CFM Really Required

room:
3'x4'x8' - painting white now
lighting -
400w HID veg light
Intake
4" round flexi tapped into duct work
Exhaust
50 cfm bathroom fan, 4" flexi below mobile home.

Hello All, Well researched newbie here ...

I have noticed some inconsistencies online about choosing a cfm for your exhaust.

I have heard that you want to be able to change the air in the room within 5 minutes ... is that correct?

If so, than why do so many people a higher cfm rated fan, that the cf of the room?

With my above setup do I really need a good blower fan, and does it need to be more than the cf of my room.
Room is about 100 cubic feet.

never read any warning about cfm being too high and air fully circulate within 2 min wouldn't worry me, if it was in a few sec I would worry and expect a possibility of creating a gusty environment in that small box for the poor plants.

no you want 1-5 air changes EVERY minute..
Cubic Feet Minute
not Cubic Feet every five Minutes

@ frizzle. I just seen this on High Times ...

"Measure your grow space to determine its total volume in cubic feet (i.e., length x width x height), then get an outtake fan capable of moving that air out in three to five minutes or less. For example, a 2,000-cubic-foot room needs, at minimum, a 400-CFM fan to push out the hot, spent air."

Are they wrong?
Any other professionals want to chime in on this?

Plus the cfm is un restricted airflow. Figure between pulling through a filter pushing through a hood/extracting pus every inch of ducting not to mention any bend you will have to make in order to vent will all reduce that cfm..

IMO, and it will save you money time in the long run, go with a decent inline fan for your light and even a 600 a 4in will work fine for both odor and cooling/exchange..

A good rule of thumb for me has been be prepared to spend an equal amount on ventilation as you would lighting.

I went ahead an ordered me an m-6 blower. Claims 530CFM.
http://www.ebay.com/itm/4-6-8-10-COMMERCIAL-GRADE-Inline-Fan-Speed-Control-Hydroponics-Blower-Inch-/290927453266?pt=US_Hydroponics&var=&hash=item43bca0a452

Problem is I already had my room setup for 4" ducting, but couldnt find a high enough CFM in 4"
So I will have to reduct it.

At this stage in the life cycle, first TRUE leave set, Im not running a filter, but will be when flower comes.
Im afraid this 530CFM blower isnt TOO loud.

Don't reduce it you will reduce the cfm. Just spend the 20$ on 6in duct.

I know had bad it sucks when all the 20,30,50 $ purchases start adding up, but believe me it is better then buying things twice.

Plus the fan will run better and have a longer life span.

If it is too loud for you buy a damper. sorry brother I feel your pain with starting up but promise you it's worth it.

I had a question about fan position. I searched all over the net and couldnt find anything relating ...

Is it possible to run the fans, with the flanges up and down, and opposed to horizontally, like you always see?

yeah wont hurt anything except creating a 90degree bend to get to your cooltube. Unless you are running a vert setup then it would be rather efficient that way.

You got a big enough fan though the extra bend isn't gonna change your temps...should be enough exchange to keep your area at whatever temp the room is, so long as you extract outside of the room your grow is in.

You want complete control of your environment. Exchanging air once every 5 mins is a minimal, IMO. You need a fan speed controller-yes you NEED one to be able to control your environement. What if your humidity goes really high and need to actually exchange air at double that rate? You need a fan rated at a much higher CFM on a fan speed controller for you to have control of the room.

Also, as soon as you start running ducting, and making bends, possibly running through a carbon scrubber, the CFM of that fan goes out the window.

@urban
No Cool Tube. Just bat wing reflector.

@Squid. Nice explanation.
Does the fact that I have air conditioned ducting running into the room, and the ac blower motor runs 24X7

I consider my room a small space, and yet, it sounds like even the most powerful 6" blower wouldnt cut it.
How would you exhaust a large grow area, like a 1000 cf. That would need 10000+ CFM air flow ...?

Did you say your intake is ducted into your a/c for your house? That's cool, but it's not cool if that is your intake!

When your exhaust is running it's gonna suck all the cool air out of all the rooms in your house, that's not gonnan fly. Now if you have an a/c vent in there for heat that's cool, but you need a seperate intake.

I had a buddy make the same mistake. He couldn't figure out why his a/c was running so much but not really keeping the house cool for more then 10mins at a time. He set his thermostat lower and lower, and could feel cold air coming out of the vents when it blew. The he realized that since the exhaust was running in the grow room 24/7, and his only intake was an a/c vent he ducted into the room, that he was sucking all the cool air out of his house. Your exhaust creates sucking through that vent, and your vent is connected to everything else, your creating a vaccum in your house.

I just use a damper to reduce my airflow since I purchased one of those shit active air controllers that aren't really made for these type fans..still havent shelled out for one that I was recommended but it;s like 65$

All the above poster are absolutely correct...

The filter, the ducting, and the length and angle of the ducting
Are all factors...and u always want more than less.

good luck with ur grow the ventilation is the toughest part...

I get away year round in a 4x4x7.5 with a 4in ventech inline but with a cooltube and temps are never above 79.. used to be 82 untill I worked out a few flow problems.

It exchanges plenty of air but my extraction point is 1 foot from the tent so not much ducting to go through.. You will also wanna figure out the amount of negative pressure to regulate or you will rip your tent apart...or is it a box/closet

urban1026835 said:

I just use a damper to reduce my airflow since I purchased one of those shit active air controllers that aren't really made for these type fans..still havent shelled out for one that I was recommended but it;s like 65$

Click to expand...

Variacs work awesome if your willing to shell out like $55.

im stupid "painting white now" no need to worry about pressure destroying your tent.

Big Trees said:

Variacs work awesome if your willing to shell out like $55.

Click to expand...

It was something similar, wanna say DST recommended one to me.