If the intake temps are lower than 70, I would think you can get away with one fan extracting from both closets, but you might struggle to keep in the optimum temperature range of 76-78. Obviously it's best to have one fan per growroom, so each room's temperature can be managed independently, but if that's not an option right now, wait til you harvest what is in the flower chamber, then turn the hps on with no plants, leave it running for a good few hours during the hottest part of the day and check the temps.

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Originally Posted by ceestyle
Don't be a martyr wasting $100 or more on CFLs to prove they're inferior for flowering pot.