Abstract: An electronic system for monitoring the condition of the filter in an HVAC unit includes a controller which issues at least one command to a circuit for the motor of the fan producing airflow through the filter. The command to the driver circuit is associated with a desired run speed for the fan. The controller is operative to compute the speed of the fan following issuance of the command to the motor drive circuit. This computed speed is compared to a predetermined fan speed that would normally occur when the filter is dirty. The electronic system is operative to generate a warning in the event that the computed fan speed does not compare favorably with the predetermined fan speed.