A control circuit for a motor driving a fan for inducing a draft in the exhaust of a heating, ventilating and air conditioning (HVAC) system. The control circuit may be used in conjunction with either a variable pressure sensor or one or two discrete pressure sensors. A controller receives information defining the pressure set point. The information may be from a switched register or low or high pressure input signals. The motor speed/torque is increased until the pressure set point is reached.