Abstract: A system for monitoring the service life of an HVAC air filter is disclosed and includes an airflow sensor that is positioned in an HVAC duct in relatively close proximity to the air filter. The airflow sensor output is sent to a processor that is pre-programmed with a filter evaluation algorithm. Each time the HVAC blower is activated begins a new duty cycle during which airflow signals are generated are sampled by the processor/algorithm. Selected sampled values are averaged to calculate a peak airflow velocity, Vpeak, for each duty cycle. The peak airflow velocity, Vpeak, is then compared to a base reference, Vreference, to determine whether the air filter requires service/replacement. The value of the base reference, Vreference, can be established during an initializing procedure and thereafter updated using the peak airflow velocity, Vpeak.