GREENHECK GB-300-VGD-20 EXHAUST FAN W/ 460/3 ODP MOTOR W/ BUILT IN VFD, BACKDRAFT DAMPER, DISCONNECT SWITCH & FLAT ROOF CURB, 8,500CFM. FAN HAS AN ON-BOARD POTENTIOMETER DIAL FOR MOTOR ADJUSTMENT. IF A 0-10VDC OR 4-20MA SIGNAL IS NEEDED, IT MUST BE SPECIFIED AT TIME OF ORDER