My ambient room temp is always between 79-81 and I currently keep my four 600watt hps in air cooled hoods they are ran together in twos each pair has its own vortex six inch inline fan taking heat out of the room. I Keep them at 26 inches away from the canopy and that puts the canopy temp around 78-80. I still get great yields but I always hear so much controversy on the distance the lights should be at. I wanna know what every1 here has to say. How hot would you let your canopy get without co2 gas? I have done trial and error and I can say that I don't have any heat stress at 26 inches like I do at 20 inches or 22 inches.