Due to the typical dissimilar metal (copper/aluminum) construction, evaporator coils also lose efficiency over time. As with condenser coils, corrosives in the air and acids created by the breakdown of debris accumulating on the coil attack the copper/aluminum bond. However, this doesn't happen as quickly as with the outdoor coil because of filtration and because the indoor air is usually less contaminated. However, cleaning techniques that employ foaming acid or alkaline products definitely reduce the coil's efficiency and life. This is one reason why it is important to change the evaporator coil whenever the condensing unit is replaced. An old evaporator coil matched to a new high-efficiency condensing unit (split system) will never reach the condensing unit's maximum possible efficiency. And built-up dirt accumulation always impedes airflow.