Aaargghhh please help?!


Hey all so I’m a beginner and having trouble figuring out my air circulation/ventilation system. So Iv a 1.2 m x 1.2m by 2m tall tent, a 400w MH lamp for now (without cooling hood) and a 400w HPS bulb to be put in for flowering. Room temps r cool Iv a strong fan In but due to the girls only being 4 weeks from germination can’t have it on high. I thought I’d figured out my inlet/extraction system so I could seal the tent up at lights on hours but now Iv set it all up the fans don’t seem powerful enough at all :frowning: so il post a pic to the fans I bought, same for both inlet and extraction but now um worrying maybe it won’t be enough to keep the tent cool when light is on? Iv got a humidifier coming as due to not having the ventilation equipment until today Iv been keeping the tent open during light on hours, but the humidity has been low as a result. When the girls are sleeping, humidity and temps are fine without any fans. Pls any ideas, suggestions are greatly appreciated Iv got them growing well so far want the best for my girls




I use an inkbird to control my temp. I have the opposite problem so I have it hooked to a heater. You can connect to an A/C unit too but you probably could get a bigger CFM (CMM) exhaust fan. Provided the room your tent is in is cooler than your target temp. Your fan is undersized for your tent I think. Roughly you want to exhaust the area of your tent every minute so 2.88 cubic meters per minute. That 4" only does 105 per hour or 1.75 cubic meters per minute. i’m new to this too but that’s my understanding. Hope this helps.



Why do you seal tent up?


I would say its once every 2 or 3 mjnutes because a plant wont use up that air in that time. R u suplimenting co2? U dont need ur tent sealed during lights on. When the lights r off i would make sure u have no light leaks. Also you need a bidder exhaust fan i would say around 200 cfm fan would work here one i got for a decent price


This one has a speed controler on ot so u can just turn it down or up if u need to. I have basically the same set up a 4x4x7 is the size of mine and this fan easily cools off my 600w hps and 2 450w leds with a carbon filter on it.


For humidity purposes and for better sleep it’s in my bedroom :joy:


This is awesome advice everyone thankyou so much!!! I was completely lost with what to do.


You’re going to increase the chances for mold/mildew. And choke off much needed co2 if you seal tent during lights on.


But what about odour control? Iv got vents at the bottom but I thought that was the whole point of the ventilation system. Sorry I’m confused by it all


if i m reading this correctly, u have 2x off brand 4’’ axial ‘booster’ fans…???
these r not the correct fan for your tent.!
4x4x6.5 = 100 cubic feet x 4 (magic factor) = 400 cfm of centrifugal inline fan needed on a speed controller.
u would need ~10 of the axial fans u r currently using.
do not buy from Amazon.!!
more and more growers r getting screwed with bad purchases.!
if u r buying the wrong equipment or getting low quality equipment U R NOT saving money u r wasting it.!!

without an air cooled hood your intake air may have to be as low as 15c to keep top of canopy at 27c or lower.
a 400w MH will veg all of a 1.2x1.2m, but a 400w HPS will flower less then 1m x 1m,
2.5’ x 3’ (75cm x 90cm) area is more ideal for a 400w HPS,


What is the magic number and why is it 4?


to be honest, the 4x factor is what works, idk why, but it does.!

we need complete air exchange every minute or 2,
so if u have a 100 cu ft of grow area u need 50 - 100 cfm of air exchange,
in an open room, with the grow area in it, a simple oscillating fan will be enough,
when we enclose the area, like for flowering plants, more powerful fans r needed,
this is due to the loss of cfm from restrictive air passages and heat build up,
that’s why we use squirrel cage or centrifugal fans,
with lots of circulating fans, no air cooled hoods,minimal ducting, and no carbon filter 2 times volume is enough centrifugal fan, but more duct work and 2x may not be enough,
with cooled hood, carbon filter, and duct work 4x seems to be the magical cfm factor.!!


Make sense to me…I’m sure if I looked hard enough in one of my books it would explain why an engineering factor of 4 is needed. I think they used a magic number of 0.5 when they built our house 10 years ago. It is darn cold in some rooms during the winter and warm during the summer.


no, i doubt u will find it in books, it is a Slow Old Guy magic factor.! LOL
the only other option is to exact calculate the cfm needed,
this would be challenging even for an engineer, so many variables and the variables change with plant growth.!

the 4 x volume does not apply to me, i m 7.5 x volume.! LOL
i m not in a tent, it is a custom miniature grow box,
the 630 cfm fan is ventilation and circulation for the veg and bloom rooms…


Very nice set up!


It’s not sealed as such but I would like to zip it up to contain smells and light leaks


So I’ve ordered a variable speed 190cfm fan to exhaust (il post pic) could I use both my 4” fans as intake fans? No not supplementing with co2 but I’m thinking of buying some co2 bags to try out. Iv got 2 small oscillating clip on fans blowing in two back corners of tent and a larger one blowing at the front. Do you think the new ventilation will suffice? Iv not got a cooler hood yet but I do intend to buy a 600w HPS with cooler hood, not sure if it will be for this grow. My girls will be being put into flower by the 18th at the latest as I have an upcoming vacation in May. The girls are looking great though since starting my nutes.