How much CFM is to much??!?

[drdawson]

Hey everyone, this is my 1st time on this forum so im hoping you guys can help me.

I have an 18 cubic foot grow space and a 140 CFM bathroom exhaust fan, this means that my fan will change the air 7times a min!!!

Im using a 400watt HPS and the dimensions are 3 ft high x 3 ft wide x 2 ft back.

Thanks in advance.

 

[regrets]

That will be fine especially having a 400w in such a small place, the exhaust will help to keep your temps down.

 

[my7k]

As long as the plants arent windburned or the medium isn drying too quickly, you cant have enough ventilation. With that much CFM you can be sure your scrubber will work well too

 

[gangjababy]

is the 400 watt air cooled, what is your intake cfm rated at? I would make the 140 the intake and get a 250 for outake, that might be enough, the 400 watt gets pretty hot...

 

[drdawson]

As long as the plants arent windburned or the medium isn drying too quickly, you cant have enough ventilation. With that much CFM you can be sure your scrubber will work well too

Bit of a newbie question... but ive never herd of windburning, any chance someone could shed some light?? cheers

is the 400 watt air cooled, what is your intake cfm rated at? I would make the 140 the intake and get a 250 for outake, that might be enough, the 400 watt gets pretty hot...

No its not air cooled, and im going to have a passive intake just abit bigger than the diameter of my fan.

Im thinking of maybe putting a glass pane close to my light and having the fan suck air from there .... then leave a gap on the opposite side of my fan so it will still exchange my lost CO2. any ideas?? cheers for ure comments guys.

 

[gangjababy]

A 140 cfm with a passive intake will not be strong enough to keep the area at an adequate temperature. Buy an inline fan at homedepot, use the bathroom exhaust fan for an active intake, you will also need a fan moving air around in the cab

 

[drdawson]

A 140 cfm with a passive intake will not be strong enough to keep the area at an adequate temperature. Buy an inline fan at homedepot, use the bathroom exhaust fan for an active intake, you will also need a fan moving air around in the cab

Ok thanks, i will try to sort sumthing out with an active intake. i dint relise the heat from one 400wat hps would cause this much hassle lol.

 

[gangjababy]

a 3x3x2 space a pretty small, your pots will take up a foot and the tops of the plants will need to be at least 8 or so inches from the bottom of the light so you really only have 12 or so inches of usuable space. I am using a 400watt as well and have a 3x3x4 space and I feel it is still too small...

 

[bonz]

bathroom fans dont pull the air wellenough in my opinion. unless it`s a real small room with a small light. i would go with a 250 for it.

 

[drdawson]

do use think it would be better to rig up an air cooling system with it just over the bulb itself and use something smaller just to exchenge the air of the room?
or would this be the same?

 

[gangjababy]

yeah plus the inline fans at homedepot are cheaper than the bathroom exhaust fans rated at the same cfm.

 

[drdawson]

but i currently have the bathroom fan,its an old one because ive just re-done the bathroom.and i have a 400watt hps from my friend otherwise i would use a smaller one. so i will try that and just test without plants and keep an eye on temp for a day. cheers

 

[drdawson]

one more thing... how many plants would you suggest for a space of that size????

 

[bonz]

you just answered your own question dude.

" so i will try that and just test without plants and keep an eye on temp for a day. "

ganga baby have you ever tested these bathroom fans to see how accurate the cfm realy is. it seems to me that the design is more for pushing air rather than pulling air, sort of like a comp fan. i had a comp fan rated at 80 cfm and there is no way it was. another push fan to me. if i knew how to tesat the actual cfm i would. hey another project for me.

 

[drdawson]

you just answered your own question dude.

" so i will try that and just test without plants and keep an eye on temp for a day. "

ganga baby have you ever tested these bathroom fans to see how accurate the cfm realy is. it seems to me that the design is more for pushing air rather than pulling air, sort of like a comp fan. i had a comp fan rated at 80 cfm and there is no way it was. another push fan to me. if i knew how to tesat the actual cfm i would. hey another project for me.

Constantly pushing the frontier of bud cultivation. ..this forums ace.

 

[gangjababy]

you just answered your own question dude.

" so i will try that and just test without plants and keep an eye on temp for a day. "

ganga baby have you ever tested these bathroom fans to see how accurate the cfm realy is. it seems to me that the design is more for pushing air rather than pulling air, sort of like a comp fan. i had a comp fan rated at 80 cfm and there is no way it was. another push fan to me. if i knew how to tesat the actual cfm i would. hey another project for me.

No, I haven't, how do you test the accuracy of the cfm's? I have never bothered to buy a bathroom exhaust fan

 

[FullMetalJacket]

I agree, you definitely need to test. Not test actualy test CFMs per se; but hook up the fan and see what your temps get to in 1 hour, 2 hour, 3 hour etc. with the lights on with specefic ambient temps. Almost like a mock grow day w/o any plants. You really wont be replacing the air 6 times a min due to dispersion vs displacement by the way. Alot depends on fan type, SP, placement, carbon filter, and intake.

I actually bet your fan is not nearly enough.

I run 2 inline 478 cfm fans, one for my exhaust and one to cool my 600 hps light in 36 cubic ft. Obviously i dial them down a ton. Going too big on a fan allows you more control over noise, temps, and intake options....

 

[bongrippinbob]

Those inline fans at home depot do not have enough force to pull through a long duct run or for exhausting.
When I was using a 400watt light, I was running 465cfm exhuast and my cab would stay about 4degrees above ambient temps. This was a blower though, and not a duct fan.
Your fan will work, depending on how many degrees over ambient temps you are looking to be at. If you want it 20degrees above ambient, i.e. your bedroom is 65degrees and your cab is 85degrees, then a small fan is fine. If you don't want to have your a/c set down that low, get a bigger fan that will keep it closer to ambient. By being able to keep your a/c at a higher temp, you will save tons of electricity and your fan will pay for its self in no time.
Also, that short of a space is going to be a bitch to use a 400watter. My old cab was 6' tall just to make sure I didn't run out of room.

 

[moon47usaco]

Too much cfm is when your plants get pulled from the roots into the fan and expelled out the exhaust like a tree chipper... =]

LOL... =]

Really anything over what you NEED to get the temperature results is a waste of energy...

You can always put a controller knob that can limit the voltage used and therefore lower the cfm to only what you need...

Keep in mind if you plan on using any type of air filter that it will cut down the effective cfm of the fan pushing out through the filter...