Fresh air/fan cycle. Fresh air all the time?

Hey all, not sure if this is the best section for this question, but my question is related to cycling fresh air for the plant. I will say that I have no issue with light escaping through any portion of the fan or vent tubing so that is not an issue.

I have an inline fan with carbon filter for my grow tent. My question is, does the plant, through young veg to bloom, does it need a constant flow of fresh air through it’s entire life or should I put the fan on similar to the light cycle

Currently I have the light set to 18 hours on and 6 hours off. Should I keep the fan on the same cycle or should I just have the fan on for 24 hours a day at a low speed? Currently I keep the fan off while at work, but when I get home, I turn it on for a few hours then turn it off for bed. Overall, the fan/filter cycle is about 20 hours off 4 hours on.

Thanks all

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@kyberky I have my fan on 24/7 higher speeds during the light cycle to reduce heat. Then back to low during lights off.
Also when the fan shuts off, and no air flow, as your girls mature, they’ll be a little smellyier. Having the fan in all of the time, keeps the smells away.
Happy growing.


young seedlings need little air movement, 4 hrs a day is plenty,
during veg they need to be swaying in the breeze all day,
in the flower stage they need fresh air almost 24/7 to keep bud rot at bay.!!
this is for your circulating fan/fans, the ventilating fan/s can be run to keep an ideal environment.


Yeah, I have heard the smell becomes an issue - I can already smell it in the tent when I get home haha. Just wasn’t sure if I am wasting electricity having it run all day for a plant that is roughly 4 inches in diameter and about 4 inches tall. When it gets a bit bigger and starts to really grow up I will go ahead and leave it on longer. I appreciate the info, thanks guys!

i have been w/o an exhaust fan for the first 4 weeks of my plants. this is my first grow. but i’m not installing the filter for my veg tent at all. i have a humidifier and a fan blowing in the door of tent to help keep humidity up and temp down. in my flower tent i will have exhaust fan with filter. listen to @Covertgrower and @SlowOldGuy , @kyberky both are very knowledgeable and a great help!


You’ll have to check this out, might be beneficial. Love mine and it’s quiet.

AC Infinity CLOUDLINE T4, Quiet 4” Inline Duct Fan with Temperature Humidity Controller - Ventilation Exhaust Fan for Heating Cooling Booster, Grow Tents, Hydroponics

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With the fan running 24/7 is serving to purposes your pulling hot stale air out also pulling fresh air in. And also creating a negative pressure inside tent so odor does not Escape. Good luck

That is a sweet looking fan. I purchased an iPower fan and duct set. It is pretty dang quiet at medium-low but once you get to high it gets pretty audible. I mean, for the price, it isn’t terrible and I expect it to make a decent amount of noise. So it sounds like 24x7 won’t hurt anything and neither will 12 hours a day. Thanks all, appreciate the help.

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It is very, even on high I can’t hear it at all once my tents closed. I like that you can’t set a high temp and a low temp. So it’ll slow down if it’s too cold. Mines in my basement so automation is key because I live in a very cold area.

I was reading more about that fan and that will definitely be my next. Question though, have you had experience with using the temperature control module and coupling it with a speed controller or does the temperature terminal have an option for speed control?

Update: NM, I am dumb, I just had to read some more hah

I was wondering what type of fan to use on the fresh air that I am trying to blow into the tent. Do You blow it in from the vent at the bottom of the tent ?

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Do you have an inline fan hooked up to remove air from the tent?
On my set up I just use the passive vent/s on the bottom of the tent opposite of where my inline fan exhausts air so while it is running (mine runs 24/7) it forces fresh air to be drawn in from screeened vents on tent. I also hooked up a 4” piece of duct to a lower round 4” opening plus the screened vent gives me plenty of fresh air while maintaining a slight negative pressure on the tent.

Hope this helps,



So, I have an iPower 6 inch fan with charcoal filter setup. Filter resides in the top of the ten, and the ducting goes from the filer, through the top of the fan, into the intake portion of the fan. Basically, the fan is sucking air out of the ten and blows fresh air outside. I do need to get some ducting ran from the bottom of the tent to where it can easily suck in fresh air from outside.

If you aren’t having any high temperature issues you can just open lower screen vent on opposite side of tent of where the 6” goes out and have that vent bring in cooler fresh air. If your having temperature issues and need to lower then you can hook up say a 4” inline fan to a lower port again on opposite side if you can and that will help with temps.
If you run that 4” outside and the temps are cooler that will help.
All comes down to how temps in tent are.

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Hi K, et al.> My observations in Bloom 12/12, you learn fast in micro setups, which is mine.

I have two 36x14x10 inch eco friendly micro grow boxes. *suitable for Veg, Bloom, Mother, seedlings whatever.
Main light are AMAZON Low watt ECRU 150 Full spectrum and 200 watt Blurple red/blue, with 1 sm. supp cfl.
Ea. unit has two quiet, USB 4 inch i/o fans. Love them, keeps the cabinet temps both at 78-81 F. max. (69-72F dark)
40 % RH ON higher when OFF.
self made carbon exhaust filters
self made remote inlet air duct and low restriction dust filter.

Q.1. I had timer turning OFF fans with the lights but was thinking I need to leave i/o fans on 24/7 because:…(pls. see below)

Condition 1 concerns; That with FANS OFF (12 dark) the RH goes UP and so does The Bud SMELL, concentrated for a while, when starting ALL, or Lights+Fans back ON. (eco USB fans)

Q.2. During this period will CO2 be enough in such a small space with ALL circulation stopped?

That said plants are 1.5 months old and looking great.! (seedling pic)
Any feedback and/or Qs appreciated,


Any feedback is appreciated

You’ll want the circulation fans running 24/7 to keep air exchanging and this will help with spike in humidity when lights off.
Plants use C02 when lights on and they exhale C02 when lights off

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