Originally Posted by rhizome

We have a 4x4x6 room. Our total duct length is 13'. We're using a filter. Our math now looks like-

4x4x6 room= 96 CF. Divided by 4 minutes is 24 CFM required.
24 CFM + 130% ( 10%x13')= 55.2 CFM ( I'm gonna round to whole CFM, to try to minimize decimal over-runs)

(55CFMx1.5)x1.5= 124 CFM – to allow for our two 90 degree bends.

124 CFMx1.3 ( to allow for air velocity lost to the filter) = 161 CFM.

So we're looking at a 161 CFM fan.

But wait- we haven't even thought about how our light's going to effect this. We could go off into a discussion of determining system effeciency by measuring intake and exhaust temperatures so that we could calc differential temperatures, but I don't know how to make the little “ delta” symbol on my laptop keyboard, so I'm gonna skip that and assign yet another load factor...

For a 250HPS- multiply by .75
For a 400- multiply by 1
For a 600, multiply by 1.3
For a K, multiply by 1.6.