Speed Control is hard-wired to the system 1 blower whether in-line or the EXT weatherproof version. Looking like normal light dimmer the speed control was designed specifically for use with the system 1. In the full counterclockwise position the fan is turned off. As the knob is rotated clockwise the fan turns on at full speed, slowing as the knob is rotated further. At full clockwise rotation the fan turns at approximately 1/3 of full speed.