• Open the front panel and see if the two prong amp connector is is place, this connector is connected to a pressure switch down near the motor(s) if this switch is broken or debris is inhibiting its movement or action, then it can't close telling the circuit board that there is medium movement. (fluidization)  somewhere between the pressure switch and the connection there may be an open connection.  causing the heater not to turn on. Now if that's not the case then there may be a calibration issue, but usually the system won't drift that far out of calibration. If it is a calibration issue click here for the calibration procedure.