passive vent intake questions

[Wookieefoot]

Hello Everyone, Long time lurker, first time poster. Just in the final stages of getting my grow cab up and running. Dimensions are 39x28x17. I have 8 - 2" passive intakes and a 4" S&P 100 CFM inline fan pulling through a DIY "pencil holder" scrubber. I'll be running a 150 watt HPS ScroG with some T5's for side lighting. I ran the HPS minus the T5's last night to get some temp readings

I got a steady 79 degrees over 9 hours with the temp gauge about 5-7 inches below the light with no PC fan blowing below the HPS. So far it doesn't seem like I'll be getting temp problems.

The only thing concerning me was the air exchange. The 100 CFM S&P should be enough for the space right? The reason I ask is I put some paper up to the passive intakes and it wasn't being pulled as much as I thought it would. It really wasn't being sucked to the vent at all. I can feel air moving but not enough to hold the paper. Did I underestimate the size fan I would need in combination with a scrubber?

This is my first grow so if I'm being a idiot let me know. Thanks for reading.

 

[potlatch]

Sounds like your scrubber is quite restrictive.

Looks like you've done the math, so the intake cross section is twice the area of the exhaust duct.

 

[DiscoBiscuit]

Wookieefoot, if I did the math right, you've got 2x and change passive-intake. Well done. Sucking a piece of paper to the intake hole is relative to the gauge/size of the paper [and] the way your fan responds to static pressure. You've still got the necessary negative pressure for your scrubber to work (even though it's a bit small for the stinkers.)

As for your exhaust fan-size, you've got more fan in a smaller space than I do so you're probably fine. Your temps sound good too. IME you're getting at or better than 8 epm.

But here's the thing. Proper air flow is a combination of exhaust and circulation, every bit as important as close light proximity. Us 150 growers like to crowd as much canopy as close to the bulb as we can get away with. Before you know it, air flow is compromised and a 120mm pc fan or a couple of smaller ones helps plants absorb CO2 in addition to cooling temps.

Speaking of efficiency, it's best to not space your lights too far apart. A 150 has between 3 and 12" of canopy penetration capability, depending on canopy density. To maximize efficiency in a smaller space, make sure your supplemental lighting isn't distanced farther than the usable periphery of the 150. Otherwise you're just extending the envelope, not necessarily intensifying it.

And then it comes back to air flow. Your circulation and exhaust will have to handle to additional heat of your supplemental lights. If this heat prevents you from taking advantage of close bulb proximity, you're paying more for less. More lights is like a six string vs a twelve string, more strings to tune but a richer environment when well tuned.

 

[Wookieefoot]

Thanks for the quick response guys. I thought I had my math right, no mathematics whiz here.

DiscoBiscuit, I'm pretty sure I've seen you in BCChronic's grow thread. I'm pretty much trying to recreate his success. I'd be happy with half of his success on my first grow.

Again, thanks for the input.