Ventilation FAQ

Discussion in 'Grow Room Design/Setup' started by Bohbo, May 4, 2010.

  1. #1 Bohbo, May 4, 2010
    Last edited by a moderator: Jul 19, 2010
    I am putting together a Ventilation FAQ to help answer commonly asked questions.

    Please share your knowledge if you see something you think is wrong or could be explained better.

    I would like for this to become supported by the community. So if you know anything about ventilation (even just setting up your own) please subscribe (you can do so without posting too if you want). Hopefully this will turn into our own GC Ventilation HQ and the speed and quality of answers will be better.

    If you have a ventilation question post it here and bask in the knowledge of your peers!


    ---=== Notes ===---

    Size of Fan is measured in duct diameter size 4" Fan = 4" Hood = 4" Ducting
    (Try not to mix different duct sizes and fan sizes using reducers.)

    CFM, CFM = Cubic Feet / Minute
    (L*W*H = Cubic Feet) all in ft

    ---=== End Notes ===---

    Q: What Kind of Fan do I need?

    Answer: In-Line Duct (Not Duct Booster)

    There are 3 Main types of fans we encounter building our grow room

    Axial Flow: PC Fans, Duct Boosters
    Suitable for wall, window and roof applications where they can extract directly to the outside. Short duct runs are an option but the length of duct should be limited.

    100mm dia : 2-3m length
    125mm dia : 3-4m length
    150mm dia : 4-5m length

    Centrifugal Flow: Squirrel Cage (not best) Vortex Style (better)

    Pressure developing fans which achieve good flow against the resistance created in a ducted system.
    Particularly useful in venting areas like internal bathrooms/ laundries, multi-room extraction systems, heat-transfer systems, etc.

    Mix Flow
    Powerful in-duct (inline) mounted fans, designed for applications that require high air flows, with low relative noise.

    Commonly Available Brands and a model to get you started

    A. Vortex CFN-400 High Power 172 CFM 4 inch Inline Fan

    B. Valueline, Active Air, EcoPlus (Probably an OEM Brand) 4" In-line 165 CFM (ITS BRIGHT GREEN) 113W

    C. Soler and Palau TD-100X – For Me Best Balance between price, performance, noise My Final Top Choice
    4"/100MM Inline Mixed Flow Duct Fan 135 CFM 60W /100 CFM 44W CFM

    The Vortex and Valueline will be a stronger fan but will most likely produce more noise (but not by much) For me the decision came down to these factors: noise, low wattage, decent pressure, decent airflow trailing not too far behind for. The S&P has a MixVent design that is even more hybrid style Axial Centrifugal intended to literally mix the benefits of both without highlighting either. Another benefit is it is lightweight extra recommended for tent growers.

    So which to get, well I'm not going to tell you that check prices ebay, the **** sites are good. Hydro stores etc. Check the power consumption if its important to you. Pick whatever fan you think is going to fit your need the best.


    Q: How big of a fan / hood do I need for my bulb?

    4" Should cover
    6" Should cover
    600W-2000W *Need more with experience here
    8" Should Cover
    2000W-?000W? Again out of my range just my guess.

    250-350 CFM (non axial, IE non duct booster) / 1000W should work

    Personal experience and conjecture based on that. People with experience chime in and I will happily give this more accuracy.

    Q: How big of a fan do I need for my room?

    Answer: Big Enough to replace the air in the room every 5 min.

    Explanation: The air needs to be replaced every 5 minutes if you are not using CO2, the leaves need fresh CO2 and the the roots need oxygen. Use this formuila Take your total cubic feet and divide by 5.

    LxWxH / 5 min

    10'x10'x10' / 5 Min
    1000 ft^3 / 5 Min
    200 ft^3 / Min
    = 200 CFM

    NOTE: This is NOT a calculation meant to control heat, this is only for fresh air for the plants.

    Additionally you may want to double CFM rating if you use a carbon filter because of the added resistance.

    For heat Calculation see this post by Cyrate:
    (discussion led to this not always being accurate, use it as a starting point give it a 20% margin of error)

    Q How big does my intake fan NEED to be?

    0 CFM is the answer to that one. (Cut a hole instead)



    You absolutely do not NEED an intake fan. How do I know, I made the same mistake you did if you are still reading this. I wanted to be extra sure wired in a bathroom fan i had into the wall, not its usually a passive intake to SAVE POWER! Let the exhaust fan do its job and let liquid flow dynamics do its job and pull air in with negative pressure.

    On the topic of Intake fans.
    "It doesn't promote good airflow and can be very inefficient. Room for air to get in + sufficient outlet fans is the way to go." - Oldskoolgrower

    The only time I see a need for active intake is if you have to move fresh air a great distance or you are trying to place fresh air in a certain area of the room for overall airflow... again intake fan is not recommended.


    Q: What is negative Pressure?

    The pressure inside room is less than pressure outside room. (Think being underwater in a Plywood box, in this example the water would be clean air and the actual air in the box would be dirty air being exhausted)

    Essentially when the room itself is sucking air from anywhere it can. under doors, cracks in walls, sockets anywhere. So if you cut a hole to relieve negative pressure it results in essentially larger airflow (don't cut too big and lose the negative pressure it is a good thing when it comes to odor)

    Q: Okay I don't care what neg pressure how will it effect me?

    Answer: Odor and Fan Placement

    Explanation: See Below

    Q: Odor, Am I really going to have odor issues? I mean how bad is it?




    Unless nobody lives within shouting distance you are most likely going to have to filter or mask the smell. You can start by using my filter DIY turned DIY compilation of carbon filters and Odor maskers. If you check them out and decide to buy one awesome you will have a WAY better knowledge on what to look for when researching what to buy.


    Q: Okay I got my Carbon Filter now what do I do?

    Answer: Attach it to the intake and pull air through it
    Answer: Attach it to the exhaust and push air through it
    Answer: Plug it in and turn it on

    1) Pulling air is usually advisable to pushing air.
    2) Suppose you don't have room for the filter inside the room, or its more convenient to have it in another like garage or attic instead of flowering space
    3) Some units come with a fan and are built to vent out or filter air back around inside room just absorbing odor reducing odors levels in room and circulating air


    Q: Should I blow air out or suck it out push it out what?

    Answer: You want to pull air out. or Blow air out. NOT push it out.



    You want to have your fan inside our room close to the exhaust point pulling air from right there or across the room or through hoods whatever then pushing it only a short ways through to the exit. Pushing is a lot harder than pulling on the fan so you end up with more air circulation pulling or blowing out. You don't want for example to put your fan on the intake side of your hood pushing air through hoods (losing air through leaks around glass etc).

    This goes back to negative pressure inside your hood just like your growroom as a whole.

    Q: What kind of ducting do i need?

    Answer: Flexible Aluminum Ducting

    I first bought insulated thinking it was way better, well it cost more and I had to cut a bigger hole, needed? Probably not, worth it, eh if it helps you sleep sure.

    These are in order of recommendation based on cost and performance:

    Q: I got my ducting anything I can do to help with performance?

    Answer: Smooth out ducting (stretch it) and reduce number of bends needed.


    Smaller duct = greater resistance to airflow.
    More bends = greater resistance to airflow.
    Longer duct runs = increasingly reduced airflow.

    Wrinkled ducting causes turbulence and reduces airflow

    Smoother, extended ducting allows smooth, increased airflow

    Q: How do I light proof a Fan?

    Answer: Spray paint inside of ducting elbow 90 black and make a turn or two as needed.


    Q: Okay I narrowed it down to 2 fans a 4" and a 6" (substitute your sizes) which should I get?

    Answer: When in doubt go with the bigger fan (as long as you don't fear the extra noise).

    Explanation: You can't really have too much ventilation. To have too much air movement (barring the plants getting pushed over and effecting growth) is like saying oh I hate the beach the air is so fresh and easy to breath. If you are on the fence put your mind at ease and get the bigger fan.

    Q: Wow a good blower is expensive are there any DIY options?

    Answer: Yes fairly easily, especially if you are mechanibus.

    Explanation: Modifying a Stanley Blower for Cooling

    Questions in progress or that are unanswered

    Q: How do I ventilate a bedroom without cutting any holes in the walls?

    A: Light trap window, carbon filter for exhaust, intake filter for pollen and spores, outside facade for security.

    • Like Like x 3
    • Informative Informative x 1
  2. <Reserved> I am sure ill come up with something and wish I had this.
  3. This FAQ answered every question I could even think to ask as a newbie to the indoor scene. And I am definitely going to be reading that DIY smell masker when my ladies start stinking. Thank you quite alot, good sir!

  4. Thank you for the feedback fox, I hope it helps others out as well.

    I would read that BEFORE they start stinking :) "An ounce of prevention is worth a pound of cure"
    • Like Like x 1
  5. Touche, my friend. But, like you said, I do not live within shouting distance of anyone. I could hoot and holler and stink a mile in radius and no one would know.
  6. Those fans that you mentioned were slightly out of my price range. Would one such as this 6" In-Line Duct Fan be acceptable, until i can afford a better one? CFM of 160, free air and boosted.
  7. Thanks for the info
  8. #8 Ramones1234, May 4, 2010
    Last edited by a moderator: May 4, 2010
    To the OP, I run a 6'' 540cfm through 1600 watts and exhaust from that is a constant 75f (drawing air at 70f room temp) and I have no doubt that the temp would rise only a tiny bit if I were running 2 1000 watters. My conclusion is that to be safe 250/300cfm per 1000 is optimum.

    These will move maybe half the CFM rated. Only in an emergency.
  9. How would I go about putting an intake in to my room. Where would I put it?
  10. Thank you, ramones!

    A question to anyone willing to answer: I'm looking for the cheapest viable intake or centrifugal fan for a 4'x4'x6.5', 500W grow room(200 ish cfm, i suppose?). Any ideas or opinions on cheaper ones? I'm not really concerned about the sound factor. power usage is a priority, but not as much as price is.

    One thing I do NOT want is my plants to have heat stress. Right now i have some personal fans, one at the door of my grow tent blowing outside air in and one behind the plants circulating the air that comes in. My temperature keeps at a constant 83, but I need a real exhaust fan for legitimate growing purposes of course.
  11. Ebay always has some fair deals, personally I would go bigger than needed and get a speed controller. Its easier to go big and not need it than to try to get a deal only to make you spend more than you would have fixing a problem than by getting the right tool for the right job.
  12. I'm looking at getting this fan for a 2' x 4' x 5.5' wooden grow cabinet, running a 400 watt HPS. Do you think it would do the trick? -

    High Tech Garden Supply

    Alternatively, I thought about making my own vent fan out of a 7" fan I already have, but with material cost it might be almost as much, and would be way bigger. Space is obviously an issue in grow cabinets.

  13. That should be more than enough running at half CFM (loss due to limited static pressure) you should easily be able to use that fan. Or the 7" you have and get a reducer fitting for it.
  14. Awesome, thanks! +rep to you, good sir :)
  15. using 4+ pc fans for a 2x3x4-5 (ish) box with a 400w hps

    which would be the most efficient method of putting them in?
    i was thinking 1 intake at the bottom then 3 at the top pushing?pulling? air out

    do pc fans pull into a box or push? and do they pull or push when they eject the hot air??:confused:
  16. #16 Bohbo, May 4, 2010
    Last edited by a moderator: May 4, 2010
    I have my PC fan mounted on the back wall of my clone cab inside the cab blowing air out the hole. Need a picture? If so I will try and take a picture later if I remember.
  17. thanks alot for the fast reply.:D
    Do you think having 6 pc fans will be sufficient to keep the temperature ,from a 400w hps low bay with an inbuilt ballast, ok for the plants? and would u reccomend having one intake fan with holes or just holes?:confused:
  18. They should be enough to ventilate the space. Say 10 CFM each and you have 3 essentially. That means you could effectively ventilate a room with 150 CFT lets be conservative and say 100 CFT 2x3x5 = 30 CFT... now about the 400W HPS WITH ballast (ballast has heat too) I would say no that wont be enough. But before you buy another fan light it up and see how the temps get.
  19. will do thanks again for the fast reply
    so essentially if its tooo hot i can just stick in more PC fans or is there a limit to how many will be efficiet
    so what i mean is 1 fan =10cfm or summin then 3= 30 then 4=39 then 6=50 or such a trend

    My last question btw (i think)

  20. I only use a single PC fan to cool 4 13W CFLs and my aero cloner in a cab. So I am not an authority on PC fan usage, but having used a lot of them in my work I would think there would be diminishing results. Plus you are getting more and more holes that you would have to filter for odor, light proof etc. Thats the reason why I suggest the investment in a single good fan. Most people overlook ventilation until "OH SHIT MY ROOM IS HOT!" I thought my 2 centrifugals (squirrel cage Dayton 400 CFM blower or whatever it was I can go look and correct later) things were loud, lots of pressure, not the greatest airflow, hot, high wattage, anyone want to buy them off me? :cool:

    Use the resources you have should budget to spend 30-50% of what you invest in HID lighting on ventilation.

Share This Page