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Whatever
There is no need to exchange air in a closed system like you're thinking especially with the setup you have. For one even in an open system you do not need to exchange air lights off cause your plants are sleeping IF you can control humidity and temps and easy to do.Lucy Diamond said:The point is...getting fresh air to the plants.
It's also a way to reduce night time humidity.Moves that humid air out,and fresh air in while the plants sleep.
My thinking(and this is just a hunch,nothing more)is that changing the room air every 24 hours while the plants sleep,will reduce pathogens,bacteria etc. that could build up in a "stale" air environment.
My suggestion to you would be to add a dehumidifier for the lights off cycle and you can always add a carbon filter for inside the room with a fan and just recirculate the air to help control possible problems. Like others have said pests are almost always brought in through the air or carried in on your personage. Obviously another source, and significant one, would be bringing in contaminated cuts.