Low evaporator airflow is one of the common faults for rooftop units. It can be caused by dirty filters@ evaporator fouling@ or loose belts. Low airflow could result in frozen evaporator coils@ reduced cooling capacity@ and indoor comfort issues. Accordingly@ more fan power is consumed as longer operating time is required. With the widespread use of variable-frequency drives (VFDs) on rooftop units (RTUs)@ low evaporator airflow can be potentially detected by monitoring the fan-power variation. In this paper the principle of fan-power-based detection is introduced first. Then@ the detection algorithm is proposed@ including development of the baseline and comparison of operational data with the baseline. Finally@ the conducted field test is discussed@ verifying the proposed methods. The test results indicate that the fan-power-based method can effectively detect low evaporator airflow for rooftop units.