HVAC System Components, Operation, and Air Distribution
One system, three jobs
The A/C side moves heat from the cabin to outside air, the heater side adds waste engine heat, and the air-distribution side decides where conditioned air goes and how much fresh air is mixed in. Know each component by function and location: the engine-driven compressor raises refrigerant pressure and temperature; the condenser at the front of the vehicle rejects heat and subcools liquid; the expansion device (orifice tube or TXV) drops pressure; the evaporator inside the HVAC case absorbs cabin heat and dehumidifies; the receiver-drier or accumulator stores refrigerant and holds desiccant.
Inside the case the blower pulls air through the evaporator and heater core. A blend door proportions air across the heater core to set temperature, mode doors send flow to panel, floor, and defrost, and the recirculation door selects outside or cabin air. A cabin filter cleans incoming air, and the evaporator case must drain condensate outside through a hose.
Traps
Confusing blend door (temperature) with mode door (direction); assuming the heater core can be shut off (on most systems coolant circulates continuously and temperature is set by air blending); forgetting that defrost engages the compressor to dehumidify; and missing a plugged evaporator drain, which soaks the passenger floor and fogs the glass.
Rule of thumb: with the blower on high and doors open, center-vent temperature should run roughly 35-45 F below ambient on a properly charged system. Cold air with weak airflow points to the blower, filter, or evaporator fins, not the charge.