Dirt on the evaporator coil causes two problems: it reduces system airflow, and it directly degrades the coil's heat-transfer efficiency (which significantly cuts cooling capacity). With good filtration, a rooftop unit's evaporator coil will stay fairly clean.

Some technicians believe that specifying annual evaporator coil cleaning is an unnecessary expense, while others maintain that enough dirt gets around or through the filters to justify the expense. Those in favor of annual evaporator coil cleaning contend that as long as the equipment required to clean the condenser coil is on the roof anyway—as it should be for the unit's annual cleaning—it's worthwhile to clean the evaporator coil as well.

The evaporator coil should at least be inspected once a year to make sure that the filters have been doing their job. Shining a light through the coil is one way to inspect it (although wavy "enhanced" fin designs can make this type of check difficult). Another approach is to measure supply-fan amperage and filter/coil pressure drop (with fresh filters). If the amps are lower and the pressure drop is higher than last year's measurement (also with fresh filters), then the coil is dirty and needs to be cleaned.