Looking for fan placement feedback please…fighting humidity in flowering

  1. Indoor Grow
  2. The seeds I’m using are all progeny from a tent that had 5 different feminized autoflower strains in it, where one or more hermied from stress (possibly heat/light/underwatered at a critical time, etc…wasn’t documented well), genetics, and/or were pollinated by outside contamination.
  3. The plants these seeds came from were supposed to be feminized autoflowers, and these seem to be as well.
  4. Tap water that doesn’t appear to have chromium (based on local water quality reports); as needed, but aired-out ~48hrs for chlorine reasons, and PH’d to the mid 6s based on the testing color/using GH drops (for now, may switch to ascorbic acid in the future once I run out of this first bottle).
  5. No added nutrients.
  6. Soil - Roots Organics Lush in 5 gallon food-safe buckets with GroBucket inserts.
  7. N/A
  8. 300 Watt LED dimmed at 70%, 10” from the tallest cola and ~14” from the average canopy.
  9. 18/6 light on temps are high 70s to low 80s F, and light off temps are mid 70s F.
  10. RH at canopy is ~mid 60s% lights on and ~high 60s% lights off.
  11. AC in the lung room, and it stays between low to high 70s F currently and is low to mid 50s% RH for now.
  12. Ventilation system - 4" at 80% 24/7
  13. No Co2
  14. A couple grows

Any thoughts on better placement for my fans to help with humidity? The plants are at 48, 46, and 49 days…For reference, I battle humidity issues every grow due to my environment, but just thought I’d see if any of you knowledgeable folks would recommend anything different in terms of placement.

Currently in a 2’X4’X71’’ tent using the white 6” stationary clip fan at the canopy that is on high 24/7, and the black AC Infinity S6 oscillating that is at the top left pointing down (the range it currently oscillates is between blowing down to the right, and blowing down to the front). Intake from the lung room is at the bottom right middle, below the white fan, with 4” ducting…hope these pictures help for recommendations, thanks:


When i was growing in a tent i used a couple of those oscillating tower fans. They usually have remotes. I put them in corners and it worked out great for me. I’ve never had an issue with WPM. Maybe I’m just lucky, but i hope not.
Good luck cause they look great.


From that pic it looks like your placement is fine. I upgraded my 2 x 4 tents exhaust fan from a 4" to a 6" a couple of years ago because I had trouble keeping humidity down. Another thing that helps is to use a filter that’s a size bigger than your exhaust fan with an adapter ring. A lot of times the filters can reduce the CFM’s that your fan can move, and we need to move as much air aspossible. I have 8" filters on my two 6" fans, and a 10" filter on my 8" fan.


If you want to test that theory, and you can get away with letting some smell out of the tent for a short time, try disconnecting the filter temporarily and see if the fan does a better job bringing down the humidity.


Needs to be exhausting from top of tent., static ambient input to tent available from bottom, required and is more effective cooling. Better cooling is obtained with lung air exchange vice tent blow-job cooling. (Sorry, couldn’t resist that one).
I run a dehumidifier in a 4x4 connected to my 4x4 grow tent. Multiple AC-I controlled pieces for trim dial grow temp. Lung room is garage being sun baked with no AC, this year.
AC-I filters on exhaust. Additional exhaust available thru roof, (not presently filtered).
I should not grow from June thru Oct, for indoors. Imagine that. That room dehumidifier that is being used has 8 years of operation behind it. Exhausting air should be filtered internal if noise or space issues , or external for more tent grow room. 6 and 8 inch fans from AC- do the job for all aspects.


Thanks, I appreciate the input…I gave the plants their most significant pruning yet, to help with airflow before things get worse later into flowering:

Hopefully my butchering doesn’t kill anyone…I don’t mind much lower yields (preferable to any mold issues), but I am trying to avoid hermies this time from excessive stress, so will try to practice more restraint and patience day-to-day as much as possible going forward.


Sounds like you are leaving flow on the table. Why not turn up the exhaust fan to 100% power? But first, your exhaust fan is most likely being restricted by the shorty carbon filter, & also by the small 4" intake area, & it sounds like you have a 4" hose bringing intake air to the intake port, so that is going to be a restriction as well. How long is that?
You would want to go to a less restrictive yet still effective carbon filter, either a much longer 4" carbon filter, or step up to a longer & wider filter like Capt Ron did. I tested a shorty 4" filter on a similar 4" fan & lost 60% of the air flow from that alone.
Also, centrifugal & hybrid fans want to see more than their own port size in intake area, in order to reach max flow at 100% power. Even the filter-restricted 4" fan that I tested wanted to see 2x in open intake area (intake area equivilant two 4" ports) to reach its filter-restricted “max” flow. The exhaust fan without the carbon filter or with a less restrictive filter would have needed even more intake area. Also, if you have ducting on the intake ports, you would have to add more ducted ports than you would open ports, or provide bigger ports & ducts, to make up for however much drag the ducts end up adding.

You can bench test the fan at 100% power with a diy flow-gauge & check for losses from filters & whatever else is in the setup, & tune your intake area to hit the sweet spot of flow & velocity at the intake. Too little intake area & cfm suffers. Too much intake area & intake air velocity suffers.


Thanks, PF.

It didn’t seem like the trade-off for noise, energy consumption, and fan longevity was worth the difference the last 20% would make on extraction…but I’m always open to being wrong.

I had stopped considering that the single 4" intake was too small since I had no problem creating negative pressure even on lower extraction settings… yes, there’s a couple feet of hose outside the tent as well.

I setup the intake that way to restrict light from escaping since it is in a room that is slept in during lights on…sounds like I may need to rethink how to get a less restricted intake, without leaking light externally now (in addition to a different extraction fan/filter potentially).

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This is what I do, defoliate often, also I put a fan outside my tent blowing into one of the bottom vent squares… with a dehumidifier in the room outside of the tent set to 40%
Good luck happy growing


The potential for more flow is right there.

The fan will start to make negative pressure as soon as it starts to see restrictions from anything in front of its own intake port, could be anything from a restrictive carbon filter on the intake port of the fan, to a restrictive undersized intake port in the tent.
See the chart again: not exactly related but I made a change in that I extended the green line to show the intake air velocity falling off once you are at zero static pressure.

If the intake area increases, the negative pressure decreases, the cfm increases, & the intake air velocity decreases, until you hit zero negative pressure & are at zero static pressure. Then you are at max cfm with no negative pressure & slow intake air speed. That’s why I put the sweet spot below static pressure & into a little negative pressure, to keep some velocity on the intake air & to help keep the smell in.

A couple feet of straight hose isn’t going to be that much more of a restriction, although start putting bends in it, &/or make it longer & it becomes more restrictive. If you only need the hose to act as a light trap, then Jardin makes some port flanges with light trap inserts, & you can delete the hose. I think they are available in 4" ports & 8" ports. The only thing is that they are a little expensive, especially if you need multiples to let the fan flow freely. If you flow your setup ahead of time, you’ll know around how much intake area you need & can buy some extra space so you can tune down into the sweet spot from there. Also keep in mind that the light trap itself will flow less than an open intake, so buy a little more because of that too.

You might end up needing a bigger fan, but first I’d try to get as much flow out of your current fan as possible, & either way it’s going to want to see less restriction at the carbon filter, more intake area, & 100% power if you want to get close to whatever static flow rate the fan is rated for. There are a few ways to test your current fan to see what it can do before buying more stuff. Turn it to 100% & run it without the filter, but I’d also give it more intake area via opening a flap corner down low or more ports, until you lose most of your negative pressure, & see what it does rh-wise with those changes. If it’s better, then you can get a filter that flows as close to static pressure as possible (not restrictive), & add up all of the square inches of intake area you now have, you’ll know around how much intake area to add in light traps or hoses, however you want to do that. Again keep in mind that the traps themselves are slightly restrictive compared to an open intake port of the same size, so if you calculate that you need 12" of open area, you really might need 16" of light trap or hosed intake area to maintain the same flow as the truly open area. Easy enough to go back & check stuff like that more exactly by doing flow tests along the way.

If doing all of that still doesn’t give you enough flow, then you might need to go to a bigger fan & you would have to do the flow work again for the new fan, maybe even a bigger filter again. But based on what you are saying, it sounds like you are happy with your temps but not the rh, so I think if you free up the restrictions on your current fan, you might be able to get to where you want to get to with that fan.


Realized I didn’t describe this totally accurately - putting a restrictive filter on the exhaust fan intake would reduce your flow but you wouldn’t see that as additional negative pressure in the tent, it would only be in the filter & intake side of the fan itself. If you have the restrictive filter on the exhaust side, it would still be as restrictive, & then you’d also see the additional negative pressure from that in the tent. Sorry for any confusion.


And actually putting a restrictive filter on the exhaust side would result in the tent heading towards & into positive pressure. Sorry I’m a bit distracted & should have waited until later to post back when I could think more clearly. But don’t worry about that.

Main things to worry about to get more flow out of your current exhaust fan is to:
-Look into getting a less restrictive carbon filter, (you could go to a long 4", or just jump to a long 6" if you think you’ll ultimately need to go to a 6" fan).
-Look into running the fan at 100% power.
-Look into providing more intake area & dialing it in to hit the sweet spot on the chart. You can make a diy swing gauge if you want to flow-test the fan, then test how much flow you lose to the filter, & then to dial in your intake area to hit the sweet spot. I would do all of the flow testing & dialing-in at 100% power. Then if you want to turn it down later, you know you have your max flow at 100% power if you need it.


I appreciate all the detailed feedback, thank you.

I removed the ducting and flap from the side intake, as well as the flap in the bottom back (though that one is close to a wall); and turned the fan to 100% so we’ll see what it looks like with lights out as a first test. I also moved the sensor to the middle back, just below average canopy since it was in the front right corner recently. Do you have a link to something that details flow-testing in case I go down that rabbit hole?

How close is it reasonable to have tent humidity, compared to lung room RH, using just an extraction fan, deep into veg/flowering?

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Lung room fluctuates? Mine does.
Goal is tent RH monitored.
Lung room can bring down or UP., or not.
Air Conditioners/Dehumidifier/Humidifiers available to implement for Tem/RH control.
I use my second 4x4 for the dehumidifier and other stuff, connected top and bottom to grow tent. Dehumidifier works decreasing RH but increases temp. Air Conditioner in lung room could dehumid and reduce temp, maybe. Exceeding recommended flower RH could result in Mold issues.
Higher temp less RH gets my vote., as I wish to avoid mold at all costs.


Was on with dehumidifier.

Was off with dehumidifier.

Just turned Back on the dehumidifier connected to grow tent.
Not pictured, but expect it to drop, shortly.
Oscillating tower fan inside grow tent on low always. (lost a cornet in the tent.
T6 Unfiltered exhaust to attic.
T6 Filtered secondary exhaust
A6 Exhaust & A6 intake available on connection tubes between grow tent and dehumid tent.
I try to keep fans under 50%.

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Very helpful for perspective, thank you.

I think if you have 55% rh in the lung room, you should be able to get close to that at the top of the canopy with the lights on. Also would depend on temps & how much the plant is transpiring. But 55% going in seems pretty good to work with. My ambient rh moves around a lot though. Usually higher than wanted until fall/winter seasons, & I don’t have a dehumidifier, so my anti-rh fan is often running to hit a lower target that it isn’t going to reach. But on dry days the box can get down to around whatever the ambient is, if I run the whole-house fan for a few minutes. Then my anti-rh fan might kick off, & my humidifier might even kick on.

There’s probably a lot out there on making a diy swing gauge, but here is what I did:

First thing I did after calibrating the swing to be big enough to be useful, & light enough to swing parallel with the fan at 100% power, but heavy/small enough to start falling as soon as it started to see an obstruction (I added bits of card stock at the end to weigh it down until it started falling from horizontal when the fan was at 100% power), was to test-flow the fan at 100% power & no restrictions on either side, no filter, door wide open, & made a mark on the card lining up to where the swing was pointing. (All tests were made with the fan at 100% power, but I would shut it off any time I was working on it.) Then I blocked 50% of the fan intake port with tape, turned it back on at 100% power, & made a mark at the swing. Then blocked off 75% of the intake, & made a third mark. To block off 25% of the intake, I cleared the tape obstructions & then used a corner of foam-board & taped that in place so it was blocking 25% of the intake & not getting sucked into the fan. Then made a fourth mark. So that gives me marks for static flow (whatever the fan is rated for) at the top mark, & the marks going down are 75%, 50%, & 25% of static flow. Then I cleared the obstructions & hooked up the carbon filter to the intake side of the fan & ran the fan at 100% power to see where the swing lined up, made a fifth mark, & that is how much flow I was losing to the filter, as a percentage of my tested max static flow at 100% power. Now I knew how much flow I was losing to that filter. It is important to check to keep the fan at the same angle & the card lined up to the fan at the same height anytime you work on it during testing, adding/removing tape or whatever, or that would throw the test off.

From there I considered that fifth mark my ‘max flow’ mark, & I could close up the tent completely & run the fan at 100% power, & start to open enough of the door until I hit that mark again on my swing gauge, & I could measure the open area of the door to know how much open intake area I would need to provide. Actually a little less open intake area if you want to be in the sweet spot with nearly as much cfm as the fan will do (minus whatever restriction the filter is adding), but still some negative pressure on the tent & a little more velocity on the air coming in the intake. But, you still have to account for light traps stealing flow, which can be a guessing game until you install one to check the flow loss with your gauge. Order more than you think you need, & if you install more than you need (gauge at the ‘max flow’ mark made with the carbon filter installed), you can cover up the last one little by little with tape to dial in the flow & get some negative pressure & intake air velocity back on it.

Also don’t forget if you dial in the intake area using a restrictive filter, & then later go to a less restrictive filter, you would want to re-dial in the intake area, since the fan should be trying to flow more & would probably need more intake area to do it. If you have the swing gauge, that all should be easy enough to do if necessary.

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Very interesting…and excellent detail, thanks.

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PNW Rains hit tonight.
Dehumidifier on while lights off.
Bottom cross-over fan A6 on 50%

Top two temp sensors High canopy and low canopy positions in grow tent.
Third sensor resides on top external of grow tent (above 2 inches insulation).
May not need dehumidifier when lights are on.

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Was looking at the Jardin light baffles. They offer two sizes, a 6" & an 8".

For each 6" intake port you would need one DF16LB baffle itself, & one DF16 flange. The 8" stuff is DF25LB baffle & DF25 flange. The flanges should include a nice little cutter to make the cutout for the flange.

They also mention on their site that one light baffle cuts flow by 40%, but they don’t mention which size, & I guess that they used their branded exhaust fan, but I didn’t look closely enough, so your actual fan may react differently. I flowtested my DF16 baffle in my drying tent with a crappy 4" exhaust fan & restrictive filter, but I forget how much flow the baffle cut from my fan in my test.

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