I try and maintain an even temperature in my grow, so I plug my exhust fan into a thermostat. I would not recommend pluging your cooling system into a timer. I would rather have normal air then un-even temps or an over-heated grow. I found the best thermostats to be the ones made by Ranco. They are $45 unwired (ebay) and $70 wired to an extension cord. Very simple to wire your self.
