blow in or suck out air??

Listen to footclan, the exhaust fan should be the last thing in line before leaving the grow space. Fans work best when the path in front of them is unobstructed. Passive intake is all you need, start with one that is double the size of the exhaust and adjust from there. You want slightly less air coming in than going out in order to prevent odor leaks (negative pressure).

Just as an FYI, plants dont like sitting in a negative pressure environment. Ergo..in a setup with a large exhaust fan but small fresh air fan. People don't seem to grasp that it is virtually the same in terms of heating and cooling. If I were you I would put a small inlet fan down low and use passive vents up high. Very small fan and carbon filter scrubbing inside the tent. Another very small fan sucking air through the tent and hood. Seal your light up well and u don't have to worry about smell, the light is "external" to the tent.

legallyflying said:

Just as an FYI, plants dont like sitting in a negative pressure environment. Ergo..in a setup with a large exhaust fan but small fresh air fan. People don't seem to grasp that it is virtually the same in terms of heating and cooling. If I were you I would put a small inlet fan down low and use passive vents up high. Very small fan and carbon filter scrubbing inside the tent. Another very small fan sucking air through the tent and hood. Seal your light up well and u don't have to worry about smell, the light is "external" to the tent.

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I've never seen or heard of anyone using your method so I can't say it wouldn't work, but, i do know that a fan is much more efficient sucking air out. Also If you suck air out through a filter you only need one fan to do it vs. your intake fan + fan for scrubbing. I also know that your right about using a smaller cfm fan for intake as this would cause too much neg pressure. However, if you made the passive intakes the right size for your exhaust there would be minimal neg pressure and you would have no odour leaking.

Stonetech said:

I've never seen or heard of anyone using your method so I can't say it wouldn't work, but, i do know that a fan is much more efficient sucking air out. Also If you suck air out through a filter you only need one fan to do it vs. your intake fan + fan for scrubbing. I also know that your right about using a smaller cfm fan for intake as this would cause too much neg pressure. However, if you made the passive intakes the right size for your exhaust there would be minimal neg pressure and you would have no odour leaking.

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No stonetech im with you what your saying makes more sense then what he was saying....rep for you !

hey guys..... so I tried to take some pix but my iPhone is the only camera I have and for some reason they come out weird, really orange-yellow and it has these faded 'lines' accross all pix- anybody had any luck taking pix with iPhone in a growroom??

so for now I've installed the 400 something CFM fan as a blower of fresh air . ran a duct from it to the top if the growroom and it's blowing on the backwall which bounces back and hits the plants so that gives them some wiggling and from there it goes outta bottom of the front door, which is just a Mylar flap taped on the top to about half way down and just left the bottom free falling to let the air out.
Temps during lights on are totally dialed in @ about 75, that's the reading everytime I check throughout the day and when checking on the min&max reading on the temp gauge, it's between 71-79- SWEET!
it's only been a day of lights off and it did dip to about 59 so had to turn up the heater, we'll see what happens today.....

I'd like to know more about this negative pressure if somebody could point me in the right direction......

So your blowing air into the top of your room? you should be sucking it out. You've got it ass backwards. You need to blow air out through the top. The intake must be near the bottom and the exhaust at the top in order to exhaust the hot air. If you do this your temps probably won't climb as much.

Negative pressure just means that your removing air from the grow faster than it is being replenished, creating a vacuum. The reason why you would do this is to ensure that all the air that is escaping the grow is being sucked through your exhaust so the rest of your house doesn't stink. If you are blowing air into your grow faster than it is escaping then its going to find other ways to escape other than your designated exhaust (filter). Any pinhole or crack in your grow is going to allow the stank to get out to where you don't want it to be. Although if you aren't venting your exhaust outside or filtering it and you want your house to stink like weed then you don't need to bother with negative pressure.

And by the way I wouldn't take pics of your setup with lights on with your iphone, I tried once with my camera phone (not an iphone) and now the camera longer works, maybe camera phones are too weak not sure but iphones are expensive.

Stonetech said:

So your blowing air into the top of your room? you should be sucking it out. You've got it ass backwards. You need to blow air out through the top. The intake must be near the bottom and the exhaust at the top in order to exhaust the hot air. If you do this your temps probably won't climb as much.

Negative pressure just means that your removing air from the grow faster than it is being replenished, creating a vacuum. The reason why you would do this is to ensure that all the air that is escaping the grow is being sucked through your exhaust so the rest of your house doesn't stink. If you are blowing air into your grow faster than it is escaping then its going to find other ways to escape other than your designated exhaust (filter). Any pinhole or crack in your grow is going to allow the stank to get out to where you don't want it to be. Although if you aren't venting your exhaust outside or filtering it and you want your house to stink like weed then you don't need to bother with negative pressure.

And by the way I wouldn't take pics of your setup with lights on with your iphone, I tried once with my camera phone (not an iphone) and now the camera longer works, maybe camera phones are too weak not sure but iphones are expensive.

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i agree .......just didnt have the energy to explain it to him......He was told by someone else in here to have his intake at the top because hes sucking in cold air......i just let it go

ive had many successful grows with just a exaust fan and passive intake holes.

I'm not quite sure why you guys have the belief that fans work better sucking than blowing. All pressures being equal, the flow rates for intake bad exhaust are the same by definition. What I mean is, put the fan on the table and plug it in, the amount coming into the fan is exactly the same coming out. Any resistance to flow will effect intake and exhaust the same.

I didn't tell anyone to blow air in up top as that doesn't make any sense at all. You want to work with convection flows not against them. The advantage if the set up I described is greater climate control AND smell control. If your scrubber and ventilation are separated, you can put your exhaust fans on a thermostat and they will turn on and off as needed to maintain the desired temps. At night, when you want to keep it warm and not exhaust you can turn off the vents and run a small heater.

If you have your filter attached to your ventilation that means you have to run the filter 24/7 thus always blowing cold air in the tent. Lastly, a filter set up as a scrubber is more effective at eliminating odors than an exhaust. As canaboids are volitile organic carbons, they will easily pass though the walls of your tent, especially in the areas that have somewhat stagnant air flow if your just exhausting. Lastly, a scrubber exhaust can be directed at the plants to increase airflow, increase gas exchange, and combat bud rot; your essentially using it in place of an oscillating fan. The hydrofarm online booster fans work great for ventilation and climate control and are only $50.

If your going to have a passive inlet or outlet, you want it to be 3x the size of the powered inlet/outlet to ensure even exchange rates.

Just wanted to clarify the advantages, obviously many ways to skin a cat. I'm a big fan of sealed hoods that exhaust outdoors, especially given some thing I read recently about digital ballasts causing MH bulbs to offgas toxic metals.

right now I only have 2 plants goingand I'm not worried about smell so I'm not setting up for big yield just yet..
the reason I ended with the fan blowing on top was because I need the heat to stay in the room as is pretty cold and the idea was when the lights go off itll get pretty cold and the heater has to work more to keep up so if I was to have exhaust "holes" on top then it'll escape faster but (and this is my perspective, not saying I did it because so and so said it) if the exhaust holes are on the bottom then the heat will stay in the room longer, same goes for the humidity which is pretty low ( kinda odd for a basement....) and the humidifier runs 24/7 and still can't go past

45% so that was the idea. also., the hood is on top of the room which will eventually (spring-summer) get hot so I figured the same fan/duct set up that blows in air right now will be used to hook up the the hood when the time comes eliminating unnecessary work later on.

everything that has been said and discussed on this post has been extremely helpful and informative and I'm greatful for everybody chiming in with their opinion and without getting into a "this is the wayto do it" type of discussion, so thx y'all

so far everythingis fine and the girls are just exploding in size and they also look healthy. I'm still monitoring during lights off to make sure I'm good and yesterday after turningup the heat a little I didn't see temp dropping under 62 so it got better but it could b even better that's just after reading the min/max on the gauge but I did check quite a few times while lights off and everytime the readout was about 72 so... what do you guys think as far as temp goes? during lights on steady @75 with min/max readout @72/79.
during lights off steady @72 with min/max readout @62/74.

eventually this what I want to get:

http://cheaphydroponics.com/store/view-all/cap-air-2-atmosphere-independent-temp/humidity-controller/prod_135.html

forgot to mention, the fan blowing in air is on a timer with 45min off and 15min on every hour during lights off and constant on during lights on.

whoever said you want more air coming in that exauhsting is fucking stupid lmao