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Ventilation calculations

So I have been building my own grow tent for a mother plant, I will need around 56 m3/h (33 cfm) ventilation to get enough air changes within the structure, and my inline fan produces around 150 m3/h (88 cfm). Obviously this is far too much but it’s all I could get my hands on.

So my 1st question is if my fan is pulling 150 cmh, Is there a calculation that I can make to determine how large my PASSIVE inlet ducts should be ? I am working with 100mm ducting and the last thing I want to do is create a vacuum in the grow space thereby making my fan overwork, and the temperature rise.

I have already made 3 x 100mm ventilation ducts, and so far it seems ok.

The 2nd question is if there is not a way to slow down the inline fan without spending 1000 Dollars on a VSD or Control unit? it’s just a generic metal inline, I can’t just add a potentiometer because that will decrease the voltage, and increase the amps… kaboom.

Lastly should the fan be running 24/7? my humidity varies between 40 and 50%, temperatures are fine externally, I have a 125W LED quantum board in there which doesn’t run particularly hot, or shall I just set it on a timer to periodically change air every few hours?

Thanks in advance and sorry for the long post!

I dont know about inlet calculations but you do lose 30% off the top if you add a carbon filter to your fan and another 25% for bends in your exhaust. So make sure to account for those. Also 40 to 50% is great humidity control. Don’t have to start worrying until consistent 60 plus if you have decent air exchange.

Yeah I forgot to account for those, I have yet to put in a Carbon filter, simply because I can’t seem to get hold of one out here, will keep an eye out. Humidity dropped to 10% today so the fan is definitely drawing too much. I will probably end up having to just spend the cash on a variable speed controller.

Good luck bud, I know it can get frustrating.

Here is what i use to control the fan speeds when the fan doesnt have a adjuster.