Negative pressure

I mentioned in a previous message that my grow area is air tight apart from the inlet hole I’ve left, am I right in thinking that if my fan can pull more air out than my opening permits this will be sufficient air flow ? I don’t want to use a second fan for air input but I do have 2 oscillating fans for air movement.

Air movement is not the same as negative pressure. Moving air is a breeze within the environment, pressure is removing and replacing air in the environment. Exhausting air from point a will have to pull it in from point b. I prefer exhaust from top and intake from bottom. Intake brings in fresh CO2, which is heavier than O2 so you get fresh CO2. If you make your own CO2, you want an airtight environment. Hope this explains it. 4x4 tent with 4" exhaust fan is more than adequate and fan for circulation should be gentle and not directly on plants

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I’ve set my exhaust fan and carbon filter at the top of my grow area and I have the intake on the opposite side of the area but at the bottom as you stated. I ran the fan whilst running smoke outside my intake and I managed to fill the entire back garden where my fan exhausts, with bright green smoke (how to draw unnecessary attention to your self) . t

Well then, you are not airtight. You have flow through ventilation. Negative pressure - exhausting air from the top, in-taking air from the bottom. I thought you stated before 2 fans - might be too much air. We want a gentle breeze, gentle and not directly on the plants. Sounds like you got it right, except for the Halloween gas attack

An active exhaust and passive intake will create a negative pressure in the tent. No need for two fans unless you’re running a fully closed system and want to filter air coming in and out.

If you want to run a negative pressure tent with two fans the exhaust fan needs to pull more than the intake fan pushes to maintain that negative pressure.

I do a positive pressure grow myself.

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Sorry yeah your right. When I first set it up I didn’t have an intake and you could hear the suction as you opened the door ( my grow space is a home made wooden room within my garage) , I do have 2 fans but they’re not the most powerful things and put out more of a breeze than anything

Thanks MDBuds I’ll give this a go but I may end up going down the positive pressure route because I can see myself sticking my head in to have a look every time I’m near it .


@papahandley That’s why I do positive pressure. Active intake with passive exhaust. It allows build up of environmental c02 and heat (which i need for my winter grow since leds don’t put off a lot of heat) and it helps prevent mold and mildew by blowing the spores out of the tent instead of sucking them in when you open it.

It works pretty good for me and I haven’t had a single issue with mold and mildew since I started using positive pressure. Granted I use preventative measures too, but even before I did positive pressure even with using prevention I got powder mildew every once in a while.