This is my first grow, using a old kitchen cabinet (24dx30wx64h) with a ac filter and fan installed in the top. I started them with a 70w incandecent grow bulb and am wondering if I can start using the 600w HPS now they are popping up (keep it 30" away from the seedlings?) or is that too strong for seedlings?
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Thanks

I would advise you not to use the 600w HPS as the generate a lot a heat and light too, you may just stress them... I recommend changing you're incandescent light for a fluorescent one as they generate less heat and use less energy.

What is the temperature of the cabinet right now ? You should always be prepare to get quite a few degree up with the HPS running, this is the reason why you need to have a good exhaust.

Do you have any idea how powerful you're fan is, you should be able to exhaust the entire cabinet in less than 5 minutes to be sure to supply enough Co2 to the plants and get the heat from the HPS out ASAP !

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The fan on the AC filter is a 150CFM, but I'll have to turn it down abit for noise but it should vent evety min or so. The cabinet without the fan running is 74 F and that went up to 83 with the fan and HPS running.If the light is too strong I'll have to pick up a few cfls,but how big do they have to be before using the HPS?

For my veg from seedlings I've used this bulb, the text is hard to read, but it's a 40W, cool white. I had three of those for my vegging... You look like to have only 4 space for plants, I guess this lighting should be good for you too.
"Now, for vegetative growth you should choose the 'cool white' light. This is also acceptable for flowering, but a 'warm white' light will be better as it is stronger in the red end of the light spectrum which is more suitable for flowering." from this FAQs: Text

As for the fan you should be all right, I've convert the inch you gave into feet to have this result 2*2.5*5.3 = 26.5 (round up) this is the cubic feet you need to vent so 26.5 CF on a 150 CFM fan at full blast you exhaust the place in 2.6 minutes and even a half the speed you should exhaust in 3 min. But sometimes filters reduce the air flow... I think you'll be okay !

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Thanks for the info, I guess I have more room now as 2 of the slow seedlings died. One poked up 1/4" and fell over and one got to the same height and kept the seed husk on and is just standing there with no change in days. I'll leave it in place until it shrivels up just to make sure