Need help sizing carbon filter

I'm at 4wks flowering and it seems that my current filter isn't getting the job done. It's a diy project that I found on the web. I made it with a $10 carbon air filter from wally world and some hardware cloth. It's hooked up with a 6" in-line duct booster fan rated at 250cfm. I'm thinking that there isn't enough surface area and the fan is pulling the odor right past the filter. I only used half of the filter so I'd have a replacement. My room is 40 cubic ft and there is no ducting from the filter to the fan, nor is there any from the fan to the outside. Also my 400w hps is cooled with a cooltube and outside air so I don't need the filter to exaust heat. What surface area do I need for the filter to be effective with this setup? It's definitely going to be 6" in diameter but how long should the filter itself be?

bump. anyone??

DIY carbon filter's don't work anywhere near effective enough. Real carbon filters are packed my machines and thats what makes them effective.

Maybe just get a ~160 cfm filter to match the fan and just do it right. Should last for a bit on such a small area to filter.

If I wanted to spend the $ I would have already. Not trying to be rude but no one has answered my question. What size would the filter need to be in order to accommodate my 6" in-line duct booster (250cfm)?

I saw a formula somewhere for this specific application but for some reason I can't find it anymore. All I'm looking for is a length that will make a 6" filter able to handle this fan.

I guess I'll just add 2-3in and see if the extra surface area makes a difference.

Your duct booster fan will not work with a carbon filter very well at all. That cfm rating is in an open room with no resistance either side. Those duct booster fans are well known for having no pressure therefore dropping air flow immensely when any restriction (even just ducting) is put on them. Look into a better fan if you really want it to work, otherwise plan on around 100 cfm of actual air movement at best.
Look at how commercial carbon filters are made, most diy carbon filters are a joke, and if they are not they cost as much as just buying one. This is one area you do not want to skimp, if the cops happen to come by this could be the one thing to truely keep your stealth grow stealth. This should NOT be a case of wanting to spend the money, but having to spend the money as it is absolutely essential for a stealth grow.
I know this isn't what you wanted to hear, if you want to fill a piece of ducting with panty hose and carbon pellets for a fish tank, it can work, but its going to slow the hell out of your airflow, and is not as cheap or as easy as it sounds. If you want to go this way, you will need a bigger fan. If you buy a commercial air filter, you will need a bigger fan. I do not mean bigger than 6" nor higher than 250cfm air movement, i mean a strong fan that can handle pressures.

freethetree said:

I'm at 4wks flowering and it seems that my current filter isn't getting the job done. It's a diy project that I found on the web. I made it with a $10 carbon air filter from wally world and some hardware cloth. It's hooked up with a 6" in-line duct booster fan rated at 250cfm. I'm thinking that there isn't enough surface area and the fan is pulling the odor right past the filter. I only used half of the filter so I'd have a replacement. My room is 40 cubic ft and there is no ducting from the filter to the fan, nor is there any from the fan to the outside. Also my 400w hps is cooled with a cooltube and outside air so I don't need the filter to exaust heat. What surface area do I need for the filter to be effective with this setup? It's definitely going to be 6" in diameter but how long should the filter itself be?

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Active Air filter the bigger the better ! If you can fine one in your local grow store there about 150 dollars for the 250 CFM one 209 for the 350 cfm.


Also there cheaper than the canair filter. If you have to order one make sure you order the 6" end for it.

If this helps you please add rep.

Honestly, I'm working with a very small grow. 2 moms and 2 clones in veg room at any one time, and 6 in flower (3 sets of 2, 20 days apart). You can't smell a thing upstairs it's just when you walk into the room. Also, I increased the size of the filter and it is working well now for my flower tent. I increased the length and attatched the 6" fan/filter combo to one side of my cooltube and exausted the whole thing thru some 4" ducting attatched to a 4" duct fan. Temps are in check, and so is the smell. Now I realize that most of the stench is coming from my green poison in my veg tent lol...

ps I don't use the pellets I use something like this: https://www.rollitup.org/indoor-growing/7074-easy-build-diy-carbon-filter.html

If it works go for it. remember, a good filter $150 dollars , staying out of jail, priceless .