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You have to vent a lot with a HID lamp, less so for fluorescents. Also, humidity build up requires that you vent at least a few times per day. For a room with a hot lamp that builds up heat quickly, the best vent would be one that cleared the room in 5 minutes, then would stop for 25 minutes before venting again, or similarly, vent 3 minutes, shut off 12 minutes, etc. The trick is to find a timer that will do this sort of thing. Not easy to find and not cheap. Once you need to regulate CO2 on and off inversely with the fan, your looking at a $100 climate controller.
Alternatives are a thermostat that turns on a fan when a certain temperature is reached, and turns it off when the temp recedes 4 degrees. But it is a bitch to coordinate CO2 release with this one, since you don not know when the fan goes on. $39 for this thermostat, but to sync it to CO2 with a voltage sensing relay is $100 for the ready-made switch, so then the environment controller at $100 is cheaper. All you really want is a fan that clears the air in a few minutes, a temperature switch that turns on and off the fan, and an inverse switch that turns off and on the CO2. If you can vent the room really quick and the heat does not build up too quickly, the CO2 could be run in a slow, continuous fashion, and would build up in-between the occasional quick exhaust cycles.
Two timers synced can be used, but the only ones cheaply available are the 30 min interval, 48 trips per 24 hours. So I could have a fan run 30 mins on, then 30 mins off. I could also sync it to the light so that I don not vent when the lamp is off. I can sync this to an identical timer that will turn on CO2 during the time that the fan is not on, and vise versa. It would be difficult to sync them closer that 5-10 mins, but at least there would be a possible inexpensive solution. $20 for two of these timers.
Fans are expensive to buy for venting, but I just go down to the local electronic parts liquidators and they have muffin fans for $5-10, so that is a real savings over the $50-70 these fans cost new at the indoor garden stores. A good vent fan will keep the humidity and temperature down, and distribute CO2 to your plants from new incoming air.
Internal air movement is very necessary as well. An oscillating fan should be used to circulate air within the growroom, to help circulate CO2. It will also keep the humidity down, allowing the air to absorb more moisture, and reduce risk of fungus. A wall mount oscillating fan will not take valuable floor space. The best grow rooms have the most internal air circulation.