I have a small 25" X 16" X 37" box that I will be completeing my grow in. I am using 1 70w HPS and 4 32w CFLs. Can I use computer 2 fans for an active exhaust passive intake ventilation system? Or would the back pressure be too strong for the fans to move enough air? Also what would the rating onf the fans have to be to do the job. Typical ambient air temp is 68 during the day and 48 at night. The lights will be on mostly at night. Any thoughts would be great. Thanks

It depends somewhat on the size of the fans. If they are normal 80mm 12v case fans, they should do quite nicely in tandem. Put them up high in your box, and run them with separate AC->12v DC converters. cut some vent holes around the surface of the box floor, or on the lower sides if you can't get venting through the floor. Your 48 degree night temp is rather low, so I'd run the day cycle at night to try and even things out a bit.

I am planning on using the 120mm fans as they move the same or more air and have less noise. The night temp is based on the winter season right now so that will go up a little. But yes I will have the lights on all night. Any idea as far as cfm rating for 2 120mm pc fans? I am guessing that two 50cfm or so fans will do the job. What do you guys think? Thanks for all input .

Does this help at all?

Contributed by: Smokey D Dope

How do I add ventilation to my grow area?
There are a couple of considerations to observe when planning your ventilation, they’re pretty simple concepts; but they are often overlooked.

· First, remember that warmer air will naturally rise to the top of any container, and that cooler air will naturally settle towards the bottom.
· Also remember that when ventilating any space, the volume (VOLUME, in cubic feet or cubic meters... [L x W x H]) of air that goes IN, also has to come OUT.
· You can’t expect to ventilate a grow space by simply forcing air in, and not providing an exhaust vent.

Since the object is to remove as much warm air as possible, and replace it with cooler air, it will be most efficient to place the exhaust as close to the top of the space as possible, and place the intake as close to the bottom as possible.


Should I place the fan in the exhaust, intake or both?
The fan should be placed in the exhaust, and the intake should be a simple hole (or light trap, if light getting out is a concern).
This type of system is known as an Active Exhaust, Passive Intake System.
Mounting the fan in the exhaust, sucking air out of the room accomplishes a couple of things...

· Since the exhaust is at the top of the area, the fan will suck the hottest air out of the area first.
· The fan is actually lowering the air pressure inside the area. Any incidental pinholes or leaky seams will simply draw air in. If the fan were blowing IN, those pinholes and leaks would allow potentially smelly air OUT.


How big should the passive intake be?
It should be slightly larger than the exhaust. Remember, the volume of air being blown out, will be replaced through the intake. Using a bigger intake hole allows the incoming air to be at a lower velocity (speed), which minimizes mixing up of the air in the area. It will also allow the fan to operate more efficiently.


How big should the fan be?
Fans are rated in either cubic feet (CFM) or cubic meters per minute in North America. In Eurpose, metric fans are rated in m3/hr - cubic metres per hour (m3/hr).

That means a 70CFM fan will move 70 cubic feet of air in one minute.
Your fan should be big enough to move the volume of your area 2 to 3 times every minute. A 70 CFM fan would be adequate for a 35 cubic foot area, and would be optimal for roughly a 23 cubic foot area.

· To figure your area’s cubic volume, multiply (in feet) the length by the width by the height.


What if I have more than one fan? Should I use one to blow air in and one to suck air out?
Not if the object is to provide as much ventilation and cooling as possible.

· If you have two 3-inch diameter fans, and you mount one in the intake, and one in the exhaust, you have a total intake area of one 3-inch hole and a total exhaust area of one 3-inch hole.
· If you use both fans as exhausts, you have TWO 3-inch exhausts and two 3-inch intakes (actually, two 3.3 inch intakes. They should be bigger than the exhausts, remember?).
· Twice as many holes, twice as much airflow.


Enhanced Blower Mods

Timer Options
If you find that the "Lights off" temps are lower than you'd prefer, you can simply run the fan from the same timer as the light by using a multi outlet power strip connected to the timer. Plug the lights and the fan into the power strip, and the fan will turn on/off with the lights. If you're using more than one fan, you could connect some or all of them, remembering that the more fans you have running, the lower the temps will be.
* SAFETY NOTE: The timer must be able to handle the additional electrical load, or an additional timer must be used. SAFTEY FIRST.

Sound Suppression
Making the ventilation system quieter can be an important consideration, and it's important to remember that the air moving through the intakes and exhausts make noise, as well as the fan itself. Some of the fan noise from vibration can be overcome by mounting the fan in a non-rigid manner. The fans can me mounted using rubber grommets to help dampen the vibration. Self-adhesive foam rubber window insulation can also be used. In some installations, it can be mounted by threading a bungee cord through each mounting hole, then attaching the other ends of the bungee cord to the exhaust hole.

· Generally, air moving through ductwork or tubing can become noisy, particularly if the air has to move at a higher velocity. More, larger diameter intakes and vent tubes will generally be quieter than fewer, smaller diameter intakes and vents. The fans also don't have to work quite as hard.
· Finally, although popular and easy to use, flexible "Accordion" type hose, commonly used to connect clothes dryers to external vents are not always the best choice, as they cause a great deal of drag, (making the fans work harder) and generally air flowing through them is noisier than smoother ductwork.

Filtering\Odor Control
Connecting a Carbon scrubber is a good method of controlling the odor that can be a dead giveaway to an otherwise stealthy installation. A carbon scrubber is simply an expansion chamber (box) into which the smelly air from a flower chamber is pumped. The chamber has a large exhaust vent, which is covered by an activated carbon air filter. The chamber must be big enough to provide a damping effect of the incoming air. If too small a scrubber is used, the fan will not be capable of pushing air through the filter. Here, you actually want the exhaust vent to be considerably bigger than the intake.

· There are also several DIY Odor killers available, which work to varying degrees to provide an "Odor Cover-up."
· Remember, the term "Low Odor Strain" is relative.
· Even the low odor strains generally still stink pretty badly near the end of flowering.

Now go grow something!

Each fan has a rating that tells you how many cubic feet of air per minute (CFM) it will move. When dealing with above average temperatures you will want your fan to exchange the grow room air 3-5 times in one minute, so for a room that is 40 cubic feet, a fan that is capable of moving 120-200 cfm (cubic feet per minute) is recommended. If you only want to replace depleted levels of co2 and are growing in a closet using fluorescents, one room change per five minutes (divide room size by 5) will be adequate.

To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute.

Air that’s exhausted will be replaced by fresh air, which is drawn from Passive Intakes-
located opposite the exhaust at the floor of your grow room (Recommended for closet grows) or forced in using another blower (recommended for larger grow rooms).

For internal circulation oscillating fans are In my opinion the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic moulds down by constantly freshening any potentially stagnant air. Home improvement centres carry a large array of various types and styles of air-moving fans, there are wall-mounted styles available and most are relatively inexpensive.

NOTE: To convert CFM to CMH use the folowing formula CFM X 0.03 X 60 = CMH

Contributed by: /usr/share/bong
Thanks to: JakeStone
Submitted: 04-29-2003

Quiet, low power and surprizingly efficient, computer fans are an ideal cooling option for smaller cabinet grows. The only problem is that most computer fans are 12v and not 120v! How do I run a 12v fan on normal household power?

Computer power supply
(A 145 watt computer power supply is over-kill for a 5-20w fan. Try for something small, like a 75watt or 100watt supply). You could drill holes and mount the power supply inside your cabinet. turn the power supply on and off with the power switch on the back.

(Carpe-Nox) … if you have any old cell phone chargers around, they use 12v, same that a pc fan uses, just splice the pc fan wires

[Editor’s note: 120v computer fans can be found. Add a pigtail to your fan and voila.]

Fan ideas
Cyberguys.com Official Site - Computer Parts, Electronics, Workplace Accessories, and more

ex.) 120mm, 90CFM fan for $11.95. it only uses 5.4 watts - a perfect exhaust fan.

ex.) 80mm computer fan that is thermally controlled. $5.95 - the hotter your cabinet is, the faster it spins.

(Jakestone) PCs frequently run their 12V fans at voltages below 12. 7 Volts is very common. Much below that, there can be an issue of having enough starting voltage to get the fan going. The volume of air being moved is reduced, but so is the noise.

(Jakestone) El cheapo $3 fans are often significantly louder than better fans (say 5+ db) and have a dirtier sound (less whoosh and more whine/ticking). High-end fans generally go for between $10 and $15. Computer power supplies are notoriously noisy (especially cheap ones).

(Jakestone) In general, a larger diameter fan moving a given amount of air will be quieter than a smaller diameter fan moving the same amount of air. So for example, one might buy a 120mm fan that moves more air than you need to move at 12V and run it at 7V instead of running a 80mm fan at 12V. The downside for cabinet growers might be the need to light-baffle a larger hole.

Thanks Mogie. I will be going with the 2 110CFM pc fans. These have an adjustable nobb that will allow me to easily adjust for air movement and sound. I want the extra cfm just in case I add another 70w hps. If anybody can suggest a better way to go please let me know.

This is more expensive but it will also move more air. Love mine.

Suncourt Inductor In Line Duct Fan - 4 in.: Allergy Relief Superstore

You'll be fine with those two fans. I used to have a similar arrangement for what is now my clone/germ box. 70w sodium, 4x23W CFL, 2 80mm computer fans.

You may find you run out of space real quick in there. I originally planned to go through harvest with mine, and the plants got way too big for it within weeks. Works great as a germ box, and I rarely needed the sodium; in fact I ended up removing it altogether.

I can get a lot of basil going real quick in there in a mini ebb/flow setup I made from two stacked rubbermaid shoeboxes with mini pumps and airstones. The temps are nice and steady, with suprisingly decent airflow. I do love pesto.

Thanks Heady. I will be finishing only one plant in this box. I will also be topping and training. What were your temps like?