucks in air, the more it pulls in air, diluted the air inside with clean air. It basically spreads the aroma molecules enough where your filter has less stress and the inside of your tent isnt as stinky.

    Point is, I want heavy air flow without needing a booster or an inline..
  12. Itll be fine but youre pulling any extra co2 out with all that air. Thats why its good to dial it down.

    Nothing bad can come of this really itll be fine. I suggest a booster isnt NEEDED though

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  13. If you want heavy airflow you need heavy fans, I'm sorry but that's how it is. There's no point overrunning your fan as like m9prime says you'll be taking your co2 away. On top of that it's louder, less energy efficient and it'll cut the life of your filter. There's literally no benefit to cycling the air more than about once a minute, promise. The idea of cycling air is you just don't want your plants sat in an oxygen rich environment. Controlling temperature/humidity is the only time you'll need to turn your fan up past replacing the room once a minute. By the way, who wants to grow weed without having to smell the weed? Surely that's part of the charm
  14. Someone who can't afford the smell.. lol. My temp isn't an issue. I'm building a pvc skeleton of my room and placing it over house vents. So aside from my filter fan, I'll have 2 ground ducts pushing central air at a stable 72 F. The insulation and 3 LED lights will hopefully bring it up 10-15 degrees. If not a small heater will do..

    I get what you guys are saying about the CO2.. I'll play with it. My biggest concern is the smell. That's why I'm trying to figure out how best to play it. I might just install two 8" filters at 400 cfm and run an inline or a booster.. well see. Never had a 15x6x7.. any recommendations on what type of canvas tarp I should use to cover the tent. Polyethylene low density or nylon high density. NO, DON'T TELL ME BLACK GARBAGE BAGS. Lmao
  15. #15 Tbone Shuffle, Jul 14, 2017
    Last edited: Jul 14, 2017
    I think you're missing the point about the purpose of the large exhaust fan. What you will be attempting to do with it is use just the exhaust fan to maintain the temps in the correct range. In order to do that you'll use the variable speed controller to reduce the fan until the flow of air in the tent maintains about 78 degrees plus minus 2-3 at the top of the canopy at the heat of the day with lights on. I do this exact thing. In the winter I run the fan all the way down and in the summer it's all the way up. Mine is only 440cfm though.

    Your intake air should just come from your already air conditioned house. You can use an intake fan but like was mentioned don't get a large one. If you do you'll defeat some of the purpose of the negative pressure. If you keep the large exhaust fan running all the time blowing all that stinky air outside none of it will escape your room. The rest of the house doesn't smell like weed at all. As long as you have plenty of air intakes they can just be passive. A room of that size is probably going to require cutting some louvered vents into the doors or wall for air intake. It may be hard to get enough air though a closed door. Interior air is far superior to outdoor for intake air. It's consistent temp and dry. Outside air is neither of those things. I could never maintain good humidity levels with outside air intake. If you have a crawl space and don't live in a hot climate sometimes the intake air in the crawl space is cooler then the home air but it's higher in moisture. I have a friend that uses the crawlspace air just for his vented hood lights. It goes through the lights and straight out so it doesn't really interact with the room.

    If you plan to run co2 enrichment the entire design of the room must be more thought out. You'll need a large air conditioner because when the co2 is on the exhaust fan turns off. You'll need a controller to manage the co2 ppm in the 1200-1500ppm range. A co2 room must be sealed with the exhaust fan off or you won't be able to get the ppm in the correct high range to see much benefit.

    If growmau5 can get 1.9grams/watt with no co2 what is the point of all that work and cost? I'd rather put all the co2 equipment cost into better lights.

    You may notice over time as you look around at successful personal grows here and on other forums, co2 enrichment done the right way with tanks and ppm meters has fallen way out of favor. It's really increases costs with the ac that has to be ran when the lights are on. There's no other way to cool a room with significant lighting and seal it to trap in the co2. The mechanics and timers involved along with the potential toxicity of a rich co2 environment cause most growers to abandon it after a short period. The fact is it doesn't grow better weed. All it really does is veg faster. I have no problem vegging plants as large as I can deal with. I don't need a boost in that arena, especially for all that trouble and cost.

    The amp draw of a centrifugal based fan is not based on how fast the motor turns. It's directly based on how much air it pumps. If it's pumping less air the amp draw is lower because the fan blade spins more free. That's why all large commercial circulating fans are of this design. Also called "squirrel cage" fans. Centrifugal is slightly different but the way the air intake works compared to squirrel cage fans is the same.

    If you completely blocked the intake of or output of the fan it wouldn't burn the motor up. That's one reason why those type of fans are used. It's almost a safety feature to prevent fire hazards.
  16. If you pull all your intake air through the home air vents in the room it will screw up the ac/heat for the rest of the house. You're almost better off blocking them off and getting intake through a hallway or door vent. Cut the door an inch above the carpet.
  17. I'll be putting in a 10'x10' pvc square to the lower left of the pvc skeleton frame of the door. To it I'll attach an 8' intake 440cfm fan that'll suck air in from my loving room floor. That air is around 70°. I'll run an 8' carbon filter fan to the outside of my house window. That out filter fan exhaust will be attached to another 10x10 pvc box frame attached to the main skeleton. You say that's enough but I'm concerned. Won't know till I try. I'll also be splitting the room into 2 at the 2/3rds line and will have to install a wall thays not removable. Will probably have to run a booster fan between rooms... I'll use clear low density sticky tarp to more or less insulate the skeleton with a heavy dense nylon tarp on the outside for total darkness with high quality mylar sheets inside. Basically 3 5x5s in harmony..
  18. The house vents I'll close. You're probably right about that. All depends on how much back pressure there is I guess..

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