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I could also sync it to the light so that I don’t vent when the lamp is off. I can sync this to an identical timer that will turn on CO2 during the time that the fan is not on, and vise versa. It would be difficult to sync them closer that 5-10 mins, but at least there would be a possible inexpensive solution. $20 for two of these timers. Fans are expensive to buy for venting, but I just go down to the local electronic parts liquidators and they have muffin fans for $5-10, so that’s a real savings over the $50-70 these fans cost new at the indoor garden stores. A good vent fan will keep the humidity and temperature down, and distribute CO2 to your plants from new incoming air. Internal air movement is very necessary as well. An oscillating fan should be used to circulate air within the growroom, to help circulate CO2. It will also keep the humidity down, allowing the air to absorb more moisture, and reduce risk of fungus. A wall mount oscillating fan will not take valuable floor space. The best grow rooms have the most internal air circulation. 

TEMPERATURE
Proper temperature is one highly variable factor. Most books state optimum grow temperature to be 70-80 degrees, but many list extenuating circumstances that allow temperatures to go higher. Assuming genetics is not a factor, plants seem to be able to absorb more light at higher temps, perhaps up to 90 degrees. High light and CO2 levels could make this go as 
high as 95 degrees for increased growth speed.* An optimum of 95 degrees is new data that assumes very-high light, CO2 enrichment of 1500 ppm and good regular venting to keep humidity down. It is not clear if these temperature will reduce potency in flowers. It may be a good idea to reduce temperatures once flowering has started, to preserve potency, even if it 
does reduce growth speed. But higher temperatures will make plants grow vegetatively much faster, by exciting the plants metabolism, assuming the required levels of CO2 and light are available, and humidity is not allowed to get too high. 
With normal levels of CO2, in a well vented space, 90 degrees would seem to be the absolute max, while 85 may be closer to optimum, even with a great deal of light available. Do not let the room temperature get over 35 C (95 F) as this hurts growth. Optimal temperature is 27-30 C (80-86 F) if you have strong light with no CO2 enrichment. Less than 21 C (70 F) is too cold for good growth. 

Low temperatures at night are OK down to about 60 degrees outdoors, then start to effect the growth in a big way. Mid 50’s will cause mild shock and 40’s will kill your plants with repeated exposure. Keep your plants warm, especially the roots. Elevate pots if you think the ground is sucking the heat out of the roots. This is an issue if you have a slab or other type of cold floor. As temperature goes up, so does the ability of the air to hold water, thus reducing humidity, so a higher average temperature should reduce risk of fungus. Contrary to many reports, high humidity is not good for plants except during germination and rooting. Lower humidity levels help the plant transpire CO2 and reduce risk of molds during flowering. 

Studies indicate the potency of buds goes down as the temperature goes up, so it is important to see that the plants do not get too hot during flowering cycles. 

• D. Gold: CO2, Temperature and Humidity, 1991 Edited by E. Rosenthal.