A little update...
So far, with exhaust plugged into my GE, 96 tab, timer...with my oil- filled, electric, heater positioned in front of passive intake and plugged into a LUX thermostat outlet set on 73F...with the temps in the barn running from 37 to 55...I have pretty easily managed to keep things between 68 and 78F. Managing mainly by varying the exhausting/venting running times. Heater is seldom on with the 600 watter providing all necessary heat. Exhaust/venting is on about 40% of the time keeping things from getting too warm...so the CO2 should be fine. These temps are nothing compared to whats coming though. Actually, "winter is coming".