Inline fan question

Hey all. NY here. The dry season is coming, and my 8x10 closet is pumping 2 hlg 600s and will be adding another. Im at 90° and 30% RH. I put in a humidfier and my RH is about 38% now, but my heat is up there by the 4th hour. Its an interioir room. I have an inline system pulling air out of the room and a 6" fan working on intake from one of the rooms it shares surface with. Without putting an ac in there, is there truth in getting a better inline ?( Its 420cu ft, i think) what abouta stronger intake? I have 2 oscillating fans and a 20" industrial floor fan. Idk why its running so hot. Im at 100% and 30", my canopy isnt even yet, i havent flipped into flower yet, but its still gonna reach temp by 4 hours.

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Can you mount the drivers outside of the grow space? The drivers output a good percentage of the total heat output.


Heat rises and collects with what your outputting. Intake from outside air. Vent outside as well if possible to expel the heat

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I dont think they come off…in the least, i dont know how. Id have to run them down the wall and out the other wall…probably 15 feet plus wiring…if its even possible. My i take is chest level and the vent system is about 6ft up and ducting goes out and into a bathroom where i run the exhaust fan 24/7. (I have actually forgotten to turn on the exhaust fan in the bathroom, ill do that right now) but i just dont see how i could run the duct work to the window and then prepare aomething to seal out the cold around the ducting.

I meant up there meaning temps, 90ish. I even have a fan on the drivers and the level of the inline is near ceiling. Its never been this hot. Always in the 80s, but, i had to bypass the dimmer and run at 100% bc the switch was bad. I bought a meter and am sitting at 800-1000ppfd at 31". I havent done anything different than i always do. Its just giving off a kot of dry heat…unless my house heat upstairs level is higher. I keep my house around 70. At 68 yheres too much fluctuation due to drafty doors, but the interior obciously has none. Idk. The lights are working up to specs, i measured very carefully. And ive got 4 thermometers in there at diff levels. And they all read around the same in the plant zones.

Could be inline be taking a crap on me? Would a better system help? I vacuumed it and cleaned it when i moved the girls in there the other day. It sounds like its working. It sucks in smoke at least. Lol. Its about 3 years old. Vivosun 720 actually and its in a 8x10x8 closet at about 6ft up on a shelf

You have a Vivosun 720 8" as your exhaust fan.
Is the 6" intake port with the 6" fan in it your only intake port?
If so, your 8" exhaust fan is going to see a 6" intake port as a big flow restriction, let alone when it has a fan in the way taking up more intake area of an already undersized intake port. The restricted exhaust fan pulls intake air velocity up farther than the intake fan can move the air. The exhaust fan is pulling air through the intake fan faster than the intake fan can move it. The intake fan sits there & burns power, maybe creates a little turbulence, but its blades are not biting the air & moving it, the air is being pulled over them by the more powerful & restricted exhaust fan.

If you had to run both fans, you would be better off running them both as exhaust fans in parallel (“side by side”), and also providing them with enough intake area so that they can reach max combined rated cfm at 100% power.

However you might be able to reach your needed flow rate by removing the 6" fan & providing the 8" exhaust fan with enough intake area for it to reach max rated cfm at 100% power. Easy enough for you to bench-flow the fan for max flow at max power with a diy swing gauge, & then try to reach that flowrate again when sizing the intake area. Probably would need 3x to 5x the fans own port size in intake area for it to reach max flow at 100% power, but you would have to flow it to see. Probably closer to 5x or more if you have ducting on the intake side, but you would have to flow it to see.


Just throwing this out, but things like filter and duct will create a static load on exhaust fan. If filter gets dirty will increase, and more duct and more bends will increase static load. This could significantly reduce the actual volume of air moving through your grow room.

If you have a lot of duct and bends i would see if you can reduce and/or try removing filter and see if it gets better. If you can’t do this may have to increase system size to account for the static load.


Here’s ACI’s recommendation for venting. If you have 8’ ceilings, your CFM starts at a minimum of 640, then goes up from there, as they illustrate in their example scenario. With both your lights running wide open, you’re looking at well over 1,000 CFM of ventilation according to their math, but that’s with no ducting or filter.

For example, a 48”x36”x72” grow tent converted to feet would be a 4’x3’x6’ grow tent. Multiplying the dimensions together would equal 72 ft3. Since a grow space’s volume is also the required CFM to ventilate it every minute, this grow tent’s base CFM is 72. Hold on to this number, as you will have to account for the ducting, carbon filter, and other accessories you might add.

For example, our 4’x3’x6’ grow tent has a baseline CFM of 72 . We multiply this figure by the efficiency percentage of each component. If we add a carbon filter (60% ), ducting (~20% ), and a silencer (20% ) to our ventilation system, we get 166 CFM. From here, factoring in your grow light’s heat output (≤50% ) will give us a required CFM of 249 . Keep in mind that these percentages are not fixed and may vary.


Where is your intake air coming from? I bring air in from the outside.

  1. use outside for air intake.
  2. move drivers outside of grow room.
  3. turn your light intensity down, until flower.
  4. hang some damp/wet towels in grow area until you move to flower.
  5. move your larger diameter inline fan to the exhaust position at the top of your grow area.
  6. oscillating fans are good to mix CO2 around your plants. They should be just below canopy.
    Up high is where heat is rising and gets exhausted out of the area. Don’t mix it with the cooler air coming in at the bottom.

Not sure if OP is running a carbon filter, but 60% loss of flow from a carbon filter is a lot to give up, if it’s on the heat extraction fan. Shorty filters seem to be the worst, & I have flow-tested one myself, & saw about a 60% loss of flow from it. I believe that ACI sells longer ones which theoretically should flow a little better while being as or more effective at odor control. Anyway, I have found this 60% level of restriction to be acceptable for my drying tent, & useful for my anti-RH extraction fan on my main grow cab. But my carbon filter on my main heat extraction fan in my main grow cab is much less restrictive, yet still effective. I plan to flow-test it one day, but I suspect that my main carbon filter might be able to flow enough as to be unrestrictive to my main exhaust fan.

For a main heat extraction fan, look for a carbon filter that will be as unrestrictive as possible while remaining effective at odor control. Mine is DIY & almost a yard long for a 4" fan, but CaptRon runs a good commercially available one. Another trick is to step up to a larger filter built for a larger fan & use a step-up collar from the fan to the filter.


I have a question about carbon filters . If I’m not worried about the smell could I just use the inline fan without the carbon filter ?


@Blazeup You Sure can. I don’t run a filter and only time it smells like weed here is once I’m in flower.
But def gets strong smelling then.
I exhaust to my basement tho so you may want to consider how close neighbors are if exhausting outside.


Sure, but you might consider still running some kind of particle filter to keep things clean. A lot air gets moved through those things.

Ok, thanks, growmies. So, ive yet to begin working on a fuller understanding of environmental controls now that im able to take my hobby to the next level. Lol. I love growing. I grow some bomb weed. Honestly, if i could make a living of it, i would…i need to understand more on this front. Ive never had issues with the plants, they are all just a seed i nurtured. ive battled with outdoor and indoor, cannabis and other plants, disease, deficiencies and lighting woes…4 years with no breaks…lol. 110 plants now (yes i count them) and its a passion. but i know that if my environ was tweaked, i would really have some fun. I understand your suggestions, but all ive went on as of today, are gauges, thermometers and meters and what i think will work (and reading/google). Heres a few pics of my set up. I wish someone could come in and set up a humidity/ac system. I do the towels thing and run a humidifier to keep it at 35-40% in veg. I sit around there mostly. The 6" is a fan in the wall pulling air into the grow space. Theres an oscillating fan about 5ft pointing on them, then on the right theres an industrial fan blowing right off the leave edges of the plants and around. That one moves air! Then about 6ft up theres another oscillating fan oointed on the canopy and the drivers. Above and to the rear, 6ft up, is the inline rear, connected to ducting and enters the bathroom wall. My lights are 2 hlg600rpecs and 2 300rspecs i dont have hanging. I use a photobio light meter to keep the plants over 700, the widest part at 950. I dont have the outside filter on,( i actually just realuzed that.) and last week i vacummed tons of dust out of it. My temp is still sitting around 90, now that im on 12/12 its actually maintaing 90. Its not feeling hot like that tho bc theres a lot of air movement, i guess? I need someone to come and physically show me/teach me, the dynamics of environmental control and set me up w the right equipment, probably… I wanna learn it for myself!

@LateNightGardner — I’m curious about this as I live in a cold winter climate; we can get well below 0 F for several weeks at a time. Indeed, pulling in that arctic air would be a negative for plant health, correct?

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Hey @Blaze_Up_Ahab that would be bad for your plants. I try and keep my grow area above 65F.
If you can throttle your intake from the outside by temp, once tent is above 65F it turns off the intake fan. You could use a separate intake fan for fresh air from another location
I’m using an environmental control that has 110v outlets for: temp, humidity, CO2 and light.

It needs to be adjusted for veg and flower.
I’m in Texas and haven’t had to many of those below freezing days. During the snowstorm 2021 we lost power for over a day. The cold weather shocked my girls and some developed naners and produce seeds in some of my plants.

Your current setup has some inefficiencies.
One is the hose between exhaust fan & the wall. Delete the hose from the exhaust fan to the hole in the wall, & mount the fan “directly” to the hole.

One definite major inefficiency is the small intake area, made worse with the intake fan in the way. Delete the intake fan & increase the intake area until the exhaust fan reaches max CFM at 100% power.

Another probable major inefficiency is the carbon filter, although you would have to bench flow it to see how much if any flow it is stealing. You can buy or make a carbon filter that will flow as much as a fan can pull, yet still be effective at odor control.

Those are the inefficiencies that I can see. What’s on the other side of the intake hole & the exhaust hole?

P.S. you might free up enough flow for that exhaust fan to pull its max CFM, & it still might not be enough for your grow space. How big is the room again? How hot is the intake air again?

Hey @Lunabudz929 90F is kinda hot for you plants to thrive.
Heat rises the oscillating fans are blowing the hot air down to the plant level.
The oscillating fans are for moving air around your plants so they can take in CO2 to grow. By blowing the hot air down from the ceiling can cause the leaves to close off their stomata and cease to take in CO2. Move the oscillating fans down to canopy level and move your exhaust vent/filter up high on the ceiling. Let the hot air stay up high and be pulled out the exhaust.