Basically i have a large 20ft long by 4ft wide n 5 ft high cellar. I just ordered my 400W HPS /MH Growlight setup and im wondering with such a large space, even if i enclose the "box" area i want to surround my plants in mylar reflective stuff, will it be too hot?
You pretty much always need ventilation when dealing with HIDs. I would say a little 4" inline would take care of it just fine assuming its cool not warm down there to start.