Fans - Not just blowing hot air!

Discussion in 'Growroom Setup' started by Tokudai, Apr 17, 2007.

  1. Tokudai

    Tokudai Registered+

    I apologize in advance for the length of this post, but you clicked it - you must be interested!

    There have been many, many topics posted here that are basically the same questions - can I use this fan? Will this fan work? Which fan should I use? What's the best fan? Hopefully I can get all the information together here in one place and finally "blow the lid off" this whole fan thing.

    Please note - I am not talking about sealed rooms with CO2 enrichment. These setups will require very specific fans and air routing, to ensure the benefit of the additional c02 is recognized.

    First off - Lets dumb it way down - there are basically 3 uses for fans in a growroom.

    1. Air Exchange - This is the process by which fresh air is introduced to and removed from the grow room, in order to provide the plants with fresh CO2. Most air exchange setups will have to be coordinated with #3 - cooling and odor control to ensure optimum results for stealth.

    2. Air circulation - This is internal air movement in the cabinet/room - plants waving in the breeze. It is required to help strengthen stems and promote air movement to make sure the C02 is actively moving around, able to be taken up by the plants.

    3. Cooling and/or odor control - These are the fans used to power a carbon scrubber (to exhaust warm stinky air from the room) and to blow across the lights to lower temperatures in the grow room.

    Static Pressure - the fans natural predator

    This definition from Wikipedia defines Fluid Static Pressure, but the applicable excerpt for pneumatic duct pressure is highlighted

    "In fluid mechanics, and in particular in fluid statics, static pressure [1] is the pressure exerted by a fluid at rest.
    Examples of situations where static pressure is involved are:

    The air pressure inside a latex balloon is the static pressure and so is the atmospheric pressure (neglecting the effect of wind).
    The hydrostatic pressure at the bottom of a dam is by definition the static pressure and so is the pressure exerted on one's thumb when stopping the water flow in a garden hose.
    The pressure inside a ventilation duct is not the static pressure, unless the air inside the duct is still.
    On an aircraft, the pressure measured on a generic point of the surface of the wing (or fuselage) is not, in general, the static pressure, unless the aircraft is not moving with respect to the air."

    ^ Thx to Wikipedia

    In other words, the higher static pressure the intake air ambient state is, the harder it is for a fan to pull the air, the less the output of the fan will be. Fan CFM numbers quoted by manufacturers are almost always advertised for the flow at 0" static pressure, since this will always be the highest number. The better manufacturers will give you ratings at various static pressures. Ducting, bends, filters, and reductions/objects(i.e.lights) in the intake (or exhaust) create a harder path for the fan to suck and blow through (suck through a McDonalds straw and then through a coffee stirring straw). Put a box fan 1/2 inch away from a wall, blowing towards you. When it is next to the wall, the fan needs to pull air through the small gap all the way around the fan casing, thereby limiting the amount it can blow out. Now, move it farther from the wall (making this gap larger). The fan finds it easier to pull air through this larger gap and will blow harder. Now put it in a window or the middle of the room, and the fan will be working as it was intended. This same principle of "restriction" is demonstrated by large duct runs, ribbed ducts, bends, carbon filters, and is the single reason two fans that move the same CFM can become vastly different outputs when dealing with a small amount of restriction.

    Types of Fans

    "There are three main types of fans used for moving air - axial, centrifugal (also called radial) and cross flow (also called tangential).

    The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air across the axis of the fan, linearly, hence their name. This is the most commonly used type of fan, and is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels.

    The centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades form a spiral pattern. Centrifugal fans blow air at right angles to the intake of the fan, and spin (centrifugally) the air outwards to the outlet. An impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing. A centrifugal fan produces more pressure for a given air volume, and is used where this is desirable such as in leaf blowers, air mattress inflators, and various industrial purposes. They are typically noisier than comparable axial fans.

    The cross flow fan has a squirrel cage rotor (a rotor with a hollow center and axial fan blades along the periphery). Tangential fans take in air along the periphery of the rotor, and expel it through the outlet in a similar fashion to the centrifugal fan. Cross flow fans give off an even airflow along the entire width of the fan, and are very quiet in operation. They are comparatively bulky, and the air pressure is low. Cross flow fans are often used in air conditioners, automobile ventilation systems, and for cooling in medium-sized equipment such as photocopiers"

    ^ Thx to Wikipedia

    For our purposes, we will only be interested in the first 2 types of fans -

    Fan Descriptions

    Centrifugal Blower Fan - Commonly referred to as a Squirrel Cage fan, also technically known as a shaded pole blower, the exhaust is offset from the intake and requires a flange to mount to ducting - Dayton and Active Air are examples of these. Blowers are modestly priced and can be excellent for any of the three purposes we need them for.

    Centrifugal Inline Fan - This is a Centrifugal Fan which has identical inlets and outlets on each side, and as such is ideal for use in ductwork. Fantech, Vortex, and Panasonic Whisperline are examples of manufacturers. These can also be used for any of the three purposes, although generally higher priced (and MUCH quieter) than a comparable CFM squirrel cage fan..

    Axial Fan - PC Fans, Muffin Fans, Gable fans, Attic Fans, Box fans, Duct "Booster" Fans, Oscillating fans - Any fan that blows air directly across the axis of the fan is considered an axial Fan. Highly affected by static pressure, for the most part, axial fans are only efficient for #2 - air circulation

    While a cross-flow fan could be utilized, output, size and price are not attractive for this purpose.

    How much fan do I need?

    In one word - more.

    Personal Opinion Warning!
    Note - this section is from opinion and experience cobbled together with rudimentary pneumatic/HVAC management knowledge and common sense learned over my 38 years. I have constructed ~20 or so different grow rooms/areas, from 150W cabinets to 5KW rooms and each one had its own problems and solutions. The only one size fits all solution is to buy the biggest baddest fan made - hardly an option

    This question is extremely hard to answer without accurate duct static pressure readings, configuration, room dimensions and plans, intake/ambient temps, desired results/temps,light wattage,acceptable noise level, number of plants, reflector design, stealth issues ... All these differences add up to make each growspace and setup unique in their requirements. 2000 watts in a 10x10x10 room in Alaska is a lot different than 600 watts in a C-13 in Brazil! The smaller cabinet will likely have more heat problems, and use a single fan for more than one purpose, hence a single powerful fan. The 10x10x10 room you might get away with 2 box fans, one circulating air and one pointing out a window (not very stealthy though) The only safe answer to the question "what size fan do I need" is "a larger one".

    In cabinets, for air circulation and exchange, I like to shoot for an air exchange of at least 3x per minute. I also always put a small fan inside the cab simply for circulation - no intake/exhausts, just to move air, blow across the tops of the plants. Computer fans can work great for this. Say you have a cabinet that is 2' x 2' x 6' - this equals 24 cubic feet (LxWxH = Cu Ft. - If using inches for measurement, then divide the answer by 1728 (24x24x72= 41472 cubic inches. 41472/1728 = 24 cu ft) The minimum size fan I would use for this box would be ~ 75cfm. That does not take into account any ducting, cooling, or routing requirements - just air exchange and circulation. If I was using a scrubber (and not concerned about anything else), personally I'd double it to a 150cfm fan, either squirrel cage or centrifugal, depending on budget and other needs. This is my starting point.

    After you have your starting point, put the lights on in the cabinet WITHOUT the plants, and take accurate temperature readings inside, once the lights have been on for a few hours. Note that if you leave the thermometer in the direct light, it will read much higher than the ambient/actual temperature. Shade the thermometer probe and locate it about mid cabinet to get a more accurate temperature reading. Check the temps of the room the cabinet is in, taking care to note situations such as a box inside a closet with the door closed - with no place to exhaust the air outside of the closet - the temperature of the whole closet will gradually raise, raising the intake temps more, which makes hotter exhaust, and continues this cycle.

    If your cabinet temp is less than 80 degrees, congratulations - you are done. Get a fan the size of your starting point and go go go!

    The rest of us will have a, in some cases, huge temperature differential (74 degrees intake air, 110+ degrees in the cabinet in the shade for example). Lights and ballasts put out a lot of heat. So not only does the fan need to circulate and exchange air, it needs to remove 30ish degrees as well. Heres where it gets tricky, and is very important to know where the exhaust is going, what the intake temperature is, how the fixture can be cooled. Most reflectors have fittings on either end of them, making it very easy to hook up to ducting. My rule of thumb is an additional 75 cfm for each 10 degrees differential. So if using a scrubber in this 2x2x6 cabinet (150cfm starting point + 150cfm for light cooling) I would size ~ 300cfm for cooling, odor control and air exchange. Then we would determine noise requirements, budget and mounting options, and decide between a squirrel cage or centrifugal fan in the desired CFM range.

    Though it may seem overkill, using this formula I have rarely ever had to resize a fan - and never cooked a plant to my knowledge. This formula accounts for static pressure from typical corrugated duct routings, usually 6-inch in diameter, and a couple of 90 degree elbows, and a passive intake (or 2) of 2x the exhaust size exit. Slight adjustments should be made for convoluted duct runs, 4-inch tubing, and any other special requirements. Temps are almost always in control from day one, making less hassles in the long run. Don't cheap out and buy the least expensive fan you can find, make sure its from a reputed manufacturer with a warranty and you will be much happier.

    Complete rooms are a different task altogether, although the same methods are used for determining ambient temps. Since the air is not contained in a cabinet, I wouldn't be as strict on the 3x a minute exchange rate, nor will there be the same intense heat concentrated in a small area. A larger room will almost always be easier to keep cool and fan sizing is generally determined by circulation and odor control requirements

    I can't get a new fan - what can I do?

    Some people cannot afford a new fan, or have no room, or are trying to use an inadequate fan and refuse to upsize - whatever the reason. Theres only a couple of solutions here - keep cabinet doors open, increase intake size (if inadequate), lower the intake temperatures, or remotely locate the ballasts outside the cabinet. other than that, theres not much else you can do to remove heat from the growspace.

    To be continued...
    • Like Like x 1
  2. bud luv

    bud luv Registered+

    hey tokudai, nice work so far.

    one question - why shade the thermometer you're using to guage temps? the plants aren't shaded, wouldn't you want the same reading the plants are getting?

    smoke on,
  3. Tokudai

    Tokudai Registered+


    The reason I shade a thermometer is because if not, it can cause an inaccurate measurement when checking actual temps. Since the light radiating down directly onto something feels warmer - think about being in an airconditioned room at 75 degrees next to a window where its 100 degrees outside and the sun is shining through - It feels much warmer, sitting in the spot where the sun beames through, but your actual temperature you are feeling is 75 degrees on the majority of your body. Even though it feels (and is) warmer in the sunspot, your actual temperature wont change much.

    Plants have the ability to withstand 80, 90, even low 100's with no problems when measuring with the thermometer directly in the beam of light, as long as the ambient (shade) temperature is low enough. When its 85+ in the shade, thats when you see problems start to pop up from temps
  4. GoldenGraham84

    GoldenGraham84 Registered+

    extremely helpful post. much appreciation for posting and sharing.
  5. Hydoboy

    Hydoboy Registered

    Quiet Non-Vibrating Intake fan for closet??

    Hey Guys,

    My neighbor can stand my 4" Hydrofarm In-Take fan that's connected to my light's reflector but not the squirrel fan at the bottom of my closet. It's louder, and it vibrates at a very annoying harmonic hum.
    I even have both of them inside cardboard boxes which is then packed with Perlite.
    I've got to replace the squirrel fan. Should I just get another 4" In-take fan and us that at the bottom of the closet to suck in air?
    Or is there a quieter/non-vibrating fan out there?

    Thanks for the help.
  6. stinkyattic

    stinkyattic CultiModerVatorAtor

    Inline fans should be set to pull air out as exhausts; adding an active intake actually is a waste, and can even interfere with the efficiency of your exhaust, because turbulence decreases air flow.
    With a nice big unobstructed intake hole, you don't need the intake fan.
    Hang your exhaust fan in midair from thick soft rope to dampen vibration and de-couple it from the structural members of your apartment. Use flexi duct to get the air where you need it.
    Thanks for bumping this thread, it should be a sticky in growroom setup and I'm moving it there.
    • Like Like x 1
  7. the image reaper

    the image reaper Registered+

    if venting directly from the lamp, do not use your fan on the exhaust-side, unless it is made for hot-air transfer ... like a furnace duct fan, for instance ... heat and electrical motors aren't a good idea, unless the fan is specifically designed for it ... :smokin:
  8. sdholic

    sdholic Registered

    Quick question: I recently abandoned a grow cab i made due to heat issues. The ambient temps halfway up the 24"x16"x48" cab were in the 78-82 F range but the temps under the light (150w cfl) were close to 90-94 F.

    Considering the ambient temps would it REALLY be ok to have plants in there? I cant imagine this being ok with the 90+ F temps under the light. But please correct me i have a lot to learn...
  9. Rusty Trichome

    Rusty Trichome Registered+

    I flower in an outdoor shed, with an undersized a/c the southwestern desert.

    You can check my signature for dealing with heat might help. :thumbsup:
    • Like Like x 1
  10. cavadge

    cavadge Registered+

    A tip which may help some:

    I am using an inline (axial I suppose) fan with the carbon filter for ventilation and odor control. I mounted it to the floor joists above with the supplied bracket, plugged it in, and went upstairs.

    Sounded like a lawn mower running in the room above. Vibration was being transferred to the framing of the house. Did a search and found these antivibration mounts, perfect for ceiling mounting - Vibration Mounts eStore Cylindrical Rubber Anti Vibration Mounts Rubber Neoprene Urethane Sorbothane Silicone Gel Ring Mounts. Click on the pic for steel mounts with rubber, rated to 110 lbs. I ordered the unthreaded units rated for 25 lbs. Problem solved. I still feel a little vibration if I lay on the floor above, but no noise.

    Last edited: Mar 1, 2009
  11. bigsby

    bigsby Banned

    I know this thread is cold but a lot of people view it so if you have the answers please post!

    1. Tokudai mentions that the calculation parameters are more relaxed for a "complete room." His cabinet size for the example was 2x2x6. At what size do the parameters shift? 4x4x6? 8x8x7?

    2. Does anyone know the multiplier for adding a reduction in this scheme? What does stepping down from a 6" to a 4" duct do? Obviously this creates additional load on the fan but how can that be calculated?

    One additional thought - you could add a duct iris to the passive intake. Coupled with a speed controller for the fan you should be able to micro adjust air pressure to an optimal point. They are a little pricey though...

    Also, the Noob's guide to grow room setup also has some good examples of how to calculate fan needs. Find it here:
    Last edited: Apr 4, 2010
  12. bigsby

    bigsby Banned

    More fan thoughts related to a reduction in the duct diameter.

    How would these two scenarios play out?

    1. 6" duct with 6" inline fan. The duct reduces to 4" half way through the run. Assuming the run is 10' with no bends.

    2. 6" duct reduced to 4" duct with 4" inline fan.

    How does a reduction impact the venting capacity?

    Silly questions to some I'm sure but it is worth adding to the knowledge base here. Further, I have venting issues. I can probably get a 4" duct through but want to be sure that my venting strategy will be adequate to vent my 4x4x6 space.
  13. KillerBudG

    KillerBudG Registered+

    Well here is my :twocents:

    Just spend the extra $$$$ the fist time round to get a kick ass fan/w speed controller and no worries who cares.

    And it will add some stress ya fan with the reducers how much all depends on number things. I was told awhile back but don't remember the cfm count tho. I would say just add 30-40 more cfm if you plan on using 1 reducer Imo.

    But get a good fan and controller is the way to go. You can always adjust your fan speed and upgrade your lighting while not having to worry about it later. And ventilation is a key thing Imo with growing, It will either make your grow or break it, you decide.

    Why would you wanna use reducers anyway, As this will reduce airflow.

    Lets say your fan can more 60 cubic feet/min air moving with a 6in line, but you only allow 40 cubic feet/min to move as the 4in line is 1/3 the size smaller than the 6 so the fan is only moving 2/3 of the air the fan is capable of moving.... Makes sense, Sorry if it don't in :S5: the middle of my cigar.:D

    Only thing that makes sense to reduce is a line off you Ac hood which would be stupid.:(

    Last edited: Apr 6, 2010
  14. bigsby

    bigsby Banned

    My ducting options are SEVERELY limited. I have a 2" opening that I may be able to widen to 4" but that is the most I can do.
  15. DeeMar10

    DeeMar10 Registered+

    It looked like some people knew what they were talking about here so I thought I'd throw this out there to see what info I could gather :jointsmile::jointsmile:
    I have a 7x9x10 room. So that 630 cubic feet

    I plan on having two(2) home-made grow tanks

    3x3x6 54 cubic feet x 2 = 108 cubic feet

    3x3x8 72 cubic feet x 2 = 144 cubic feet

    ( Have not decided on the height yet so both figures are given )

    From what I have gathered I need to cycle the air 1 to 2 times a min. for best results ? I want to connect both hoods together and run through one carbon filter before it goes up in the attic where I plan to have it vented towards my bathroom air drain having it pass through one smaller carbon filter in the attic before attaching it to the bathroom air vent (stink pipe?)

    With this information can anyone give me a little insight on what size of can fan or vortex power fan I should be aiming for ? Or better yet what kind of CFM's I need to be pumping out!

    I appreciate the input and advice! If you have a better idea or tips I will keep them all in mind.
  16. GaGrown

    GaGrown Registered+

    You are soooo right! Take a Hydrofarm 465 or a 265 cfm fan,for instance.. Both these type fans are what they refer to as active air blowers. They are meant to be used inside the grow area and blown out of the room.A furnace duct fan keeps itself cool by being inside the cabinet. If you use an active air blower then it is advised that you not use it as an exhaust fan. You have 3 ways that air can be used to serve your needs.

    Passive air
    Exhaust air
    Active air

    Each one has it's own application.Passive air will occur when used with an exhaust or an active air blower/fan. Exhaust is where the fan is placed on the outside of the grow area.

    Ga Grown!
  17. GaGrown

    GaGrown Registered+

    I would just leave it vented into the attic at the soffit vent. You'll get a sewage smell in your grow as gas from the pipe your planning to use as a vent. A backdraft damper won't even help you in this situation. Hope this helped ya!

    Ga Grown!
    • Like Like x 1
  18. GaGrown

    GaGrown Registered+

    I should mention this since this thread is about ventilation... If an active air blower is used other than by instructions,you could risk a fire.

    If you pull exhaust air through an active air blower your heating the motor and damaging the bearings.If you hook it up right and use it the way it is designed to be used it will serve you well.If not? They get wobbly and noisey.A 1000 watter is an active air blowers worst enemy!! Be safe not sorry!

    Ga Grown!
    • Like Like x 1
  19. Ocotillo

    Ocotillo Registered+

    Ach, what a bunch of wind....
  20. bronc76

    bronc76 Registered+

    I am currently using a 440cfm inline fan to pull air through my 1000w hps and out the roof of my garage. I have 6" from the hood of light to the fan then 4" from the fan out the roof. This system has been running for about 9mths now 24hrs 7 days a week with no problems. Noise level is very low and air really moves well.

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