The GAF Master Flow 12 Hour Timer, when installed on GAF Master Flow Belt Drive and Direct Drive Whole House Fans, can be set for any time from 10 minutes to up to 12 hours. When installing with Direct Drive Fan Model, you must have the BWS2 2 Speed Fan Control Switch as well. The timer automatically turns off when time has expired. The timer knob and gray finish wall plate are included. Detailed installation instructions and diagram included. Size: 3.05 In.