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I have a 4x4x7 grow tent and use a 400MH for Veg and 400HPS for flower;
I am 4 weeks into flower and just put the 400MH back in with the 400HPS; 800w for the last 4-5weeks.
Just 400HPS my temp is 81c(day) 69c(night)
both 400HPS and 400MH my temp is 89c(day) 69c(night)
Where do we draw the line with heat vs wattage. do i keep the extra 400W and take the increase to 89c??
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I think you have reached the limit as far as heat is concerned with 800 watts. In my opinion if you add the extra 400 watts, you will need to get more ventilation and extract more heat out of the tent. On the other hand 800 watts is overdoing it in the 16 sq ft you have. If you leave it like that and have a temp of 89-90F on a daily basis I think you would be wasting the extra power for little gain.
keylime
Just my humble opinion, but your girls in the pic look a little bit stressed. I'd probably forego the second 400 simply because you don't have enough plants under the light to justify the increase. I'd use one 400 for the time being and get your environment dialed in before you make a habit out of dealing with high heat.
since you are using an open reflector I would drop the second 400W and just stick with 1. I would also drop the light about 8" or so. If it was a cooltube or such u might be able to do something a lil different but like it was already pointed out that 3 plants be fine under 1 light.