n00b ventilation question? I don't know...

I've been researching like crazy as I get ready to start my first indoor grow. My tent got here yesterday, set it up, love it.

It has 3 holes, all 4" in diameter. From all the research I did (4" exhuast fan site:rollitup.org) and reading page of threads, people say obviously you need to create negative pressure by having more exhuast than intake and for a small grow tent (such as mine, 3' x 1.7' x 5') you don't need an intake fan. Fair enough. But, everybody keeps saying "Have your intake hole twice as big as your exhaust hole" but obviously with my tent this isn't possible since the holes are pre-cut.

I'm obviously using the top hole for exhuast, bottom hole for passive intake.

So, here's the scenario:

My tent requires 25 CFM to replace the air once a minute. We want to replace it 2-3 times a minute, so I need a 50-75 CFM fan right? I'm not asking anyone to show me examples of fans I need to buy, I can find them on my own, I just need to make sure I'm on the right track.

Anything else to look out for as far as fan quality? Wattage? etc?

Thanks for any help you guys can give me!

I have the same size tent and this is my setup.

4" inline fan 190cfm
4" Carbon Filter
400w MH/HPS
6" cool tube with 6" to 4" reducers

You'll want a fan with 190-200cfm minimum because when you attach it to a filter the cfm will drop. I assume(I don't have an anemometer) it's 90ish cfm with the filter attached. It creates nice negative pressure within the tent. The intake is passive. It has flaps at the bottom on 3 sides. I taped all but one shut.
View attachment 2567100

Do you tape them shut on the inside or outside? I saw the vents but I'm not exactly 100% on how they work or their purpose other than passive airflow? And do I need to plug up the third 4" hole?

36OhBuds said:

Do you tape them shut on the inside or outside? I saw the vents but I'm not exactly 100% on how they work or their purpose other than passive airflow? And do I need to plug up the third 4" hole?

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Inside. I have everything sealed. Except the intake hole to increase negative pressure as much as possible.

are 2 of your holes on the bottom and 1 at the top? you have 3 holes 1 for exhaust and the other 2 4" holes for intake which would be double your exhaust.. i run a 1000 watt hps in a 5x3 area with a 6" exhaust 440 cfm fan and 2 6" intakes and temps stay at 75 degrees.

also when i had a dresser grow going i would use the carbon filter that you can get at homedepot, or menards that is used for vents in your house to cover the intake holes to light proof them

mr2shim said:

Inside. I have everything sealed. Except the intake hole to increase negative pressure as much as possible.

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Gotcha... so it seems my best bet would be to close up the other two 4" holes and all but one of the vents like you did and it kind of solves the issue blue is talking about below needing an even bigger fan since there's another intake.

There isn't a rule for wattage is there?

bluedreamzy said:

are 2 of your holes on the bottom and 1 at the top? you have 3 holes 1 for exhaust and the other 2 4" holes for intake which would be double your exhaust.. i run a 1000 watt hps in a 5x3 area with a 6" exhaust 440 cfm fan and 2 6" intakes and temps stay at 75 degrees.

Click to expand...

bluedreamzy said:

also when i had a dresser grow going i would use the carbon filter that you can get at homedepot, or menards that is used for vents in your house to cover the intake holes to light proof them

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Temp isn't going to be as much of a concern for me since I'm running an LED that stays pretty cool... but I do like your idea to lightproof them.

Can't I take reflective foil tape and just make a patch over the hole too?

if you dont need the holes then yes.. but i guess im confused cause you said you have 3 holes in tent.. i would use 1 for exhaust and the other 2 for intake leaving them open (with filter pad covering them) whether heat is an issue or not you still want a good exchange of air in your tent..

Yeah for sure. I was thinking of plugging at least one of them so that the fan is actually pulling more negative pressure since them both open would require a bigger fan as you said (and my tent is already really small)

do a little testing, depending on your fan i would mayb only cover up half of one of the holes so you have 1 and half time the size of your exhaust.. depending on your situation i used to push through a small filter mounted on the outside of my dresser no problem.. used the pen cup method for the filter.. worked great no smell and was able to use 100 cfm pc fan.. other wise hpsgrowlight store has a 4" inline 175 cfm fan for 47 bucks if you want to pull air through filter.. but with small space it was nice to mount pen cup filter on the outside to give more room and push with pc fan.. i was running 14 cfls during flowering.. not sure how hot ur led gets but you said its runs pretty cool..

mr2shim said:

I have the same size tent and this is my setup.

4" inline fan 190cfm
4" Carbon Filter
400w MH/HPS
6" cool tube with 6" to 4" reducers

You'll want a fan with 190-200cfm minimum because when you attach it to a filter the cfm will drop. I assume(I don't have an anemometer) it's 90ish cfm with the filter attached. It creates nice negative pressure within the tent. The intake is passive. It has flaps at the bottom on 3 sides. I taped all but one shut.
View attachment 2567100

Click to expand...

how much did you spend on the fan, light and filter altogether? I've been trying to find a 400w HPS with a cooltube for cheap. And I can't find an inline fan for under $85
and I only have about 300 to spend on my grow right now.

hpsgrowlightstore has 175 cfm 4" inline fans for like 45 bucks

HolySmoke420 said:

how much did you spend on the fan, light and filter altogether? I've been trying to find a 400w HPS with a cooltube for cheap. And I can't find an inline fan for under $85
and I only have about 300 to spend on my grow right now.

Click to expand...

cool tube ballast and bulbs were 175, fan and filter were 145.

So after reading a bit... is there any advantage to me using a 6" fan and ducting down to 4" to the exhuast hole other than not needing a 6" fan later when I get a bigger grow?

@m2s was I right in understanding your post that you closed up the other two 4" intake holes as well as 2/3 vents? Or did you leave one open?

Thanks to everyone for the help thus far. Ordering a fan and filter today -- got notification my light is on the way and then I'm ready for plants!

what lights are you going with? you can do alot with a 3x5 space.. thats the size of my room and ill be running 3 20"x30" scrogs perpetual harvesting 1 scrog every 3 weeks.. should pull atleast 1/2 lb every scrog.. now im able to mount my fan in another room so i dont have much clutter in my room as my ballast is outside as well which all gives me more room.. if your getting a 600-1000 watt id say go with the 6" fan 440 cfm.. no need to reduce to a 4".. cut the holes in ur tent bigger! make 6" holes run ducting and mount fan outside tent.. the fans are fairly good sized and will take up alot space in your room.. cut both bottom holes to 6" and your good to go!

The space is actually only 3 x 1.7 but 5 feet tall. Sorry if I didn't make that clear...

But the light is an LED light with a custom ratio built by a guy recommended by my friend who is a vet grower. It's a 300w LED with ~230 real watts of output. I also might be supplementing with some low watt 6500k cfl's during veg and 2700k during flower

my bad i thought ur room was 3x5 for some reason.. i just re looked its 5' tall lol.. 6" fan would be over kill but u could get a fan speed controller and dial it down you should be fine that way u can upgrade your room later.. however the fan is fairly large as i stated so mounting outside of tent will be best

Haha yeah no big deal I'll probably just get a 4" and use this as a veg tent when I set up a flower room that is bigger... It is my first grow and I'll prob do two or three plant cycles before upgrading anyway. Super stoked, seeds came today, light shipped today. It's getting so CLOSE! I want to hook it all up!

haha hell yea bro i know the feeling! i grew for about 2 years then had a 2 year break and now i have my first batch 3 weeks in flowering n their budding nice i cant wait either!! lol im worse than a little kid on xmas eve!!

Amazon is your best friend, especially if you have Prime with free two day shipping!

Someone mentioned not having a large budget, but this fan is perfect for any budget.

http://www.amazon.com/dp/B006OQTY6U/?tag=chrome-purchreview-20

440 CFM for like $72 bucks, with free two day shipping with Prime. Or you can spend $30 more and get almost 300 more CFMs!

http://www.amazon.com/Output-In-line-Exhaust-Scrubber-Filter/dp/B004OH0DIE/ref=sr_1_10?rps=1&ie=UTF8&qid=1362972110&sr=8-10&keywords=inline+fan

Most of these fans only use like 1-1.5 amps max so you should be fine OP. Connect the fan to your exhaust and the light so you can vent out the heat from the light. Then you'll need another fan for an exhaust to the room itself instead of just the light. Definitely get yourself a fan and with how cheap they are on Amazon you can get two for the price of one compared to those damn hydro stores :/