Too much cfm?

Im about to add a second tent to my room.

After some calculations, im pulling air too fast !! Smh..

Rule of thumb is to scrub room every 1-3sec

probably scrubing room twice as fast as recomended?? <1sec

if i use recommendations how do i keep my lights and room cool ??

is there a sweet spot here ??

am i starving plants of co2??

what am I missing in these calculations?

75deg f / 49% rh

... plants look decent happy but slight overly transpiring during veg...

About to flip

Would love input..



8" inline style duct booster fan must move more air than I accounted for , especially with little to no static pressure?

Nerdy details below..

hot humid location ,lots of ac..

current setup:

2x4 tent
600w hps air cooled hood style light ss2
Using 8" 400cfm duct booster fan for tent / light (into room)

6" 500cfm can fan/filter for the room itself. (Into atic)

room is 7x7x10

ac duct plumbed into the middle of room room.

planned setup (even more cfm yikes)

two 2x4 tents
Two 600w hps
one aircooled hood
One cool tube

8" can fan / filter 700cfm exhausting room (to attic)
6" 500cfm can fan
" y " connection exhausting both tents / lights (to room)

As of now...

Can touch ss2 glass . Ir thermometer is like 85deg at glass bulb is 149deg .

(Plan on testing different bulbs temps, and lumins ( hortilux super hps etc)
ss2 vs cooltube..

stay tuned for that ..

but , yea . Would love to hear from the pros??

Dennydank said:

Rule of thumb is to scrub room every 1-3sec

Click to expand...

1 - 3 minutes. So if you have a 3000 cuft room you want a 1000 cfm fan minimum.

Dennydank said:

8" can fan / filter 700cfm exhausting room (to attic)

Click to expand...

Venting hot humid air into a cold attic can and will cause black mold. This is no bueno. Stop venting into the attic if the outside temps drop below 60F. I dunno if you are equatorial or something lol.

Dennydank said:

.. plants look decent happy but slight overly transpiring during veg...

Click to expand...

probably the low rh%

Dennydank said:

am i starving plants of co2??

Click to expand...

It takes very little air exchange to maintain co2 PPM.

Nice catch. Minutes not seconds.

Hot attic / cool air . Solid advice tho... either directiion can cause condensation ur 100% right.

You do positive pressure setups then?

also my vent temps arent high becuse room is cooled..

Renfro said:

probably the low rh%

It takes very little air exchange to maintain co2 PPM.

Click to expand...

Yes but can u overly exchange co2... ?

(Over thinking? But can u exchange air too quicly , u want fresh cool air to hangout?)

not-equatorial but subtropical.

as u can see tents over crowded... hence adding second one .

plan to smash down canopy with trellis..

An 8" fan will turn that little space into a wind tunnel. You actually need 10-15 ach (air changes per hr) for cannabis scrubbing - too many cfm and the odor doesn't get enough "dwell time" in the carbon and the odors won't be scrubbed properly.

Your tent is 2 x 4'...but since we're calculating volume we also need height. I'll use 6' as a placeholder.

Here's your formula: CFM x 60 / volume. So...vol would be 2x4x6 = 48 cu ft

6" 182 cfm fan x 60 = 10,920 (converting mins to hrs).

10,920 / 48 = 227.5 ach

That's too many ach...and that's with a 6" fan. For that volume I would use a 4" exhaust and a speed controller. I doubt you will run it maxed. I used 6" fan for my calc because That is what I have and I know it does 182 cfm.

I design odor mitigation systems for plastic and rubber extrusion venting. For cannabis I recommend Alphasorb8 absorption media and not carbon.

No you can't overly exchange co2, you can make the air move too fast which can increase leaf water loss to a point they are not happy. You see the same when you put a fan on your plants,

In my experience you can’t have too much air exchange unless you’re having transpiration problems, in which case buy a thermostatic/hydrostatic fan speed controller which will allow ur fan to run slower then speed up to take care of temperature or humidity spikes

I’ve had a similar set up to you, what I do is use the big fan to cool the lights independently. Then use a smaller fan to take care of air exchange, when doing this you can allow the air to hang around ur room a lot longer which helps with transpiration depending on what stage ur at.

I run an 8 inch high power fan blowing room temperature air through the duct from outside my tent, through 3 600w cooltube s inside my tent then out through the duct in the other side of my tent, I hide my fans in the loft and blow all my hot air into my bathroom.

Doing it this way means your fan and filter set up can run much slower and quieter cos the heat issues are being dealt with independently, allowing you to use the fan and filter to deal with humidity and odour control

Considering we all live in different parts of the world there's no hard rule that says x amount of air exchange is right for everyone, despite what online calcs tell you.
What's missing from those calculations is wattage and ambient air temp, you could add altitude in too but that gets a lot more complicated.

coreywebster said:

Considering we all live in different parts of the world there's no hard rule that says x amount of air exchange is right for everyone, despite what online calcs tell you.
What's missing from those calculations is wattage and ambient air temp, you could add altitude in too but that gets a lot more complicated.

Click to expand...

Not complicated at all. That stat comes from General Carbon and is based on EPA and OSHA recommendations. There's an ach number for every type of airborne VOC and particulate known to mankind. Height above sea level is a major factor in airflow calcs too. 15 ach is a good average anywhere on planet earth. The pre-filter wrap is to mitigate moisture and particulate and their capability range makes them suitable pretty much anywhere. Call any industrial carbon supplier tomorrow and ask to speak to an applications engineer. They'll tell you 15 ach. is a good average. At least then you'll know it's not just the internet

McStrats said:

That stat comes from General Carbon and is based on NFPA and OSHA regs. There's an ach number for every type of airborne VOC and particulate known to mankind. Height above sea level is a major factor in airflow calcs too. 15 ach is a good average anywhere on planet earth. The pre-filter wrap is to mitigate moisture and particulate and their capability range makes them suitable pretty much anywhere. Call any industrial carbon supplier tomorrow and ask to speak to an applications engineer. They'll tell you 15 ach. is a good average. At least then you'll know it's not just the internet

Click to expand...

I don't think he scrubbing the tent, pretty sure just venting it with a 8 inch duct booster. Then exhausting the whole room through a carbon filter.

As long as the fan and filter he bought are rated to work together he shouldn't be moving too much air to scrub the smell.

I've always ran tents with a lot of negative pressure and always had great results. I wouldn't worry about it too much. I'd rather have a few wind burnt leaves than mold or heat issues.

Apalchen said:

I don't think he scrubbing the tent, pretty sure just venting it with a 8 inch duct booster. Then exhausting the whole room through a carbon filter.

As long as the fan and filter he bought are rated to work together he shouldn't be moving too much air to scrub the smell.

I've always ran tents with a lot of negative pressure and always had great results. I wouldn't worry about it too much. I'd rather have a few wind burnt leaves than mold or heat issues.

Click to expand...

The title of the thread is "too much cfm?" He also asked: "what am I missing in these calculations?"

I'm a novice pot grower but my day gig is applications engineering. The cfm x 60 / volume was an answer to both of those questions.

McStrats said:

The title of the thread is "too much cfm?" He also asked: "what am I missing in these calculations?"

I'm a novice pot grower but my day gig is applications engineering. The cfm x 60 / volume was an answer to both of those questions.

Click to expand...

Whether it's a room or a tent, the calc is the same. The ach requirement is the same too.

All I'm saying is I used tents for years with a ton of air flow moving through them, and that it worked great. I see people worrying about too much cfm through a tent sometimes, but I don't think it's really a big issue and in my opinion is beneficial to keep things from molding.

As far as whether the air has enough contact time to work with the filter, it shouldn't be an issue if you have a filter rated for the size of your fan. The filters are made to use with certain size fans, if you follow the recommendation of the filter company of what cfm fan to run you shouldn't have to concern yourself with the math as they have already done it for you and tells you what works best.

Apalchen said:

All I'm saying is I used tents for years with a ton of air flow moving through them, and that it worked great. I see people worrying about too much cfm through a tent sometimes, but I don't think it's really a big issue and in my opinion is beneficial to keep things from molding.

As far as whether the air has enough contact time to work with the filter, it shouldn't be an issue if you have a filter rated for the size of your fan. The filters are made to use with certain size fans, if you follow the recommendation of the filter company of what cfm fan to run you shouldn't have to concern yourself with the math as they have already done it for you and tells you what works best.

Click to expand...

Sorry but that last statement is factually incorrect. Carbon doesn't scrub odor if the air is moving too quickly through it...and it is *all* about math. The carbon filters people commonly buy on amazon are barely adequate...all the more reason not to have too many cfm..

More here:

• Air flow is vital. To remove the most pollutants possible from the air, the air needs to spend the maximum possible amount of time passing through the carbon. In the air filter industry, this is known as “dwell time.” A filter with a good amount of carbon of sufficient thickness and high dwell time is going to be far more effective than a filter with a thin layer of carbon.

A starting point with all VOC and pollutants is knowing the requisite number of ach that pertain directly to it. These are all averages that are relevant no matter where you live.

15 ach is the number for cannabis odor and cfm x 60 / volume is how you get there. The carbon filter you buy on amazon isn't operating outside the laws of basic filtration.

McStrats said:

Sorry but that last statement is factually incorrect. Carbon doesn't scrub odor if the air is moving too quickly through it...and it is *all* about math. The carbon filters people commonly buy on amazon are barely adequate...all the more reason not to have too many cfm..

More here:

Activated Carbon Air Filters: Everything You Need to Know

If you are looking for an activated carbon air filter to get rid of strong odors or harmful gases in your home, you have come to the right place to get all the details. Learn more about why people use carbon air purifiers, whether they work, and if you should use one to filter the air in your...

molekule.science

• Air flow is vital. To remove the most pollutants possible from the air, the air needs to spend the maximum possible amount of time passing through the carbon. In the air filter industry, this is known as “dwell time.” A filter with a good amount of carbon of sufficient thickness and high dwell time is going to be far more effective than a filter with a thin layer of carbon.

Click to expand...

Don't buy cheap equipment. I've been growing for a long ass time. I buy the correct fan filter combo and never have an issue. I don't think you understand what I'm saying, I'm not saying you can blow more cfm than the filter can handle. What I'm saying is that if you buy the correct filter for your fan you won't be pulling air faster than it can handle. You don't need any math when buying a CAN brand filter cause it has a chart right on the packaging. I'm sure phresh and phat have some kind of rating on the box as well. Those are 3 brands I recommend unless you like wasting money on junk that needs replaced too soon.

Apalchen said:

Don't buy cheap equipment. I've been growing for a long ass time. I buy the correct fan filter combo and never have an issue. I don't think you understand what I'm saying, I'm not saying you can blow more cfm than the filter can handle. What I'm saying is that if you buy the correct filter for your fan you won't be pulling air faster than it can handle. You don't need any math when buying a CAN brand filter cause it has a chart right on the packaging. I'm sure phresh and phat have some kind of rating on the box as well. Those are 3 brands I recommend unless you like wasting money on junk that needs replaced too soon.

Click to expand...

You're being obtuse. I use the same math for a $50k exhausting system that I use for a cheap amazon system. The "cheap" part is merely a function of how long it will last. The manufacturer doesn't know the size of the space their fan will be used in...that part is the "unknown" here. Yes...the motor/fan and filter are matched to one another...but they aren't matched to a specific volume (size of grow space in cu ft)...that is why they make different sizes and different cfm ratings. Figuring out which one is correct for your space involves a little math...something you seem terrified of.

Go back to the thread title...the question was "too much cfm?" the answer in figuring that out is cfm x 60 / volume.

Okay I'll use my room for the math but I don't believe your using the correct formula to get cfm for a grow room.

Cfm 1050 x 60=63000/2580= 24.41

Exactly what was this math supposed to tell me though. Where as most grow room calculations for cfm go like this.

Total cubic ft = 2580 /3 (one exchange every 3 mins) = 860 cfm

Then you add on lights at 10% for every 1k watt light. I have 6 lights so that's 86 X6 = 516. Total cfm so far is 1376

Then you add in for filter resistance so 20% for that =172

Total cfm 1584
Then you have to add in 1% for every ft of ducting so let's say 10 ft for easy math= 8.6

Total cfm needed equals 1592.6

So I'd need roughly a 12 inch fan at 1500 cfm to exchange air every 3 mins. I actually use a 10 inch fan in the room I got set up, at 1050 cfm and it cools the room well as long as it's below 65 outside. I only use my exhaust in case of ac failure anyways so this works well for me.

Apalchen said:

Okay I'll use my room for the math but I don't believe your using the correct formula to get cfm for a grow room.

Cfm 1050 x 60=63000/2580= 24.41

Exactly what was this math supposed to tell me though. Where as most grow room calculations for cfm go like this.

Total cubic ft = 2580 /3 (one exchange every 3 mins) = 860 cfm

Then you add on lights at 10% for every 1k watt light. I have 6 lights so that's 86 X6 = 516. Total cfm so far is 1376

Then you add in for filter resistance so 20% for that =172

Total cfm 1584
Then you have to add in 1% for every ft of ducting so let's say 10 ft for easy math= 8.6

Total cfm needed equals 1592.6

So I'd need roughly a 12 inch fan at 1500 cfm to exchange air every 3 mins. I actually use a 10 inch fan in the room I got set up, at 1050 cfm and it cools the room well as long as it's below 65 outside. I only use my exhaust in case of ac failure anyways so this works well for me.

Click to expand...

The carbon filter/fan assembly is designed for filtration and not for cooling. Cooling the space is a whole different thing. Does the filtering fan also help to cool the space? yes...but that isn't its primary purpose.

My responses were for the OP who seemed to be asking how to calculate cfms. Ach are a calculation to determine filtration in order to scrub odors....hence "dwell time" etc.