I see... A few problems.
First of all you gotta make a choice between one of the following: - Forget both the a/c and co2 in order to use active ventilation
- seal the room 100% and use solely air conditioning and co2
- aircool your lamps on a sealed ducting system and attempt to seal the room, using co2 as well as an air conditioner
It appears as though your attempting to mix the first two options and that just doesn't work. You wind up sucking all your cold air out (and your co2) with your exhaust fans that you spent so much electricity to keep cool in the first place with the air conditioner.
Let me get this straight though; your using an air conditioner and the outside temps are 78?
Are you just using the ac because your trying to use co2??
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So the first option increasing the size of your exhaust fans, to about 2300 CFM total exhaust, and have a passive intake twice the size of your exhaust holes. Forget using the AC & co2 entirely. This should keep the room temp at about ~2 degrees above ambient intake source temperatures.
Second option you need to get rid of your exhaust fans entirely and buy another 12000 BTU AC, (or preferably, as
lou mentioned, a 2 ton split ac system... Using that to cool your growroom... This allows you to have no glass for aircooling your lamps, creating a 10% increase in yield helping offset the increase in air conditioner costs + 15% increase from co2.
Third option is more difficult to finagle and you have to really work on sealing up your ducting properly, but if you've already got aircooled hoods this might be an option for you; basically get that aircooling working, and have the ducting suck outside air and exhaust outside air, so the ventilation on the lights is completely separate from growroom air... Then you inject co2 and use the air conditioner you've already got... What can I say; i've tried this one and gave up. Dont do it haha. Its not worth all that trouble for only a 5% yield increase...
If you can just use active ventilation its worth forgetting about co2. Because you don't have to use glass in your hoods for aircooling them, so even though you get a 15% increase from the co2, you get a 10% decrease in yield from using glass in your hoods; basically not worth all the hassle of refilling tanks etc. for only a 5% yield benefit over active ventilation.