fisrt time 400 hps

hey i was just wondering the rule of thumb is keep the light 18" obove plants. is that just for heat beacuse im using a cool tube so is it okay to have it like a foot?? or can it be even closer ((without burning of course)) or is there a totally different reason on why 18 inches??? thanks +rep

yes, it's because of heat. and with a 400W you don't have to do 18", they aren't as hot as a 1000W. With a cooltube you could be 6 inches or less. With my standard non-aircooled 400W I only keep it 10-12" away.

I'd keep it about a foot away. With my 250W hps I stay about 6 inches away.

6in to a foot depending on plant respons