Need help with ventalation and duct question

I'm getting my grow tent in this week and it has multiple 4" inch holes but 4 inch inline fans are not that strong. is there a way to use a 6" inline fan with the 4" hole. I saw 6" to 4" duct REDUCER i'm wondering if I can just flip it around and that could fix my issue.

I would really appreciate any help my house is starting to get stanky. By the way 3/4 where female! just had to share that.

Also how much CFM do i need for a 720 square foot room (3 plants)

You don't want the REDUCER you want an INCREASER. It would be on the same shelf at the store. The difference is the cut is designed to only have airflow 1 way so the REDUCER would require alot of duct tape to seal properly. Don't get me wrong though it is usable just kinda ghetto lol.

I can't find an increaser anywhere if anyone has a link would much appreciate it

Don't mess around with trying to find different parts, just get an exhaust fan that has the proper sized outlets/duct work. Square footage only gives one part of the equation, you need your total cubic feet. I dont' know what indoor growers use as a rule of thumb for exhaust rates, but in aquaria, depending on what you're keeping this is how we size pumps.

So, let's say your room is 30 cubic feet, I'm gonna guess that you would want the exhaust to turn over approximately 3x that volume, equaling 90cfm (I'm just pulling that number out of my head, you only gave square footage, not total volume). That gives you your base cfm, yeah? Googling 'best price' + the cfm you want will give you lots of results, especially of bathroom fans that have the 4" ducting already installed.

Also, there's no reason why you can't work with something like Schedule 40 pvc to make the duct work, is there? Should be able to find reducers that would take you from 4" to 6".. now that I think about it you would probably find that in the black pvc section of your local hardware shop, and I can't remember what the schedule of that stuff is, but you get what I mean, yeah?

Well I'm not much of a handy man so the pvc talk is a bit out of my league (i'm not too proud to say it). The room is 32 cubic feet. this is the reducer i am talking about Hydroempire, Organic Hydroponic Nutrients, Indoor Grow Lights and Supplies - 6" to 4" Duct Reducer [FADR6/4] - $10.95

are you saying for a 32 cubic foot room I only need aprox 90 cfm? I have no problems finding a 4" 170cfm fan. I just want to make sure its good ventilation I'm gonna vent it through a carbon filter.

much love for all the help.

Seriously just go to a hardware store and buy an increaser. You'll pay 1/2 the cost and won't have to wait for delivery.

Dont "rig" your setup like that. Just use a good 4" fan.

I like the right size fan idea from a performance aspect.

But a 250CFM 6" fan vs a 4", the 4" will have a much higher tip speed. Which will be a lot louder. (I like quiet, but consider it a perk over function)

If you have a 4" opening and a long run, increasing the duct size will help flow. Flow reduction is proportional to duct length. So for every foot of duct (even straight) you will get x cfm loss.

I'd say Mr. BigBudBalls there is more knowledgeable in the movement of air than I. I can move your water.


Wait.. that didn't sound right. I mean in a totally aquarium way.

Seamaiden said:

I'd say Mr. BigBudBalls there is more knowledgeable in the movement of air than I. I can move your water.


Wait.. that didn't sound right. I mean in a totally aquarium way.

Click to expand...

Actually they kinda work the same. Water is *just* slightly more dense.

Thank you for not making a water sports joke.

Seamaiden said:

Thank you for not making a water sports joke.

Click to expand...

Well, since you brought it up.........

No, no no, you're being a perfect gentleman.

Oh, and while we're at it, what, in your opinion, is the usual optimal turnover rate for indoor grow rooms? Based on cubic feet per minute if possible, please.

Seamaiden said:

No, no no, you're being a perfect gentleman.

Oh, and while we're at it, what, in your opinion, is the usual optimal turnover rate for indoor grow rooms? Based on cubic feet per minute if possible, please.

Click to expand...

No real opinion. I think it depends on how fast the plants in the room use up the co2, and the humidity from the transpiring.


i'm still a noob grower, just have an ok tech background.

Growin' outdoors, no ventilation issues as yet.

BigBudBalls said:

I like the right size fan idea from a performance aspect.

But a 250CFM 6" fan vs a 4", the 4" will have a much higher tip speed. Which will be a lot louder. (I like quiet, but consider it a perk over function)

If you have a 4" opening and a long run, increasing the duct size will help flow. Flow reduction is proportional to duct length. So for every foot of duct (even straight) you will get x cfm loss.

Click to expand...

Are you saying I should stick to the 4" which is the proper size because adding any extra ducting/increasers will loose cfm anyway and i'll get a louder system as well?

I could always just use 2 4" fans if it becomes a problem i'll just have to save more money to get 2 fans which could be a waste or I might be ok with 1 fan...Maybe i'll have to play this by ear and either save or end up spending...

Fans are worth there weight in gold IMO in a grow room. I think everyone has done a cheap grow before, but look at the setups of the more experienced growers. Youll find both intake and exhaust fans as well as fans used only to cool lights, then more fans to blow on the plants. In total, I have 7 fans in a 2 x 4 x 10 area. One 250 cfm intake, one 110 cfm exhaust, two 250 cfm fans on the 600w cooltube and three fans blowing on the plants. May sound like overkill but I like to get my temps in the ideal range. Everyone knows the lighting is the single most expensive part of the grow. And many throw all there chips on a badass light then go the cheap route on ventillation, a big mistake if you ask me. Your plants will be much happier with a proper vent setup.

theeggman said:

Are you saying I should stick to the 4" which is the proper size because adding any extra ducting/increasers will loose cfm anyway and i\'ll get a louder system as well?

I could always just use 2 4" fans if it becomes a problem i\'ll just have to save more money to get 2 fans which could be a waste or I might be ok with 1 fan...Maybe i\'ll have to play this by ear and either save or end up spending...

Click to expand...

I\'d go for a 6 inch fan. Will be quieter (if in the same cfm range as the 4 inch) The fan size and cfm will *basically* determine noise level. Small fan with high cfm will have to spin faster, more noise. Longer ducts should be increased to keep the flow going. There is restriction due to friction on the walls of the duct. Then there is laminar flow, but we won\'t go into that. Using a 4 inch and a 6 inch duct after the fan will not decrease your flow. It will actually increase. Not past the cfm of the fan, but you will loose less cfm from restrictions and resistance. Your prob is that the flow *to* the fan is 4 inch. Can you make the opening 6 inch? After the fan its best to increase the size. Prior, I\'m not sure. got dig into that a bit. (what I work on needs velocity along with cfm) Its a tough call as to what one needs to do. but blackcouple01 summed it up very well.

Guys: I'm in HVAC, if you want quiet, a 4" is the way to go. A 4' duct will flow 100 CFM and stay quiet. and if you have a 10' x 10' room w/ 8' ceilings, that = 800 cu FT, so you will make a COMPLETE change of the air inside in 8 mins. Further more you do not want ANY duct over 25" in length, and any 90 deg turn will add 5' to the legnth. If you do exceed 25 step up a size to 6" (150 cfm) or 8" (250 cfm)