Venting is a significant aspect with an installation that includes HID lamps, thou venting is less required with the fluorescents. Due to the higher chance of humidity build-up it is necessary to vent a space several times each day. In a room equipped with a hot lamp that is likely to see a quick build-up of heat, a vent is certain to have the room cleared within a period of five minutes, which then waits a certain period prior to repeating the vent cycle. A venting system with built in timer is certain to be the ideal option but they can be quite expensive to locate the ideal climate controlling unit. 

An alternative set up includes the thermostats able to operate a fan once a predetermined temperature is achieved and switches off once the temperature lowers 4 or 5°. But it can be quite difficult to control the release of CO2 with this type of system, since it can be difficult to establish when the fan might operate. All that is necessary is a fan unit able to produce clean air within a couple of minutes and comes equipped with a inverse switch for controlling the CO2 and temperature control that goes on and off in line with the fan. If able to vent the grow area within a short time which doesn't heat up excessively, the CO2 is likely to be able to run in a continuous and slow pace and likely build-up in between the venting cycles. 

It might also be possible to operate two separate timers which are synced and this is likely to give the ability to operate a fan system for 25 minutes on and 25 minutes off. A timer can also be synced to the light system to avoid venting when the lights are off. A third option is to sync to a further timer to make certain the CO2 is turned on during those times that the fan isn't in operation. It is likely that you will be able to sync the timers so that they are close to five to 10 minutes apart should be ideal if looking to start with a low-cost solution. 

A venting fan is likely to be a quite an extensive investment, but if you are able to research the best deals in the market you might be able to get those at the more attractive rates. A high quality venting fan is perfect for controlling the temperature and humidity, and distributing CO2 to the fresh plants from pumped air. 

Having the ideal internal air control is also highly beneficial. Use an oscillating fan to help with circulating air inside the room which should help with circulating CO2. A fan also helps with lowering the temperature range, reducing the potential of fungus development, and allows air to soak up the moisture content. An oscillating fan mounted on a wall is likely to require minimal space. An ideal grow room is certain to have the right level of air circulation.