am i doing this correct? please help! CFM calculations..

Discussion in 'Grow Room Design/Setup' started by luv2grow420, Feb 7, 2011.

  1. hey guys! I decided to start to grow in my spare room in the basement. However, a lot of work must be done. I'm trying to figure out the CFM required to ventilate my room. Here are the dimensions: Length=14 Width= 10 Height= 6.5 So, I multiply 14x10x6.5 and get 910 cubic feet. Now this is where I get confused. During my research I heard the fan should be able to adequately exchange the air in a grow room once every three minutes. Is this true? So I would take 910 and divide it my 3 to get the minimum cfm for exchanging the air in a grow room. 910/3= 303 CFM So did I do this correctly? Any advice would greatly help me out. Thanks guys and happy growing.
  2. if its 910 cubic foot room you need a huge fan.

    303 cubic feet is not enough i think to get passive air going in it.

    id suggest you just have a fan that cools the lights and another for air scrubbing.

    if not you need like a 10" fan rated at like 1000 cfm
  3. ok, thanks pigpen! so I should get two fans instead of one, got ya. what do you suggest as far as the size of these two fans? sorry if its a dum question dude. I'm assuming a giant fan like that is going to cost me a crap load of money lol so i believe i will take your advice up for consideration. Thanks again, much appreciated.
  4. Sorry dude, I know you're trying to help; but that is absolutely incorrect. A 300cfm fan moves 300cfm regardless of room size so long as the ducting is properly sized. It will pull 300 cfm from an intake duct across the room @ 14' with no problem whatsoever. 300cfm is adequate ventilation for the PLANTs needs. Heat is an entirely separate issue that can't be accounted for until the OP provides details on lights (how many watts, open or closed fixtures?), intake air supply (from outside? or conditioned air from an adjoining room?), and HVAC capacity. In any case, with the average outside ambient temps being below 70* for 8 months out of the year at his location, he is not going to NEED a 1000cfm fan if he designs his room properly.

    Actually, a 1000cfm (12") fan will only cost you about $40 more than a 300cfm if you shop around a bit. What will kill you is the carbon filter (and the electricity to a lesser degree). As for the question of how much air you need to move, please provide more detail on your intended grow setup as listed above.

  5. how was it completely incorrect.

    you said excactly what i did?

    its a better idea for two fans.

    when you attach a carbon filter to a a 1000 cfm fan it almost cuts 30-40% of the cfm rating.

    so he would need a big fan for such a room.

    but if you had a seperate fan ontop of a carbon filter and having it duct out of the grow and have the same thing on a light it will run nice and cool.

    I have a 375 cfm fan with a 15 lb carbon filter and 1000watt light and it keeps my 3.5x3.5x6.5 tent nice and cool.
  6. hmmm wow thanks jamesreed! yeah, i'm trying to design my room properly, maybe you can help me out. :D I want to use MH lights for veg and HPS for flowering. What do you suggest for a room at this size as far as watts? should i get two lights or one? also there is window in my room thats sealed rite now but can be reopened. Since its quite cool where i live here in ronkonkoma long island new york, i didn't think an air condition was necessary. I'm not sure how exhaust works. could you explain if you don't mind? maybe direct me to a link? perhaps i should figure out the lighting before i figure the exhaust? thanks for all the help. Like i said i'm trying to do this correctly, I don't care how long it takes. any suggestions you have is going to go a long way. sorry for my newbness lol let me know what else you need to know to help me.
  7. thanks pigpen, good stuff. like i told james, perhaps i should figure out the lighting first before i figure the exhaust? either way thanks homie.
  8. See below

    \t\t\t \t\t\t
    Not true.

    Still not true.

    True, and a good suggestion. So you are right, not completely incorrect.

    And back to not true.

    Not true.


    Not true. A persistent and widely repeated myth. If your carbon scrubber airflow is rated at 400cfm @ .2"w static and your fan is correctly matched and rated to move 400cfm @ .2"w or higher static pressure, then you will experience negligible losses in air movement (if your fan is rated for significantly higher pressures you could even experience an increase in cfm as the fan forces more air through the scrubber). This myth was likely created by people who had no idea what to look for in fan/filter ratings. A 400cfm rating is not actually 400cfm, unless the static pressure used in the rating test match the installed conditions. The cfm rating on the box is only a suggestion of the capacity of that item and was derived using a very specific set of circumstances. The test methods are not regulated in anyway, not universal between manufacturers, and not likely to match actual field use (unless those specs are taken into account when designing the ventilation system).

    Still not true.

    True, if properly designed and installed

    Sweet, glad that works for you. Doesn't make it the most efficient method. You are possibly using more electricity and burning through carbon filters faster than is necessary. Though without specs on your equipment I can't be sure.

    Have a good day. :smoke:
  9. #9 jamesreed, Feb 8, 2011
    Last edited by a moderator: Feb 8, 2011
    My suggestion for your room would be to split it in half with a wall and use one side for veg and the other for flower. In each room:
    I would use 4000 watts (4x1000) in enclosed vented XL hoods, and leave a 1.5 foot "+" shaped walking path between the 4 planting areas. Thus resulting in (4) 4.5'x2.75' planting areas holding 2-3 full grown plants each, with enough space to move around and tend to them. I would have 4 vents to the outside (2 exhaust (one for the lights, one for the room), 2 intake (same as exhaust)) and 1 vent from my AC "T"ed into the room intake with an adjustable dampener so I could switch intake air to come from the AC system during those hot summer months. I would run 2 8" trunk lines (1 intake, 1 exhaust) down the center of each room and feed each light individually with fresh air from outside and exhaust to outside in a closed loop run by a 800cfm fan at the end of the exhaust line controlled by a thermostat plug with a probe @ the hoods. Then I would install a 200cfm room exhaust with a speed controller and carbon filter to vent outside with intake air as described above.

    Basically exactly what I did do with my own space with tweaks for the different ambient temps associated with our locations and the fact that my rooms are each only 7x6x9.

    But then, I'm not the one paying for your setup and I have zero Idea of what your goals are. Why don't you start with those (remember that talk of dealing is prohibited on this site), and then we can design something for you. ;):smoke:

    Sorry for the runon sentences.

    ETA: you also want to install a 1600 micron intake filter at any intake penetration in the walls, 12x12 for room air @ 200cfm and 12x16 for light system @ 800cfm. Filters oversize to provide as little restriction as possible.

  10. wow bro, that is a lot to take in lol My goals are to grow 4-8 plants for myself, family, and a few close friends. I just want to be able to harvest every month or so once I get everything all situated. It would be nice to have bud every month :smoke: I like your suggestion about dividing the room. one side for veg and flowering. That sounds like a plan. I don't have any carpentry skills, so perhaps I can get two of those grow tents? My budget is around 2,500 or so. Thanks for the suggestions dude! Do you have any pics of your set up? happy growing :D
  11. #11 jamesreed, Feb 9, 2011
    Last edited by a moderator: Feb 9, 2011
    How much do you smoke a month? I'm running a perpetual grow myself right now and pulling an average of 3-4oz/plant. I would recommend not starting out in perpetual grow, it will be easier to accomplish that once you get a few grows under your belt and know what to expect. $2500 is a good budget point for what you want to accomplish. Building a wall doesn't really require any carpentry skill, just a saw and some nails, a few studs and some Sheetrock. I would only suggest the tents if you had no other options. You could always run smaller if you only want six plants a month.

    I don't like to post pics for security reasons, and posting one of my setup would reveal to much about my location. I do have one in the photo gallery of my flower room last month, just click on "photo gallery" at the top of this page, then search for my user name. If you want more details about my setup just search my posts, there is one about perpetual harvest that gives some info.

    ETA: a link to the pic mentioned

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