A monitoring system for monitoring operation of a refrigerant-cycle system is disclosed. A first temperature sensor measures a first temperature of air at a first location downstream of an evaporator. A second temperature sensor measures a second temperature of air a second location upstream of the evaporator. A processing system determines a temperature drop across the evaporator based on the first temperature, the second temperature, and a humidity of air. The processing system that determines whether airflow through the evaporator is restricted based on the temperature drop across the evaporator.