I'm not sure what you mean.. so.. A carbon filter is meant to filter the smell from flowering. Lets say you have a fan with are flow like this [ ]~. The ~ is the side the air is blown OUT of (the side you'd sit on to feel the air.) You would put the filter (which is labeled *) like this: *[ ]~. So, the carbon would be on the side of the fan that SUCKS the air.
As far as CFM, you want a fan that can exchange air at or around 5 minutes. So, let's say you have a grow room of 5 foot long, 6 foot wide, and 8 foot tall ( 5x6x8 ). That's 240 CF (cubic feet). So, you'd need a fan that could exchange 240. So, you'd need a 48 CFM fan. This is pretty easy to find. I usually go with 120 CFM fans for this type of room just because I like a 2 minute exchange, which overall keeps it cooler.
Up to you.
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Also, you can go to a pet store and pick up carbon filter blocks for cheaper than a carbon filter, then just go to your hardware store and grab some metal mesh (usually used for plumbing) and just wrap your carbon filter blocks in that. It doesn't work as well as a real carbon filter, but if you buy more carbon blocks, it'll work. I usually have to buy 3 bags of carbon filter blocks (they are for fish tanks). Any kind will work.. filters, blocks, chips, pellets. Whatever.
