if you use these bags or any co2 with an exhaust fan on, your wasting your money and co2 as its just gonna get sucked out the exhaust fan. and if you only run an exhaust w/o an intake fan or some other opening in your grow structure, your going to have negative pressure issues. when guys are running co2, they do it in a sealed grow room to utilize their co2, otherwise its a waste of money and your plants wont get the desired amount of co2 you think they are getting. iirc, the co2 guys are getting 1200-1500ppm, with a bag, you might get 300-500ppm.
bertaluci, I had some heat issues when I flipped to hps. I run a 600w a/c tube hood w/440cfm fan in a 4x4x7 dr120 tent. I was running 86-90* lights on and 74 lights off. I bought an 18" oscillating fan, made a new mounting support and now have it blowing across the tops of my flowering plants and under the hood. this helped drop the temps by 5*. if you put one of these fans like I have mine, and completely seal your grow room (no exhaust or intake fans going), then co2 would be beneficial.