CFM help ASAP

What up. So I'm about two to three weeks away from harvest and I'm starting to flip a little. I've got a couple afghan plants that smell like 10. Everyone that comes to my house thinks there is a skunk outside !!!!!(external building ) . I can smell the shit at my mailbox , not good. My cab is 2 1/2 ft wide , 3 1/2 ft long and 3 1/2 ft tall. About 40sq ft . I've spent like 50 bucks on ona pro but its not working. I will be going to the hydro store by this evening to get another fan and a carbon filter.

My question is , the fan I have works perfect for my cab. I have a 400w hps . The fan is a 4.5" axial fan that pulls 112cfm. The carbon filter I'm getting is the smallest they have. 4" by either 8" or 10". It is 200 cfm and the new fan I'm getting is a 6" axial it pulls 235cfm. Will this be too much for such a small space ? I'm worried I will exhaust the co2 too fast. There is an intake that draws in fresh air but there isn't a fan on it if that matters , its just a hole cut in the box with a light trap. There is also a small fan in the box to blow air over the plants. Any advice would be awesome. Just don't wanna buy the wrong stuff. Thx. Peace.

Hmmmm.....so I don't get it. You need a carbon filter... you're getting one. Why the smallest? The Carbon filter is the pickup with a fan sitting one it, pulling in air, and exhusting from the room. You can use a 4" fan on an 8" filter or an 8" fan on a 4: filter. You just need to have a couple of reducers to match the sizes.

Or is it matching the sizes to the pipe and CFM flows you worry about? Don't worry about that. You can get a speed controller to set the carbon fan lower speed. so it is just pulling slightly. That is what do. You want a longer transit time over the carbon bed.

Then the comment about the CO2....I don't get that. Are you adding CO2? If not you want more CO2 in through the intake, not less.

So, a 235 fan and a 200 carbon? Perfect, I think. You keep your 112 for something else. But, don't put it on the intake. Push-pull for fans in a cabinet doesn't works so well.

The problem with smell, it is probably cause. You knew that.

I would be sure the filtr is bigger than the fan or it will be loud and restricting causing the fan to run hot. there is co2 in the air coming in that's how it works your pulling out the oxygen rich warm air and replacing it with cooler air air. more air flow = more gooder

I think you mean fan larger that filter capacity.

Doer said:

I think you mean fan larger that filter capacity.

Click to expand...

I don't know maybe. You ever try to suck a golf ball though a garden hose? if the fan is larger than the filter capacity wouldn't that make it less effective and fail sooner since your trying to suck more through it than is intended. wouldn't it just be not designed to allow air to pass though at such a high rate??? I always thought an oversized filter would be better than under sized

I just meant a fan larger than capacity can be turned down to adjust the flow for long transit time.

Doer said:

I just meant a fan larger than capacity can be turned down to adjust the flow for long transit time.

Click to expand...

oh gotchya. I wasn't taking future expansion into account. I have a fan speed controller on mine but havn't used it much

Ok so from what I understand u want a fan that will exhaust your room / cab atleast three times in a min. So take ur cubic feet of your room and times it by three. Which for me would mean I want a 120 cfm fan and u have to match the cfm rating of the filter to the cfm of the fan bc if the fan is too big it will overpower the filter and the filter won't be able to keep up and will allow some odor to pass thru. If the filter is bigger than the fan you will get heat build up in your room and possible fan problems bc its having to work at a higher load to push /pull the air thru.


i don't want to spend any more cash than I have to. This shit is expensive and times are tight right after Christmas. Ill be using this setup for along time so if i need something bigger in the future ill burn that bridge when i get there. I guess my question is , is this fan/filter setup way to big for my cab ? I could have swore I read somewhere that if u have way to big of a fan for your room you'll be sucking the co2 out faster than needed and the ladies won't have adequate time to absorb it . They also say that it is good to have the fan a little bigger bc cfm get lost thru the ducting so hopefully this will work. I have to do something fast . Thx for all your responses. Peace

tusseltussel said:

oh gotchya. I wasn't taking future expansion into account. I have a fan speed controller on mine but havn't used it much

Click to expand...

I have 3 fans but, one speed controller. And what I mean is that the carbon filter specs are max air flow but we don't need it. And if we don't need it, it is very desirable to dial that down to minimum flow, for transit time. So, not talking about for later.

I have my carbon fan set so I can barely feel it with the back of my hand at the carbon filter.

So the filter I got is a mountain air its rated for 188cfm and the fan is a sunleaves dura breeze it's rated for 171cfm. I installed it a couple days ago and holy shit. The smell is gone. I spent 170 bucks on the fan/ filter. I know I could have ordered stuff off line and saved money but this was a priority and it needed to be done ASAP. I also made my own exhaust muffler and I can barely hear it now. I made a box filled with aurolex sound isolation panels that I robbed from my studio walls lol to put iver the fan and with that and the muffler it sounds much much better. The fan was so loud I felt like I could have strapped that shit to my back and took off to the moon. All in all I'm very pleased now. . Peace

1) your grow are is not 40 square feet lol,it's 8.75 square feet...simple math bro...

2.5x3.5 = 8.75 square feet.

You cubic footage is 30.625 cubic feet...again,simple math...

2.5x3.5x3.5 = 30.625 cubic feet.

Get a 6" 440cfm inline centrifugal exhaust fan (vortex,canfan,ecoplus,etc.) and a 6" carbon filter and be done with it.

Axial fans suck donkey ass.Sure they may move alot of air when not attached to ducting,but they have shit for static pressure,so once you attach any ducting and/or carbon filter the airflow drops drastically due to pressure losses.

Here's a chart/graph you should study...



Note that with 200cfm of airflow (after pressure losses from ducting and carbon filter) keeps a growroom (normally where the footprint of the growroom is equal to the footprint of the light) with a 400w HPS about 6°F above ambient temp (ambient temp = temp of air in the room your grow pulls it's fresh air from).

Do your homework guys...read read read,then read somemore.

I should also add....

When it comes to just simple ventilation for nothing other than removing stale air and humidity to bring in fresh c02 laden air...2x per minute.So in your case...60.25cfm just for simple ventilation.

Now...when also using just one fan for both ventilation as well as cooling,you need to look at that chart as well as know your ambient/intake temperature,then look at the chart to see what airflow you need (after pressure losses from ducting,carbon filter,and air cooled hoods) to keep your growroom within optimal temperature (70°F to 78°F) during lights on in the growroom.

So once you find that airflow value,simply take that,and add your simple ventilation airflow.


Also look at fan/filter charts/airflow curves like these from the canfan/canfilter company (if your fan/filters manufacturer doesn't supply this info,they're garbage IMO)...

Can Fan performance/spec sheet (shows airflow at varying pressure losses as well as max pressure liss it can handle)


And...look at the filters...they'll tell you the max airflow per filter,and how much of a pressure loss you can expect (note that is just fan and filter,no ducting,so you have to account for pressure losses from ducting runs).

Example...




Note...try to keep bends in your ducting run to an absolute minimum as each bend creates more pressure loss.If you must have bends in the ducting,keep them less than 90° bends as each 90° bend can cause a 25% or greater pressure loss/airflow reduction.Try to use no more than a 45° bend,but note that two 45° bends in the ducting will create the same pressure losses/airflow reduction as one 90° bend.

Also if making bends...make sure they aren't directly before or after the fan.


Do your research guys because I'm getting sick and tired of typing this shit out and holding someones hand.If you did your homework I wouldn't have to do this all the time.

Also...runs of ducting greater than 25' will cause pressure losses/airflow reduction.

Use oversized and insulated ducting.Not only does it allow for less restricted airflow/less pressure losses,the sound of air rushing around in the ducting will be quieter,as well as not leaking heat back into the grow because the ducting is insulated.

Example...6" exhaust fan with 8" insulated ducting.

Exhaust out a window via a window exhaust box (look it up),into the attic,or another room...but don't dump that exhaust back into the same room as your grow tent/room/cab as all that will do is heat up the ambient air/intake air,causing the actual growroom to rise in temperature,defeating what you are trying to accomplish.

Also...

Make sure your passive intake is twice the area of you exhaust....

Example...



You are welcome...now do your homework!

Here is a great place to start...

Ventilation 101

I'll copy/paste it in a new thread and hopefully the damn mods will make it a sticky,because this site lacks alot of knowledge and good advice,which is why I usually stay at ICmag.

length x width x height x by 3 = 91 cfm min. needed to vent 3x a min

Use a filter that's designed to pull more CFM than your fan.... pull your smelly air through the filter.... dont push it out

During Co2 use the filtering system and Co2 system is on a timer or you are sucking your co2 out constantly

If you smell to much you might not want to run a sealed room.....you need 24/7 filtering

Ou8aCracker2 said:

I'll copy/paste it in a new thread and hopefully the damn mods will make it a sticky,because this site lacks alot of knowledge and good advice,which is why I usually stay at ICmag.

Click to expand...

haahaha the advise is here you just need to find it.....your info is great copy and paste but not sticky worthy

a 1000 cfm min size carbon filter. just put it onto a timer so it wont kill the electric. 10 on 10 off. also. what I do I don't even use carbon filters and you could not tell the difference. way more plants then you cant say how many , I burn candles 24/7 absorbs all the stink . also incents . this work well.

Sigh...you can tell who's done their homework,and who's got their thumbs up their asses lol.

kinddiesel said:

a 1000 cfm min size carbon filter. just put it onto a timer so it wont kill the electric. 10 on 10 off. also. what I do I don't even use carbon filters and you could not tell the difference. way more plants then you cant say how many , I burn candles 24/7 absorbs all the stink . also incents . this work well.

Click to expand...

1000 cfm??? that filter wont fit into his grow area .....did you mean 100 cfm


This is 1000 cfm filter...its over 4 feet tall

colonuggs said:

haahaha the advise is here you just need to find it.....your info is great copy and paste but not sticky worthy

Click to expand...

Well then where is the ventilation 101 thread?!

It sure ain't a sticky in the top of growrooms and design where it should be smartass

Ok so maybe I listed the specs of my cab wrong. I was stoned sitting on the couch watching full metal jacket and didnt want to get up , either way its forty Cubic feet , not square feet so to exchange 3x I would need 120 cfm. The axial fan I cad was like 112 or something. As I stated it worked fine with no filter and only about 3ft of ducting, the ducting was on the outside of the cab and the fan on the inside pulling the air out so that didn't affect its rating at all .

Anywho.... I know that the axial did not have enough torque and would have probably malfunctioned with a decent filter on it that's why I went with an inline . My original question was , would the fan (171cfm) and the filter (200cfm) be overkill for my tiny cab. I couldn't find a smaller carbon filter , I don't guess they make them. My problem is solved tho, the setup I bought works great and there is absolutely no smell now and the sound is hardly noticeable with the silencer I built and the box I built around the fan. So thanks to all of you who took your time to respond to my question. Thx for the info. Peace

Yeah, you got the idea. If the motor is big enough to spin the blades, it is too big for the pipe. Good one.

Booster fans are blocking their air with the motor. And they are too small to do the the job of a primary fan.

What they are for is for slow spaces after a lot of turns maybe, you could add a few feet per second to the near dead flow speed.

I don't have a booster fan I have a 4" inline vortex type fan ( round and sounds like a jet engine) it pulls quite a bit of air thru it. Enough that I had to silence the fan and the air that is being exhausted. I get the principle of all of this and I have done my homework , but honestly this is the smallest high quality carbon filter I could find , and def the smallest one my hydro shop had and due to circumstances it has to be bought/installed ASAP with a quickness. I just kinda wanted to know if I would have any problems with it being overkill. Thx for the response. Peace