Building a grow box, 0.7m x 0.7m x 2.0m. I want to put a 400w hps (regular reflector) but don’t want the temp to get above 20*c. I have a squirrel cage fan rated at 265cfm.

The question is should the intake be bigger than the exhaust or should the exhaust be bigger than the intake? Thinking about getting an inline fan but don’t know if I should use it as the intake or exhaust and whether to get a bigger one or a smaller one. Don’t really want to spend any more money on an air cooled reflector.

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"Much unhappiness has come into the world because of bewilderment and things left unsaid." -Dostoevsky

You don't really need an intake fan, if you use your cage fan for exhaust air will be drawn in by the negative pressure inside your grow box. I'm not 100% sure if 265cfm will be enough to keep the box under 20c but it should be close. You should also consider a carbon filter to remove odours from the exhausted air. I don't have much faith in home built filters but considering the volume of air in your box you might get away with one. A manufactured filter for your size room wouldn't cost much.....

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"If it fucks, floats or flys. Its cheaper to rent it!!"

Thanks mate. Yeah it’s pretty hot were I am. Might get a 6” inline fan for exhaust and use the squirrel cage to pump cold air into the box. I just know heat = crappy yields and everyone sets up there ventilation a little different.

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"Much unhappiness has come into the world because of bewilderment and things left unsaid." -Dostoevsky

You want a bigger exhaust fan to create negative pressure to suck all the bad air away from any possible leaks

That's what I have been told and it makes sense

And I agree with the Jedi about carbon filter, you need one

My place started stinkin after 1 week of 12/12

Yeah they do stink but right now I’m good with air freshener. Here are a couple of pics.. mother is in soil, clones are in soilless. Both get Botanicare nutrients. The temp right now is around 30*c and humidity is about 40%.

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"Much unhappiness has come into the world because of bewilderment and things left unsaid." -Dostoevsky

Question about those inline fans, do they push or pull better?

The fan will move the same amount of air in either configuration (pushing or pulling) but if your using an inline fan to ventilate an air cooled hood, its better to push so the fan isn't pulling hot air through the hood and into the fan. Its just better for the longevity of the fan......

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"If it fucks, floats or flys. Its cheaper to rent it!!"

If you do use a fan to push air into your grow box, you will need to get a fan that is at least double the cfm of the intake fan to maintain negative pressure. Like I said before you don't need an intake fan!! Its not going to have any affect on the temps in your grow box, unless its drawing cold air from an A/C unit. I don't think you understand what negative pressure means? I apologize if you do but I'm going to explain it anyways. Negative pressure means an atmospheric pressure just slightly less than normal or less than the atmospheric pressure outside your house (101-104 kpa) This means if you maintain negative pressure inside your grow box its actually under a very slight vacuum. So air will be constantly sucked in (passive intake) and pumped out by an exhaust fan.....

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"If it fucks, floats or flys. Its cheaper to rent it!!"

I was wondering the same thing myself, I just bought a "Active Air In-Line Booster Fan 6in." to go with my new Xtrasun AC Reflector 6in. hood. The fan puts out 188 CFM, uses 25 Watts.

I realize that they recommend placing this fan to the exhaust but I will be setting up a grow tent in my attic, the intake will be connected to my central ac unit. Where I live it also gets pretty hot, sometimes even in the winter & it is pretty humid usally. Would it be a smart idea for me too hook up this in-line fan to the intake and buy a motorized fan such as this one for my exhaust?:


265 CFM ActiveAir Blower System

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