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I really can't order anything until I get my ventilation figured out, right, or am I just over-thinking this? I don't really have money to waste so choosing the correct product(s) the first time is necessary.
I see you need expert advice on calculating your exhaust needs, I did this for a living (check my profile).
In a 4'x4'x6' room you have 96 cf add 10-20% for unknown variables you're looking at 106-116 cf (round off 9.6 to the next highest cfm=10). You were in ballpark on that, give yourself a great big round of applause. Let's stick with the higher rated cfm 116x2 rooms = 232cfm.
To completely exchange the air in a space we often calculate for 100% air exchange at 3 minutes we call it passive exchange which will translate to 232 devided by 3 = 77.333. You make the call.
The one thing you will notice with a higher cfm fan is that the negative pressure will increase. Whats that you ask? Negative pressure is experienced by most folks on a daily basis, ever try opening a door to a building that swings towards you but you a have a hard time doing so, thats negative pressure.
Try to choose a happy medium for your grow.
I don't mind helping you, you passed on to me a great idea about ballasts which was wire them in remotely, cuuting down the heat load in my grow room, Thanks!!
Happy growing!
B4L
