The ability of an HVAC unit to heat or cool a room is largely dependent upon the coils inside the unit itself. Without these coils, temperature and airflow could not be regulated. In units that use water coils, hot or chilled water moves to heat or cool an environment, instead of a mix of air and refrigerant liquid movement.
 
It is well-known that exposure to water causes metals to rust, break or produce mold and fungi. Since water coils operate by using water to change temperatures, these processes can happen fast leading to lower performance, making maintenance and replacement all the more important.