Sometimes a drop in cooling efficiency, where you may feel warm air flowing from the vents, will signal the need for more refrigerant. Another symptom of insufficient refrigerant is the freezing of evaporator coils, which can reduce cooling capacity. A leak within the system may also be indicated by the sound of hissing near the refrigerant lines. Additionally, prolonged AC use without providing appropriate cooling may result in increased energy costs due to decreased efficiency. A qualified HVAC specialist must be called in if there is any reason to believe there may be a refrigerant issue.