with 20 plants in the room, it will get really musky within 30 min. i thought the same thing, that u wouldnt have to vent all that much...
this is what ppl do with co2... while the lights are on, they will inject co2 up to 1500ppm for at most 20 min while the exhaust's off, any more co2 is a waste. then the stop injecting and turn the exhaust on. they let exhaust run for 1 or more hours, then repeat the cycle. no co2 during night. the co2 isnt suppost to be injected all day, and the room must be exhausted while lights are on and off. to much co2 will burn plants. not enough ventilation is unhealthy for plants, they would be better off with no co2 and full exhaust in my opinion, that is till u figure out a co2/exhaust schedual or get an environmental controller