The HVAC System provides proper in vehicle climate conditions.  It de-humidifies the vehicle interior (and helps to clear fogged windshields).  You must seal and contain refrigerant, lubricant, and coolant. There are some signs that indicates your HVAC system is malfunctioning: Improper vent air temperature, Low or no vent air flow, Smell or fogging of windshield during defroster operation, Growling/grinding or squealing noise during A/C or heater operation and overheating