I've heard that hps isn't so good in veg. If I put the hps on a 12/12 timer, along with two flourescent tubes on 24/7, would that be okay for veg, or would the 12/12 hps stress the plant out somehow? (The flourescents are on the whole time, so the plant should stay in veg I think)

Also, what gives off a lot of heat, the ballast or the lamp? I'm asking this question because it gets hot sometimes in summer, and I'm a little worried about heat problems. If I have a fan on my babies 24/7, would that be okay for a 250 watt hps?

Finally, has anyone ever looked into putting a big block of dry ice in their growroom? I think it might cool off the room, and add valuable C02. I'm not sure how expensive dry ice is though.