Im starting a grow box (4x4x4) up in my attic, and this is the one problem I can't figure out. My attic is prolly around 40 degrees around this time of year, and I need about 74 degress in my box. I'm ging to use a 400W MH/HPS light, and i was thinking of putting a 80cfm fan in for ventilation. my dilemma is, one, should i get two fans, one for intake and another for exhaust, and if i do, would the cold air coming in offset the heat given off by my light? If I do not get an intake fan, will there be enough fresh air? I was thinking of a small space heater with a thermostat on it for heat, but if i get an intake fan its going to be on constantly. Any suggestions????

I can tell you I use a closet (grow area is about 4x4x4) in my basement about 60 degrees f this time of year with one fan and it does not get warm in there. The airflow in is through the slatted door on the closet. I don't think two fans will hurt. Why not use the heater as the intake when the light is off? and keep the exhaust fan going? Just a thought good luck.

4 x 4 x 4 = 64 cubic feet. ideally your exhaust fan would be able to exhaust your grow area three times a minute so your ideal exhaust fan would have a CFM rating around 190 (64 x 3 = 192).

as far as heat/cold - my box is similar (2 x 4 x 7) - i'm using a 1000w light and 440 CFM exhaust fan and a 170 CFM fan to cool the lights. it is in an environment that stays around 60 degrees and during the winter i have to turn off the fan to cool the lights for the box to stay warm enough. In the summer however - it's all i can do to keep it cool.

with the right exhaust fan - no intake fan is necessary (passive intake holes still are though!)

thanks sounds good

if you cant keep the temps tropical you will be in trouble fast! cold air will slow growth substancially

__________________
Go Medicate Yourself..... Then we will Talk