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Thread: Airflow / Temperature / Wattage table (and calculation).

  1. #81

    Join Date
    Dec 2015
    Thanked 26 Times in 18 Posts


    Just a quick update: As I'm not currently growing, I've had the opportunity to do a few tests:

    I've switched the system on for a few hours with the window wide open. The ambient room temp was <20C..yet my groom was still at 30 touched 29.5 for a while. So, I don't think opening the window is making any difference, or at least too little difference.

    What I'm finding most confusing is, my groom was the same temperature (30 C) when the ambient temp was 22 C....Not quite sure how that has worked out? Perhaps previous tests haven't been run for long enough, therefore, perhaps my heat problem is even worse than I thought.

    After a few more calculations, I'm looking at getting at least a 300 m3/h Centri-fan. I only need about 250, but I have a small amount (perhaps a foot in length) of bendy ducting and a 5 inch carbon filter to consider. It seems to me that a CF can add about 20% to your system impedance if I'm to go by the forums (several different forums for good measure). So ideally I'd like to get a 500 m3/h fan which I can dial down or up as needed. However, I'm limited by the fact I built the system in the winter and installed 5 inch equipment, which at the time was fine. So, perhaps a little poor planning on my part. However, it's the limit none-the less.

    The maximum (within budget) air flow I seem to be able to find in this size is 355 m3/h Which is going to be prohibitively noisy.

    However, there is a particular well known manufacturer (that begins with M) that do a 310 m3/h at less than 35 dB

    So, I'm hoping this is the one for me...I don't think I can do much else.

    I did read somewhere that I could lose a few degrees by using bigger duct (say 8 inch) with reducers. Apparently this gets rid of some of the resistance from the pipe work. Does anyone have experience of this?

  2. #82
    MrChedHead Guest


    You could try a 5 inch L1 rvk.... they shift about 323 if i remember rightly and not that loud

  3. The Following User Says Thank You to MrChedHead For This Useful Post:

    Heavyhank (22-04-16)

  4. #83

    Join Date
    Dec 2014
    Thanked 1,941 Times in 843 Posts


    Quote Originally Posted by Anonymiss View Post
    We've all seen the room volume/air flow calculation* and, hopefully, we've all realised that it's next to useless for controlling temperature when high-Wattage equipment is used in small spaces.

    To address that, I made a table which shows how much air flow is required to maintain a given temperature rise above ambient.

    It's calculated using the basic heat transfer/dissipation equation, which is a standard method for estimating the cooling requirements of hot things in small enclosures (it's often used in relation to electronic devices, for instance).

    I hope it's useful

    Short (memorable?) link to this post: (I made it as easy as I could!)

    If you've found this post helpful then, please, do pass that on to when people ask the age-old, "how do I work out air flow?" question.

    Short link directly to the table:

    *The room-volume method is derived from calculations used for building ventilation. For those who don't know it, it's 20 x room_volume per hour.

    My original post is below here. I'm leaving it because, it has good keywords for the search engines and, it may be useful if people want to copy/paste the formulae but, I've removed the old tables to reduce the page size a bit.

    [edit 26-10-11]

    To make it easier for everyone I've created a table which can be used to look up lamp Wattage and temperature rise and then read off the required air flow.

    <snipped table>

    And here's one to convert CFM to m³/Hour...

    <snipped table>

    Hope it helps.


    Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.

    It boils down to CFM = 3.16 x Watts / DT(°F)

    Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
    DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.

    So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.

    An example is:
    Ambient temerature = 20 °C
    Target temperature of the enclosure = 25 °C
    DT in °C = 25 - 20 = 5
    DT in °F = 5 x (9 / 5) = 9
    Lighting Wattage = 250 Watts

    Plugging these values into the equation gives:

    CFM = 3.16 x 250 / 9 = 87.77

    This represents the actual throughput required but it doesn't take account of the static pressure necessary to overcome the system impedance (how hard the fan has to suck or blow). But for a free-air system with no ducting or filters it should be fairly accurate.

    For those who want a bit more info on how to work it out, here it is.

    First, you'll need to know the amount of heat that needs to be dissipated.

    The general equation for heat transfer is:

    q = Cp x W x DT

    q = amount of heat transferred
    Cp = specific heat of air
    DT = allowable temperature rise within the enclosure
    W = mass flow

    Mass flow is defined as:

    W = CFM x Density

    DT is the difference between the ambient air (room) temperature and the target temperature inside the grow space in °F.

    At sea level the density of air is 1.2041 kg/m3 (at 20°C) and the specific heat capacity (under typical room conditions) is 1.006 kJ/kgC. After doing some substitution and conversion this gives:

    CFM = 3.16 x Watts / DT(°F)

    is this calculation correct ? i thought 5 °C was 41°F i.e °C x 9/5 + 32 = °F

    Im trying to write a python script using this calculation thanks

  5. #84

    Join Date
    Oct 2016
    Thanked 1 Time in 1 Post


    I've got a 10 inch ducking fan as intake with a 600w Hps lamp and 3 45 w t8's got a tent but have no idea how much air 6 plants would need in my tent per cm3 my room size is 240 120 200 ? If anyone had the time to work it out would be amazing my fan has 2 speed setting usually only use the 1st setting. Thanks

  6. #85

    Join Date
    Nov 2009
    somewhere under a bush
    Thanked 40 Times in 29 Posts


    using attic at moment n will only have 4: fan going straight out top of attic thru outlet pipe just till can afford fan n c/b aas plan on 2nd tent going up-1st tent 1.2x1.2x2
    Current Smoke-amnesia lemon & cheesus

    "May The Skunk Fairies Watch Kindly Over You!"
    I don't suffer from insanity; I enjoy every minute of it.

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