Inline/exhaust/carbon filter fan advice needed

3 month vegging plants, just about to flip
Indoor 4x2x6’ tent, coco coir and perlite, 4 Gallon fabric/smart pots
2000 (450watt actual) cree cob full spectrum led
Humidity usually around 75% (high of 90%, low of 55%)
Temp with lights on around 75f

I need to get a fan system set up, as humidity is looking like it’s going to be problematic during flowering. Also to deal with smells, as it seems like it’s going to be a stinker.

The CFM requirement is only about 48, but the 4 inch inline fans I’m looking at seem to be around 300 CFM.
So I need to know if running something that powerful (possibly unnecessarily so) is going to cause any problems.

Ideally, the purpose is to lower humidity, (tent is very wet at “lights on” due to being zipped up all night), to draw in new air, and to extract hot, stinking air to a carbon filter. I think I want an active rather than passive set up, even though the space is small.

I don’t really have a clue what I should be looking for, or any rules I should be looking to obey to get the best possible outcome for my set-up. Any advice on this will be gratefully received. :smile:

@coriolis You are right on track for figuring out fan size. I was taught the rule of thumb that tends to work well is multiply your cubic feet by 4 to get necessary cfm of fan. I was light when I started and had to get a larger fan. You can always get a variable speed pot or fan and it will help.

If something like this is in your budget you won’t be disappointed.

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Hey @Bubblehead that’s the fan I was looking at, in 4inch (which is still above what I need).
Annoyingly, because I’m in the EU, postage and customs is easily going to be as much as the fan.
I can’t find any stockists in the EU. so I guess I’d be looking at something similar but made nearby.

I wondered if there was any penalty for going too big on the fan.

4" model doesn’t handle static load of a filter well in my opinion. With as good as the speed control is I would also go with 6" if you’re buying ac infinity. The only filters I buy is phresh. They are a little pricey but work very well.

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@dbrn32 nailed it. Your cfm will be hampered by any filter you put on it as well as the length and diameter of your duct. It also depends on your lighting. If you use hid you’ll want more exchange esp if it’s not air cooled. It slowly adds to what’s needed.

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(I’m using a 450watt (actual) cree cob full spectrum)

I will tell you what I would do but I built sterile rooms so I’m very attentive to air exchange rate.
A couple reasons are
-temp
-humidity
-bacteria.
There are two basic types
-Passive=just an exhaust
-Active=intake and exhaust
You know your cfm is 48
You want to exchange that air at a min of once every 3 minutes but if you go too fast you’ll create a wind tunnel. That’s why the controller is key. I go about 3 times a minute. So for you I’d round up to 50 multiply that by three to get there times a minute. Now we are at 150. If my light runs hot I add 20% and If fight humidity I add 30-40% of that. If I’m adding a filter I add 20% of that. If for some reason I have a long tube or bends to exhaust your pressure gets bottlenecked I add 10% Exhaust is more important than intake as a general rule if you only have one fan. I always use the same size fan on intake I do exhaust if I have two fans. So it really depends on your funds, location geographicly, smell and many factors. I would go with 300 6" intake and exhaust and dial in the controllers. That’s gives you 6x your cfm which should be plenty for your light and space. You always overshoot tho and dial down. You can’t go past your fans limit and you don’t want to order twice and waste money.

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@coriolis here’s a link I just read over that is better at explaining it than I am lol

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One of the reasons I like to use a 6in exhaust fan is I have a lot more filter capacity With the 6-inch filter use a variable speed on 6-inch and can run it around medium speed and still maintain a good negative pressure inside tent.

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@RandomlyRan @dbrn32 @kellydans @Bubblehead
Thank you so much, that was the kind of information I was looking for.
Knowing what to do is easy enough to look up, but knowing why you’re doing it allows for adaptation. Thanks everyone.:smile:

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@coriolis in the case for the AC Infinity fan going bigger will allow your larger fan to operate at a slower speed, increasing efficiency. (Because it’s a DC fan) the 6” infinity fan I think does 45ish watts on high. I have one and love it. Works very well.

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