Some say that for each 10% reduction in airflow, the efficiency of the equipment drops about 5%. When the airflow is reduced, that means less cooling is taking place, and building occupants may become uncomfortable. Then they start to complain.
It is recommended that evaporator coils be cleaned once a year (sometimes more, depending on the application), especially if the customer does not change the filter regularly. However, most coils are in hard-to-reach places, making it difficult for service technicians to access them, let alone clean them. It can also take a lot of time to clean a coil, which means it can be a costly procedure. That’s definitely not attractive to customers.