Imo if your just harvesting for personal reasons i would highly suggest just a 600w epistar led. Can get it pretty cheap and can do like 3 plants under it efficiently. as for your venting question just put an inline fan in your room ether 4,6, or 8 inch and take out air from the room but you have to make sure your bring as much air in as your taking out. or you will get brown spots and the bottom of your plant even without deficiencies and you will hate yourself for it. good luck buddy.
Yeah, use a cheap ass led like this guy.why don't you use a LED light instead of MH, it has less heat than MH, and you don't need a fan to ventilate
You're going to need to exhaust the hot air out and hopefully have a passive air intake into the room at the very least. If you plan on using an HID light then you'll need to procure flexible ducting and attach that to the light hood - you'll be pulling the hot air out. Obviously you'll also need an inline duct fan to pull that air. You'd typically put a carbon filter at the other end of the duct (opposite of hood), to eliminate the odor.I need help from of the more experience guys on here imnew to indoor and have a 6x6x4closet and want to run a 400w mh my only concern is heat . How should I ventilate I'm seriously clueless on what to do. at the moment I just have a t5 setup and waiting for seeds to germinate