how much CFM will be needed?

I would use something along the lines of like 150-200cfm to be sure...I beleive the fan im using hooked to my scrubber is 150 and I have a closet with a 600w light. You do want something that will change the air out in less than 5 min.

CFM is just the fan capacity thats needed to exchange the air in an entire room once every 5 minutes.

So, CFM needed = Cubic Feet of any room divided by 5.

You have a 4' x 2' x 8' room which would = 64 cubic feet

Hence, 64/5 = 12.8 CFM

I have a 4 x 2.5 x 7 room which = 70 cubic feet, or a 14CFM.
I don't even think they make CFMs that small (your 12.8 or my 14) so I went with a 130CFM odor scrubber/filter fan.

Just an IMO.

Hope this helps you:

I use a Homebox S (6' x 2.5' x 2.5') with a Sun Systems 250w HPS (enclosed ballast = more heat). I have a 90 CFM hydrofarm running constantly. This moves A LOT of air in this space, even 10" away from the light the canopy never gets over 74F. I could have gotten a 400 watt easily, or a smaller, quieter fan.

4x2x8=64 cubic feet
using HID lights and heat being an issue you want to exchange air 3-5 per minute.
192-320 cfm fan required.
if using cfl's that 80 cfm will work

read growfaq

How much ventilation

How much ventilation do I need in my grow room?



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