The infamous question regarding CFM and passive intake. All help is appreciated

I live in the north east. out side temps can be 15-20 in the winter, basement can be around high 30's low 40's. Summer can be around 90's, basement can be 80's. The box is 6'x4'x5' going to be insulated with foam board, gonna have orca grow film. The outside of the box is going to be painted with bedliner paint to prevent the wood from absorbing any moisture. The area will 4 400watt led (200watt actual draw each) read LED grows need to be 80-85, going to have a CHE just incase for the winter. Im thinking of putting the CHE by the intake holes to warm up the air in the winter if need. Also a wall fan will be in placed. The inline fan will push the air through a DIY 5gal bucket scrubber located outside the box(the inline fan will be located inside box). I have everything worked out except for the Fan size and how big/many holes I need. would like to use a couple of elbows and few feet of duct work per intake hole to reach a radiator (mainly for winter).

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So you are not planning on dumping the air outside ? because with each cycle of air your temps will go up, you might want to try that for your winter months but summer might be a problem.

All of your questions could have been answered with a simple search,seeing as this question is asked every single day,multiple times a day.

Look in my sig and click ventilation 101

By the radiator is a small window that I can open so fresh air can come in. I can extend so duct work maybe with a few more feet for the summer and during the winter shorten the ducting to the radiator and leave the window open (the window opens like a 'V')

Trust me bro, I hated asking this question. I tried looking at a few other things for a reference but couldn't add everything up with the variables. I know the elbows decrease speed so you need to add 50% for the CFM, you need double the square inch size for a hole, I was going to get a 4" 200CFM inline fan, then I read about all factors with ducting elbows hole size and got a head ache... I will look at your vent 101 sig, perhaps I over looked that one.

Would multiple passive intake holes connecting to one external PVC piping be a waste?

well if you are tying them all in to one pipe yes its a waste only so much air can travel in a certain size duct

something like this should be a good start ,when dealing with venting no one room is a like, what works for one person might not work for another .

jrainman said:

well if you are tying them all in to one pipe yes its a waste only so much air can travel in a certain size duct

Click to expand...

I want to get all my passive intakes ducts into a 'fresh air room'. My passive intake holes will be staggered then since tying them in is a waste. but staggering them isn't problem. Im thinking of a 6" inline fan with a controller and 3 6" passive intake holes with 2elbows each to reach the 'fresh air room'. I made a better layout of my area. My windows are old school and only open a little bit like '\|'

With the inline fan controller I would have to increase the speed to a higher setting and create more suction due to the velocity being slowed down from the elbows on the incoming air.

I can also put the scrubber in the bilco door space (area is not air tight) but I would have to run a longer ducting. Hopefully a 6" 440cfm inline fan will be enough for both the intake and exhaust work.

jrainman said:

something like this should be a good start ,when dealing with venting no one room is a like, what works for one person might not work for another .

Click to expand...

Once again, I appreciate the help. it feels like this part is the most important one of them all. dealing with air and smell. The picture you uploaded is to small and when I blow it up its all pixelated. Can you look at my previous post (#10) and tell me what you think?

Sorry about that having trouble with sizing my files, I based this drawing on the information you gave me , the more information I get the better I can understand your needs , Like I said this should be a good start , humidity is something you did not address . , If you have questions on the drawing ask me ,I will explain why

So at the bottom of the box Ill have a regular 12" square hole in the box. On the top is going to have ducting running across from left to right while having a Tee fitting to suck air from the bottom intake and while sucking air from the window (is there any help, or is the 6" 400CFM doing all the work?) The 2 things I only need explaining is the 6" reference and the 8" reference for the ducting, will I need adapters? 2nd thing, I will need to use 2elbows an few more feet of ducting to get to the window due to the of the how the foundation was laid out.

Humidity wise, I haven't really paid attention to it on summer months. Ill see if can pick up temperature/humidity reader today to see how it currently is.

Ok I will explain , first and most, you will need plenty of fresh air in a basement grow , if your house is old meaning it does not have air vents built In the foundation ,you must get fresh air moving through your basement, because you are below the frost line and radon gases seep through your walls and floor . and being you are in the north east radon levels can be high,

not saying you have high level, but you have to understand that radon has a high attraction for soil and it is very harmful to you. your plants will feed on the radon through the soil.

I know it is never discussed here maybe guys here don't know are understand how harmful radon is , all the earth emits radon ,you can have the levels tested in you home by a home inspection company , but in my opinion and building experience you want to move lots of fresh air through your basement if you are growing in your basement in soil.

Now lets move on to your questions yes you wll use a 8'' to 6'' reducer fitting off the tee on the exhaust side of the fan (transition) and include a manual damper on the 6'' outside air fan intake side, this will allow you to :

A) buy using a damper in the 6'' duct going to the outside the damper itself will act like a speed controller letting you manipulate the amount of air you exhaust inside your grow area.
this I called a bypass fresh air system.

B) by utilizing this bypass system you have cooler air a it mixes with your grow room air at the TEE keeping your fan cooler

C) by utilizing a bypass system you can regulate the amount of fresh air Percentage wise moving through your filter back in to your basement, resulting in a higher % of fresh air throughout your basement.

D) most basements have a higher humidity level a bypass system can help you regulate the humidity by constantly moving fresh air in that space.


its fine to use 9o degree elbows' 45 degree is better , you see though how I use 8'' on the exhaust side of the fan ,this is to compensate for the use of turns (elbow) .

I do not think you will have issues in spring , summer , fall. your problems will occur in winter being it seems you have a very cold basement , that is something you will need to figure out.

thanks so much for your help, I believe I have a good understanding now of what I have to do it. While I was in the basement going over thing you suggested, I thought of something else as far as exhausting the heat. I have a dryer and its connected with 18ft of plastic ducting and exits outside. Would it better if I just buy a carbon filter and install it inside of the box and tie (See attached picture of what will be used) in the exhaust ducting to the dryer exhaust. From the box to the dryer vent will be about 45ft. would the 400cfm fan handle the passive intake, the sucking of the carbon filter and then push the air through 45ft of ducting? I think after this I will have a solid game plan.

First off you never induce air to anything like sewer line ,chimney , dryer. potential for big health issues and or possible death.

and most likely your dyer duct is way to small in diameter for 400 cfm to pass through, So NO DO NOT ATTEMPT.

And that pic is a TEE Y witch is better then a straight bull TEE excellent choice , if you have a local plumbing or HVAC supply house in your area just go there and get every thing you need. TEE or T Y $12 6'' MANUAL DAMPER $5 . 8'' x6'' reducer $5 ADJUSTABLE ELBOWS , Duct tape, And if you wanted to hard pipe everything or most of it rather then flex , if you are handy and can measure and cut with tin snips ,your air will move a lot better then using flexable duct

Wye is definitely more effective than a saddle takeoff. The airflow is much less turbulent.