I've been doing a little reading on grow room ventilation and ran across this:
"Air Exchange
As mentioned earlier, air exchange is a key ingredient to a successful grow room. The ideal air exchange rate will vary at different times of the year and from room to room depending on things like grow room height, etc. As rough guide about 30 air changes per hour is often recommended.
In addition to simply exchanging the air in the room to replenish the depleted co2, you need to keep it cool, this requires more air movement than replenishing the co2. I use this math equation:
A 90 CFM fan with a 600 watt light will raise your temperature above the intake air temp by 21°. The 240 CFM fan will raise it 8°. 90 CFM wouldn’t be enough. It’s not the size of the cabinet that you want to take into consideration; it’s the wattage being used. Watts produce heat.
(600 × 3.2) ÷ 90 = 21.33
(600 × 3.2) ÷ 240 = 8
To find how much the light will warm the room with a different size fan you can just use this formula. Just take the wattage of the light and multiply by 3.2. Then divide the CFM of the fan to find how much the room will warm in Fahrenheit degrees.
For instance:
If the temperature of the intake air is 75° and you’re using a 600 watt light, the temperature would go up 5° if you used a 384 CFM fan. So, the temperature would end up 80°.
(600 × 3.2) ÷ 384 = 5
5° + 75° = 80° "
With my 250w light and running my fan at 100 CFM, I'm averaging 4 degrees above ambient. That formula puts me at 8 above ambient, which isn't all that far off, so this might be a useful tool.
gentlemen 
