Fan on with CO2?

Alright so I work at a gas company and get CO2 for nearly free (not that I want to blow through a ton). But was wondering when I should run my CO2 and how this will work with the fan running. I am growing in my closet and space is somewhat small. I guess my main question is what will CO2 do if I have the fan pulling air out the whole time? Also Ive read that the plant only uses CO2 during the light period but I have read others that say the plant utilizes the CO2 during the dark period. Can anyone give me some more info on this? THANKS GUYS!

If you are constantly venting then it is a waste. You'll never get the levels high and steady enough to have any positive benefit. Its still a pain in the ass even if its free. I only ran co2 when I had a sealed room and it was only during lights on.

co2 at lights on only. unless you have a monitor it's all guesswork. the light hood should be pulling air from outside the room and exhausting outside the room. this keeps the lighting cooling from sucking c02 right out of the room.
You need to explain your situation/system somewhat better before anyone can really help. if you're exhausting room air most of the time then co2 is pointless.