Intake passive vs active, CFM recommendation

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cruzer101

Well-Known Member
Nice setup.

I took a look at your journal and saw you have the lights cooled so we are talking about the environment and I understand you want to keep approx 1400 ppm CO2 in there.

What I found in regards to intakes is if the hole is large enough I didn't need an intake fan. Like you I went into the sub floor (4"x12" hole) and out the top. I have dual cabinets so only about 10 sq feet or so but with only one 80 CFM bathroom fan and passive intake the cab stays 5 to 10° below outdoor temps. (light is in diy cool tube 110 CFM fan) so not much heat.

On the other hand using CO2 I am sure when you want to exhaust you want to do it as quick as possible and by matching the CFM to the exhaust in a 9x9 room it would happen almost instantly. with 9 foot ceiling you got like 729 cu ft right?
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
cruzer101 said:
Nice setup.
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Thanks!

cruzer101 said:
I took a look at your journal and saw you have the lights cooled so we are talking about the environment and I understand you want to keep approx 1400 ppm CO2 in there.
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Exactly

cruzer101 said:
On the other hand using CO2 I am sure when you want to exhaust you want to do it as quick as possible and by matching the CFM to the exhaust in a 9x9 room it would happen almost instantly.
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Good point. If i have an active intake the air exchange will happen quicker and I can fill the room back up with CO2 sooner.

cruzer101 said:
with 9 foot ceiling you got like 729 cu ft right?
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The room is actually 10'6" x 10'6" x 8" so it's real close to 800 cu ft but only about 9'x9' is for growing area.
 
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cruzer101

Well-Known Member
Close enough, Just one of those 450 CFM fans will be enough to exhaust it in a couple minutes with say a 4 by 12 intake. Better with two. I am sure you have thought of dampeners on the vents. If you can keep those tubes cool to the touch or close to it the room wont get much hotter then the rest of the house. You will probably be exhausting mostly for humidity. Depends on what temp your starting with. I bet you could set it up for two cycles four hours each with only one 10 minute exhaust in between.
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KnowledgeSeeker

KnowledgeSeeker

Active Member
cruzer101 said:
Close enough, Just one of those 450 CFM fans will be enough to exhaust it in a couple minutes with say a 4 by 12 intake. Better with two.
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Actually the 10" max fan 1000 CFM will be doing the exhaust and the 6" vortex will be handling the intake.

cruzer101 said:
I am sure you have thought of dampeners on the vents.
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Which brings up another thread of mine that you may have some input on: https://www.rollitup.org/grow-room-design-setup/242292-backdraft-dampers.html <-click to follow link

cruzer101 said:
If you can keep those tubes cool to the touch or close to it the room wont get much hotter then the rest of the house.
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The tubes are cool to the touch but they still seem to heat up the room pretty good. Without the AC it gets much hotter than the house real quick. The 8" vortex that I used on the second set of lights is much better than the Dayton 463 that I put on the first set of lights though. The difference between intake and exhaust for the first set of lights is about 10 degrees, while the second is closer to 6. I'll probably switch out the dayton for another vortex after this run.
 
Mcgician

Mcgician

Well-Known Member
I'd say run the setup as is, see how it works for your environment, then make your decision on whether to add an intake fan. In my current situation, I use a passive intake, and it works well enough for me. I'm also not using Co2 though, so the fan is constantly going. What I have done though, is hooked up a Can Fan RS6 to run at "night" instead of the Max Fan so I can save about 120-150 watts.
 
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cruzer101

Well-Known Member
This may help with the CO2

"This form will calculate how much CO2 will be needed to fill a grow room to the required level. It will also determine how long it will take to fill at the given rate."

Click Here
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
Mcgician said:
I'd say run the setup as is, see how it works for your environment, then make your decision on whether to add an intake fan. In my current situation, I use a passive intake, and it works well enough for me. I'm also not using Co2 though, so the fan is constantly going. What I have done though, is hooked up a Can Fan RS6 to run at "night" instead of the Max Fan so I can save about 120-150 watts.
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Good advice. I'll cut a passive and test it out. I love how energy efficient your setup is. Nice work!
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
So i cut a 6" hole and installed a non-motorized damper. The problem now is that there is a lot of negative pressure in the room and the intake makes a lot of noise. It practically whistles. I really don't want to cut a larger hole (I've already made swiss cheese of the room) but I may have to. Or purchase a fan. The largest circular hole that I could cut would be an 8" diameter. I'm afraid that it would still make a lot of noise and i'll still have a significant amount of negative pressure. From my understanding (and correct me if i'm wrong) if a fan is constantly pulling against a strong negative pressure the fan will wear out faster. Hmmmm....
 
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cruzer101

Well-Known Member
Don't you think your exhaust is a little big? 1000 CFM for 800 CF?
I understand carbon scrubbers slow it down some but you are talking about exchanging the air in there every minute you run that fan.

I think thats why your intake whistles. you are moving a lot of air very quickly.
If you can adjust the fan speed down to say half and you would still exhaust the entire room in 3 minutes.

I would say if you want to use that fan you are going to need at least one more passive 6 in with dampener for intake.
get a variable speed attachment if you cant adjust the fan speed already and turn it down.

your exhaust is up top so you don't need dampener because CO2 is heaver then air. Run the lights and thermostat and time the cycles. see how long it takes for those lights to heat up the room and what your max is. Thats how much time you have for 1500 ppm shot. From what you have described that is not going to be very long.

You might have to consider just a slow release with a constant flow through the room to keep temps right.
I did that in a cabinet grow where It always got hot.
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
cruzer101 said:
Don't you think your exhaust is a little big? 1000 CFM for 800 CF?
I understand carbon scrubbers slow it down some but you are talking about exchanging the air in there every minute you run that fan.

I think thats why your intake whistles. you are moving a lot of air very quickly.
If you can adjust the fan speed down to say half and you would still exhaust the entire room in 3 minutes.
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Yes, the CFM is higher than most people use. My logic here is that the quicker i can cool down the temp the longer the CO2 uptake period will be. If it takes a long time to bring temps down and the lights heat it back up rather quickly then that doesn't leave much time for CO2.

Higher CFM = Faster air exchange = shorter cooling period = longer CO2 uptake period

cruzer101 said:
I would say if you want to use that fan you are going to need at least one more passive 6 in with dampener for intake.
get a variable speed attachment if you cant adjust the fan speed already and turn it down.
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I think that I am going to cut the hole wider to 8" diameter and install an 8" Vortex 750 CFM with a backdraft damper. That would remove any whistle and lessen the negative pressure significantly. Another perk to installing a fan is that I can run ducting from the fan to the outside of the house to bring in colder air. The temp of the subfloor air is on average 5 degrees warmer that outside air from 8 pm till about 3 am. At around 3 am they seem to equal out.

cruzer101 said:
your exhaust is up top so you don't need dampener because CO2 is heaver then air.
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That's kinda what i was thinking.

cruzer101 said:
You might have to consider just a slow release with a constant flow through the room to keep temps right.
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I would like to avoid a constant exchange for CO2 purposes but we'll see after further testing. Thanks a million for your feedback!
 
Mcgician

Mcgician

Well-Known Member
KnowledgeSeeker said:
I think that I am going to cut the hole wider to 8" diameter and install an 8" Vortex 750 CFM with a backdraft damper. That would remove any whistle and lessen the negative pressure significantly. Another perk to installing a fan is that I can run ducting from the fan to the outside of the house to bring in colder air. The temp of the subfloor air is on average 5 degrees warmer that outside air from 8 pm till about 3 am. At around 3 am they seem to equal out.
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No matter if you have passive or active intake, the whistling sound is from the movement of air. If you move air even faster, it will whistle even more. In my case, I cut a ten inch hole, and it works great for my purposes. Since you're aiming to use Co2 though, what you're going after is not the same goal as mine. My aim is to maintain a constant temperature whereas yours is to actually bring down the temperature enough to have actual swings in temperature reduction.
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
Mcgician said:
No matter if you have passive or active intake, the whistling sound is from the movement of air. If you move air even faster, it will whistle even more. In my case, I cut a ten inch hole, and it works great for my purposes. Since you're aiming to use Co2 though, what you're going after is not the same goal as mine. My aim is to maintain a constant temperature whereas yours is to actually bring down the temperature enough to have actual swings in temperature reduction.
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True. I'm expecting that a larger intake and a fan will eliminate most of the whistling mostly because I have a Vortex 8" on one of my set of lights for cooling and it doesn't make nearly as much noise. We'll find out later today.
 
KnowledgeSeeker

KnowledgeSeeker

Active Member
Mcgician said:
No matter if you have passive or active intake, the whistling sound is from the movement of air. If you move air even faster, it will whistle even more. In my case, I cut a ten inch hole, and it works great for my purposes.
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Slowly but surely I'm learning to listen. Sorry for the stubborn streak. I cut an 8" hole and installed an 8" backdraft damper and ta-dah! no more whistling and the negative pressure is greatly reduced. You can lead a horse to water but you can't make him drink. Thanks, Mcgician, for leading the horse to water.
 
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