Hey y’all! Quick question for you!
I’ve seen the CFM conversions online, but I need examples from real people using fans… My tent is tiny, yet Im afraid a small fan isn’t gonna be enough to push air through my filter.
My tent is 4x2x5’ – what should the CFMs be for each the intake and exhaust fans? Please advise!
I only use dc mixed flow fans, and have never run an intake fan. So I’m not sure I can be of a lot of help. What I can tell you is that my fans don’t deal with load of adding a carbon filter and a lot of duct work as good as the centrifugal fans. I use them because they’re a lot quieter and have the best speed control. Knowing this I typically get a little larger fan than I need.
Your tent is 40 cubic feet. An air exchange of once per minute is usually plenty more than you need. So an unrestricted 40 cfm fan would do that. If you plan on using a filter even a properly sized filter will pull that down some. As turns in duct and any significant length of duct. Another thing to consider is the possibility of wanting to exchange a lot of air to help with temps if need be. Since my dc fans can be slowed to pretty much nothing, I get them a lot bigger than I need. It costs more up front, but I also know exactly what I’m working with there.
You also have to make sure your intake fan doesn’t overtake your exhaust. If you’re filtering you want to maintain a slight negative pressure within your tent to keep from having scent leaks. I have 4” 150cfm fan for 2x2 and I’d like to say it would work fine for you. But haven’t ran it in space that big, so I’m not completely sure.
@dbrn32 Is there a brand of fan you like? I did a quick search and struck out.
@Bubblehead @Lou4494
AC Infinity is the brand dbrn32 prefers. They make a 4” all the way to a 10”. This fan is quieter than most, and energy efficient also.
AC Infinity CLOUDLINE T4, Quiet 4” Inline Duct Fan with Temperature Humidity Controller - Ventilation Exhaust Fan for Heating Cooling Booster, Grow Tents, Hydroponics
@Covertgrower Thank you.
So, @dbrn32 & @Covertgrower with that fan do you only need the one? Or do you still need a 2 fan setup?
@Lou4494 I would recommend just one, with passive intake.
Another thing to consider, if you plan on expanding, I would get a larger fan than you need, and just turn it down.
I would also say try going passive. Also, if you can afford 6” wouldn’t be a bad idea. Last I looked it was only couple bucks more when available. You don’t really need to worry about it being too big if you have a 6” port in tent. Like I said, can be slowed to nearly nothing.
For what it’s worth, I also have hyper fan that I really like. It’s just more expensive with less features. Both are great fans!
I purchased a fan similar to this from a local shop to get started in my 3x3. My first grow is in progress now. I decided to put my money into, what I considered, to be good light to first. I will upgrade the fan for my second grow and use the original for make up air.
[Inexpensive 4” Fan](iPower 4 Inch 100 CFM Booster Fan Inline Duct Vent Blower for HVAC Exhaust and Intake 5.5’ Grounded Power Cord Amazon.com)
That could very well do everything you need, and I don’t blame you on directing more money towards light. Try to keep any restrictions to a minimum and keep us posted as to how things go with it.
One of my projects after the first of the year is to design a passive air make up that I can use during flowering. I want to make it adjustable so it can pull a slight vacuum when running the fan. It currently sucks the sides of the tent in about 2 inches. The sides are not super tight but they are not loose either.
That doesn’t sound unreasonable. If you’re filtering air you kinda want that to keep from having air leaks. If you’re not filtering, it’s gonna stink when you’re into flowering anyway, so no difference.
I’m running a filter, while the odor is nice not sure the wife or guests would enjoy it. Plus it would loose the whole stealth aspect I’m trying to keep. 
Does your tent have passive intake open?
So, I actually have a 440cfm fan I initially ordered, not realizing cfms were a thing. And it has a speed control, but even on low it sucks the sides of my tent in baaaad unless I have both vents open. BUT. Im worried about those vents being open and light getting in once I switch to flower.
So, I got two small 150cfm fans, one for intake one for exhaust, I just dont know if the exhaust one could handle pushing the air through a filter.
So.
In a predicament, and don’t really want to spend another $100+ on a fan. Haha.
That’s kind of what I was talking about earlier with the difference in speed control between ac powered fans and dc powered fans. Based on electrical nature you don’t have near the amount of speed control on an ac powered fan, so you need to be more precise with sizing them.
The one thing you have to be careful with running same fans for intake and exhaust is that your intake fan isn’t putting more air in than exhaust is taking out. If you don’t have at least a little negative pressure you get scent leaks.
If you’re worried about light leaks just build a light trap around your intake. You can use cardboard and tape or something more fancy. Try some browser searching for ideas if you need to, but it’s not very difficult once you have the concept down.
Alright. Good to know. Thanks a million. This is very outside my normal field of knowledge, so I feel out of my element. But as far as the light trap, I think I can solve that easily enough. Would I be okay with getting like a 210cfm fan for my exhaust, and keep my 150cfm intake? Would that work on a 6" carbon filter?
OR.
Should I just hang my 440cfm exhaust fan, and open my vents, but make light traps?
Hard to say really. You may be fine with the two 150cfm fans if you can keep the negative pressure. If you have intake open though. You probably don’t need intake fan at all. So you could probably go either way.
No it doesn’t. There are a couple of things I don’t like about my tent. One is the only passive air opening on the floor is at the back and it is nearly 2 feet long. I’m kicking around upgrading the tent too. Maybe I’ll ask for one for Christmas next year.
That’s pretty standard. If you open it and then pull off wall a smidge that may help a little with light proofing. All you really have to do is put some stuff around it so light has to change direction a couple times but air can still get through.