Do-It-Yourself Repairs & Basic Wiring Projects:
Using a Spring Wound Timer vs. Line Voltage Thermostat When Installing an Attic Fan

By Terry Peterman, the Internet Electrician

Q:   I am installing an attic fan. 

I have a hi-low speed switch and I would like to put a spring timer in.

Can you please tell me how and where I should put that? 

Therefore, your only choice is to put the switch in line with the power to the attic fan. 

You will have to set the fan to operate in the high or the low position when the timer is turned on. 

A better device to use in conjunction with an attic fan would be a line voltage thermostat. 

The purpose of an attic fan is to evacuate the hot air that is in your attic space, reducing the affect of this heat on the temperature inside the home, and the load on air-conditioning systems. 

A 'cooling' thermostat is what you need.  You set the temperature that you want the fan to activate at, and when that temperature is reached, or exceeded, the fan will start, and won't shut off until the temperature cools back down to the pre-set value. 

This gives you automatic control, and eliminates the need to keep re-setting the spring wound timer.

Terry Peterman, 'The Internet Electrician'