This is a 5 minute timer for fan control made by Nexus. This might be easier to wire in then a thermostat for some people.
http://www.ncix.com/products/index.php?sku...us%20Technology

The standard attic thermostat has two wires; one to the fan(s), one to "hot" power. The timer has several multi-wire plugs and assumes you're using a computer power supply. IMO, the former is far easier.

I was looking at those,but came to the conclusion that,as it drops the voltage to 5V for the 5min period,it wouldn't be that effective.
P