im looking to grow in a 2 metre squared tent with 2 600 hps' blue halide for veg and hps for flower. Im sure this can get quite hot, so as long as i have good vents i shud be ok?
With 2 600's in there, you'll definitely want some cooling. With an ambient temperature of maybe 15-20C, my tent will easily go towards 30c if I zip the door up and just let the fan recirculate air. It also depends on how big your room is and if you'll be leaving the door open to the rest of that house.
Mine is in quite a small room with radiator off and the door closed 24/7.
For that much wattage in that sized tent, a 250cfm seems good. The intake duct from the window sucks lots of cool fresh air in at the bottom and a desktop fan blows it up into the leaves to make sure it doesn't go straight out the cooltube. You should watch your leaves with the air moving. If they're wiggling, even slightly, there's enough airflow over them to avoid pockets of stale air.
I tried sitting in my tent under the light with the door zipped up. I was virtually naked at the time for reasons unknown and was trying to feel how the drafts were flowing.
Using the duct idea, even if I closed the vents over, because they're on the inside they'd still blow open a bit and I could feel air coming through. If I pushed the shut with my hands, lots more started coming through the duct. The duct is about 4m long and smaller than the vents, so it's easier for the fan to suck air through the vents. Putting some sheets of paper over them on the outside sucks them tight against it and blocks all entrances but the intake duct.
Good thinking on the two 600w's by the way! I'm guilty of running a single 1kw, but I'd definitely go the 600 x 2 or 400 x 3 route next time - even with a cooltube ripping cool air through it, a single lamp that big creates a big high temperature space around it that the plants don't love. Plus 5 - 6 foot plants = a close call
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