Frost on an evaporator coil will prevent the correct amount of airflow across the coil. Anytime the evaporator coil experiences reduced airflow across its face, there will be a reduced heat load on the coil. Low airflow across the evaporator coil can cause much of the refrigerant in the coil to remain a liquid and not vaporize as fast. Some liquid refrigerant may travel past the evaporator coil, through the suction line, and eventually get to the compressor, which can result in compressor damage from flooding and/or slugging.
Figure 1 shows an air-source heat pump with a severely frosted outdoor (evaporator) coil. The frosted coil is preventing the majority of the air from flowing through the outdoor coil, which in turn, will prevent the refrigerant in the coil from experiencing the proper heat loading. A dirty evaporator coil can also cause a reduced airflow across the evaporator, which will then cause reduced heat loading on the refrigeration system.