I am in a dilema and need advise. My 400 hps heats my area up to around 90F. I have a 4" inline duct fan bringing in fresh cold outside air at 80 cfm according to the manual, and blending that with a fan into towards the plants. Without the outside air, it goes above 100 in the area. The light is about 24 inches above the plants.

So, I now purchased a larger 4" inline fan, with around 150 cfm rating. My question is should I use this in place of the other inline, or should I exhaust the room using this fan, and leave the other fan as it is....in other words, if I provide exhaust ventilation will that help to efficiently reduce temps? Hope this makes sense. Thanks in advance for the help.

well a 4" will only flow 75 cfm max if you want more flow you'll have to step it up some, 6" = 150 cfm, 8" = 300 cfm

oh these #'s are the point that they start to "whistle"

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The paradox of Life:

An empty bowl needs to be filled;
A full bowl needs to be emptied.

use both fans for exhaust and use the intake hole as a passive intake. the exhaust will pull fresh air in at roughly the same rate it is being exhausted (assuming little to no air leaks). this combined with the fan in the grow space should be drastic in change. what are the dimensions of your grow space?

I agree with sublime

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Spreading Happiness One Bud at a Time