How a Car Automatic HVAC System Works

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Here's how the automatic heating, ventilation and air condition system work in a car.

The HVAC system in a vehicle is responsible for keeping occupants comfortable by controlling the temperature, humidity and fan speed. The system can be controlled based on external climate conditions. It provides heat in the winter, cool air in the summer and the removal of moisture to defrost the windows.

In this video, a dual-zone automatic climate control system is taken apart and detailed from an Infiniti G35. The air starts from the air inlet, where an actuator controls flaps to cycle recirculating air or fresh air. It is then sent through the cabin air filter and then pushed through the blower motor, into the air mixer.

The air mixer is responsible for taking air and making it warmer or cooler, depending on what the climate control computer directs its actuators to do. It is first passed through the evaporator core, which cools the air and removes any moisture before getting mixed between the heater core or bypass. Air is then directed out through the appropriate vents through flaps that move on a cam-slider system, controlled by the mode motors.

The control system comprises of many sensors, including the cabin temperature sensor, external temperature sensor, sunload sensor and evaporator core sensor, in addition to inputs from the HVAC driver controller interface. The brains of the controller is hosted inside the display unit, which forms a network with the motor actuators that control the flaps in the air mixer assembly. These motors are controlled through a CAN BUS network to reduce wiring and aid with diagnostics and each house a DC electric motor, a potentiometer and a gear reduction drivetrain.

The A/C loop comprises of the compressor, condenser, drier and evaporator core and expansion valve. The expansion valve controls the rate at which the refrigerant flows through the evaporator core inside the mixer box. Coolant lines running from the engine head also circulate through the heater core inside the HVAC box, and act like a mini radiator.

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