Should Intake = Exhaust?

[Guest Eikel]

Another question from my newbie self.

 

I'm rigging up 4x PC fans for the ventilation on my grow-area.

 

I have 2 x 32 CFM (Cubic-feet/min) and 2 x 42 CFM fans.

 

The grow room dimensions are 2.5m x 2.5m x 2m - I've done the math and my fans should be able to cope with the room of this size (I think).

 

My question is, do I setup the room with more intake than exhaust or vicea versa?

 

I know ventiation is quite important and I'm under the impression exhaust should be greater than intake, especially when the using a carbon filter - am I correct?

 

--Eikel.

[Luke Skywalker]

You can manage with a lesser power on the active intake if you have another passive intake to prevent any strain being placed on the lesser fan.

 

Generally, I would personally suggest using no intake fan, just an oscillation fan for air movement, and extraction fans. Provided there is adequate passive inlets, the exhaust fan will be able to pull the air out adequately by itself.

 

I'd say run both fans on one side of the room, up as high as possible, pulling out.

[Guest Eikel]

Thanks Luke!

 

-- Eikel.

[Luke Skywalker]

Just realised your fan size for your room. You have to be SURE it's going to do the job. There is a lot of impedance on exhaust fans if they're axial and have ducting around them, whereas centrifugal fans are much more pressure powerful, and aren't as badly affected. So you could be getting a lot less exhausting from these small fans than you imagine.

 

Do this calculation to see if it's adequate. Multiply the dimensions of the room together and work out the volume, then see if your total fan load in m3/min will move all the air out in under 5 minutes, preferably under one.

 

I feel you'll need a minimum 250mm continuous rated axial, or a 150mm inline or centrifugal fan to vent that space. You'll also need a fair amount of lighting too, like a 600 at least, probably two.

[F420]

Also be aware that theres alot of different ways to do things.

Myself if I use an inlet fan as it does two things the way its setup.

 

It blows the plant gently so that I dont need an osc fan.

it also evens out the volume of air the exhaust is pulling.

 

eg- if you use a 250mm hydro turbo with no inlet fan it will suck too hard, and as generally most boxes are sealed it cant suck anything so it then overworks and they dont last as long.

vs- if you had a 250mm inlet fan and a 250mm exhaust fan the air is balanced, ie- the same amount of air is being forced in to the box compared to the same amount being pulled out again.

 

I use a smaller inlet fan so that the main 250 doesnt overwork, but it still works to capacity.

 

The only downside is that in some cases it will drop the temps of the growbox, in summer this helps to keep them cooler but in winter it can get too cold.

 

you will know what I mean if you listen to the fan change note from the doors open to the doors shut.

 

[Luke Skywalker]

That's why your passive inlet must be larger than the size of the fan. You can also spread the intakes out amongst several smaller inlets, or a long thin one along one wall with an overlap to prevent light escape and entry.

 

If your fan makes a straining sound when you close the door, you don't have enough intake space to allow fresh air in. What goes out, must come from somewhere.

 

The smaller intake fan is likely being overworked itself ferengi.

 

but yeah, lots of people use them, but for a small room, (under 3 m2 floor space) I'd say you shouldn't have to worry about it IMO, just ensure you have adequate passive inletting.

[Guest Eikel]

Thanks for your responses guys.

 

I've worked out that I have 148 Cubic-Feet/min of airflow with my fans.

 

My space is 2.5 x 2.5 x 2, total of 12.5m3

 

Multiply by 3.2808 to convert into feet = 41.01

 

So therefore my fans exceed my space by ALOT.

 

Is this too much airflow? (or is there no such thing?).

 

-- Eikel.

[Luke Skywalker]

Okay, that sounds fine then, I'd say you're laughing. Remember that those are the un-altered ratings for those fans, I usually drop what they say is the unimpeded amount by 20% or so to get a realistic figure on the actual airflow. And you must remember that with even a small amount of ducting, an axial fan will blow almost nothing in comparison to a centrifugal.

 

There is only such a thing as too much airflow if your ambient temps where you draw the air from is too low. This isn't a very long period, if at all in Australia, and if your temps are low and you have heavy winter rains, (like perth, well, ignore the drought we've had the last few years and yeah.... ) you should keep ventilation up anyway, and compensate temps in other ways, ( lowering lights, removing any air-cooling on the shades, even adding a heater or reverse cycle air con... ) during the coldest months. HID lamps and ballasts put out a lot of heat, so this is rarely an issue.

 

So in short, (geez, can you tell I just got home from work and have had 2 honey oil cones? ) yes, whilst it is theoretically possible to have "too much ventilation", in reality, this doesn't happen often, or even for long periods if it does, in Australia, except maybe in the coldest parts of the country.

 

A good idea for your particular circumstance may be to use one of the smaller fans as an active intake,(along with substantial passive inlets, which need to be lightproofed....) attach a length of duct and then run this underneath the plants, with holes in the latter part which is under the canopy. This provides a steady, even flow of air through and out of the canopy, allowing excellent airflow to the plants and making it almost impossible to develop budrot for sheer lack of still air in there. Just a thought, I found these to be a most excellent method of ensuring good airflow underneath the plants.

 

Good luck elkel, will be looking forward to seeing pics of this gr... if we're lucky that is. Oh yeah, and you'll want a good sized oscillating fan, (like one of those pedestal fans you can get at most hardware shops, or a box fan, or even a tower fan....) to move air within the space. This also assists the plants to develop strong stems. It should be on a low speed though, maybe moving up to "2" when the plants are in flowering. Too much movement of the plants themselves, like a very strong breeze outdoors, where the plants shake and move constantly, can be detrimental. It makes the plants put all their effort into growing cellulose and hemp fibres to strengthen the stems, rather than growing foliage/buds for you. So yeah, have a gently blowing oscillating fan, perhaps on a pedestal, or even mounted on the wall/roof, to move air around the room, and even out the temps.

 

Whew, I type sooooooo much when I'm stoned.... hee hee Hope that helps, if you can decipher it that is....

[quizywig]

after running a bit of a test on my box, changing the 2 intake fans to agitation instead of inlet, Ive found they work better as inlet fans, i havent got a temp gage but found it noticably hotter without the intake fans

[veg]

EIkel and all,

 

I don't get this the same, Im just working out my new room and yours doesn't add up. Is it me or not? can someone check me on this.

 

you say

"My space is 2.5 x 2.5 x 2, total of 12.5m3

Multiply by 3.2808 to convert into feet = 41.01" - thats Length conversion tables not Volume conversion tables.

 

I got this from Vol conversion tables:

cu feet > cu metres multiply by 0.02832

cu metres > cu feet multiply by 35.315

 

so i get 12.5m3 x 35.315 = 441 ft3

another way to look at it is to convert first to feet, so i get:

8.2ft x 8.2 x 6.56 = 441 ft3

therefore your exhaust (148 ft3/min) will clear the room within 3 min which is still good.

 

Veg

Edited February 18, 2005 by veg