How can I control the heat above my canopy in a small closet? Also, when are seedlings ready to be placed under a 400w light? I tried too soon and killed some seedlings.
Not sure about your second question, but you can control heat with a ventilation system (intake and exhaust fan), portable AC unit, or depending on how hot it is a regular oscillating fan blowing over the tops might be enough.
Don't have a thermometer so I'll just see how the three clones I put in my closet can handle the temps. I have a fan as well.
Get a thermometer. 15 bucks for a wireless one at Walmart. Controlling temps in a closet tends to be hard. Why closet, shed or attic grows can suck. Closets are often cut off from any moving air. Controlling heat is easy. Hot air out cool air in. You have to somehow get air outside the closet to exchange it with the air inside it. Thats all passive air cooling is...air exchange..cars, computers, mj grows. Very simple. If you do not do that you will never control the heat. 400w HPS in a non vented closet is asking for fail. If you can't vent it I'd ditch the HPS get LEDs, T5s or CFLs. All 3 generate a lot less heat.
As the others said you have to have a fresh intake and the exhaust venting out or you will never control the heat. As far as whwn you can put the seedlings under the 400, as soon as you get your heat under control. Light intensity doesnt kill plants, heat does. The 400 isnt anything compared to the sun seedlings face outdoors. I put mine under the 400 three days after they sprouted and had no problems at all.