Too much cfm?

Airflow / volume = ACPH

The Air Changes Formula

To calculate room air changes, measure the supply airflow into a room, multiply the CFM times 60 minutes per hour. Then divide by the volume of the room in cubic feet:

This is the formula for ambient capture of VOC and particulate. You seem to be conflating with resistance and static fan pressure which usually apply to source capture - such as losses because of runs of ducting, bends, etc. The short piece of duct that most novice growers here use won't really cause anything in the way of losses in inches of water column.

You are also mixing in HVAC and lighting temps which are a separate thing entirely from sizing a fan/carbon filter and how to calculate ACPH.

McStrats said:

This is the formula for ambient capture of VOC and particulate. You seem to be conflating with resistance and static fan pressure which usually apply to source capture - such as losses because of runs of ducting, bends, etc. The short piece of duct that most novice growers here use won't really cause anything in the way of losses in inches of water column.

You are also mixing in HVAC and lighting temps which are a separate thing entirely from sizing a fan/carbon filter and how to calculate ACPH.

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Who exhaust soley for smell control without ever taking environment into consideration? If you size your carbon filter and fan without taking into consideration the heat load being produced in the room you will be undersized and unable to deal with the heat load.

The only way I could see your calculation working is in a totally sealed room with a mini split and c02 enrichment. I run that way in one room now (but don't vent when doing so), but the majority of people on here run tents or small rooms and exhaust to control environment.

Not trying to be insulting I think it's just lack of experience in setting up grow rooms. But you kinda remind me of some HVAC tradesmen I've tried to get estimates from. They try their standard calculations and try to tell me some stupid low number of btu for my grow room. They fail to account for the amount of heat produced by lamps and other heat producing devices in the room, and even if they do try to calculate a heat load it still comes up low. I end up having to just find people that will let me size my own equipment or do it my self. So far I've been spot on with my calculations that I got from formulas I found on places like this vs the way off calculations that come from the hvac industry.

I really don't think this is even what the OP meant, I think he simply wanted to know if too much negative pressure was effecting his plants in a negative way. I say possibly, but I ran with high negative pressure for years and I think the benefits outweigh a few wind burned leaves. If it's a really high negative pressure you can always turn the fan down some with a speed controller, but I feel keeping some negative pressure is beneficial.

McStrats said:

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

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Sorry, my point was that simple calculation doesn't take into account different environments and set ups.
In the context of controlling temp by way of extraction, not best performance for the filter.
Lots of people rely on those air exchanges calculations and wonder why they are still having heat issues.

That's why I said that is what is missing .

The calculation for staying x degrees above ambient with y wattage is different and more accurate to controlling the environment and far more relevant to grow rooms since its the most common way of maintaining grow room temp.

Haven't grow in a little while & situation is different now so need air cooled hood, fan & filter. Retired so can not spend much. I've read reviews and forums until I'm dizzy. Have a 2x4x50 inch tent. Will this fan be good enough with a 6 inch carbon filter to control odors? In a bedroom.

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.

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Your math legitimately makes me want to go back to school again, so forgive my questions, and I don't mean to hijack the thread..

How exactly do you figure the percentages each time for the variables? Is that based on the equipment you have, or overall?

I L0VE W33D said:

Your math legitimately makes me want to go back to school again, so forgive my questions, and I don't mean to hijack the thread..

How exactly do you figure the percentages each time for the variables? Is that based on the equipment you have, or overall?

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If you need help figuring it out for your room can send me a dm.

I was just wondering why you're growing in those two tents in that room? Why not just set up the lights in the room, they would cover most of it, and you could eliminate a few fans in the process. Do you just want to have separate tents for veg/flower? You could grow much bigger plants in there outside of those tents. Just curious.

McStrats said:

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.

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hmm so with something like this in mind.. so a 10 x 6 x 8 foot room... if you have a 6 inch fan moving 400 CFM hooked to a 16 inch presh filter.... that would be (calculates) about 50 ACH which you are saying is too much.... but in order to achieve proper temps etc. that fan needs to move all of that air (or even more I am thinking) OUT.... would the solution be to install a separate scrubber fan that achieves that proper ACH by having the fan slowed but just exhausting into the grow room itself.. and then having a separate fan moving the air outside not on a scrubber? I have never heard of the ACH measurement before, thank you for this insight!

PS forgot to add I need to up my intake. currently I only have 2 8x8 dark room vents installed in the walls.

meangreengrowinmachine said:

hmm so with something like this in mind.. so a 10 x 6 x 8 foot room... if you have a 6 inch fan moving 400 CFM hooked to a 16 inch presh filter.... that would be (calculates) about 50 ACH which you are saying is too much.... but in order to achieve proper temps etc. that fan needs to move all of that air (or even more I am thinking) OUT.... would the solution be to install a separate scrubber fan that achieves that proper ACH by having the fan slowed but just exhausting into the grow room itself.. and then having a separate fan moving the air outside not on a scrubber? I have never heard of the ACH measurement before, thank you for this insight!

PS forgot to add I need to up my intake. currently I only have 2 8x8 dark room vents installed in the walls.

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You are welcome. The term ACH is common in industrial filtration and ventilation. I work in industrial ventilation, so ACH is something I use everyday. You provide exchange air to provide fresh source of air. ACH isn't really about cooling. That air needs to exit the grow space and be replaced with new fresh air from outside the space. Air exchange definitely can affect temp,...but its like jacking up the ceiling fan to cool a hot room in July. It might work in April/May but in July/Aug it won't get it done...so you turn on the AC. Your ACH is high but it's not hurting anything.

Rurumo said:

I was just wondering why you're growing in those two tents in that room? Why not just set up the lights in the room, they would cover most of it, and you could eliminate a few fans in the process. Do you just want to have separate tents for veg/flower? You could grow much bigger plants in there outside of those tents. Just curious.

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Opposite timers save electricity also different strain. Plus tents really help make a great light footprint the difference is dramatic. I used to do open rooms. Oh I also breed duh . Forgot to mention that. So yea just have tons of tents .