I currently use four 4' long florescent per shelf. One of these days I'm going to add another 2 bulb fixture to each shelf. They are T12's. Ratter said the other day that T8s are the same price as T12s these days, so if I had to buy my own bulbs I would be going the T8 direction.
xvart.
Instead of T-8 or 12s, get T-5s, they are a little more but worth it since they put out a lot more light
Heat is definitely an issue in my case. My growrack has two levels, each with 4 t12's, and it gets up into the low 80's when they are on.
So - any idea if I were to use 6 t8's per level vs. the 4 t12's what the heat output might be like? I'd like to keep the light level high, but lose some of the heat output. Would t8's allow this?
What kind of ballasts are in the fixtures? The bulbs produce some of the heat but the majority is from the ballast. If you have fixtures with magnetic ballasts they will produce alot more heat than electronic ballasts. The heat produced by a t12 vs a t8 is roughly the same. The t8 might feel a little warmer but this is due to the smaller surface area to dissipate the heat. So increasing the number of bulbs from 4 t12's to 6 t8's will produce more heat but if your t12 fixtures have the magnetic ballasts and you got new electronic ballasts for your t8 bulbs it might be possible to bring the overall temperature down.
The other thing you could do is remote wire the ballasts so they are outside your grow chamber. Would take a bit of wiring but you could eliminate the heat within the grow rack from the ballasts. You could also continue to use magnetic ballasts which are cheaper than electronic ballasts. Just don't electrocute yourself!