The fan is hung or mounted closer to the ceiling than the floor, forcing warm stale air out through the dryer hose. Cool Co2 rich air is drawn in through cracks or vents closer to the floor. The fan may be cycled by a timer or connected to a thermostat or environment control device. This allows the grower to give the plants an optimum aerial environment at all times. Blower fans come in a wide variety of sizes, and are rated according to the amount of air the move per minute. A 245 C.F.M. will displace 245 cubic feet of air in one minute.
To choose the right fan for your grow room, first determine the size and average temperature range of the area. If a cooling effect is required, which is usually the case; the fan should be large enough to change all the air in the room in about three minutes or less. A 400 C.F.M. fan should be adequate far a 1200 cubic foot area, unless the room gets unusually warm.
Depending on your requirements, insulated ducting for your fans is recommended if maintaining a temperature in your growing area is required.
Good air exchange and circulation is a vital part of controlled environment gardening. Plants increase humidity and use up available carbon dioxide rapidly. Controlling temperature, humidity, and Co2 availability can be easily achieved with a squirrel cage blower fan and a length of flexible dryer hose. 
