A ventilation system is fitted to vehicles to make the environment in the cabin comfortable for its occupants. It can duct air to the windscreen for demisting, or temperature-controlled air to the foot well and to face and side vents, for occupant comfort.
Air can be sourced from outside the vehicle, usually from apertures located below the windscreen, or recirculated from within the cabin.
Air is drawn over the fan and directed over the air conditioning evaporator, if fitted, then through the heater core. From there it is directed to the desired outlets by ducting and control doors.
Fresh air normally flows through the cabin when the vehicle is moving, however, a fan is used to assist the air movement.
The ventilation system is controlled by a set of switches located on the dash. They control:
When the air control is set to fresh, outside air is drawn from the front of the car and ducted through the cabin, where it is exhausted through a flap, located in the rear of the passenger compartment, to the outside.
The air pickup sometimes directs air through a filter to prevent dust, dirt and pollen from entering the cabin. The air is directed to the electric fan, which can be activated if more airflow is needed. The movement of the car through the air is usually enough to provide adequate air flow.
Setting the air control to recirculate closes the entry to the outside and opens a duct that directs cabin air to the fan. As there is no assistance from the vehicle moving through the air, the fan needs to be operating to circulate the air.
The Mode Switch is used to control which outlet or combination of outlets air can flow from. The air is guided by a series of ducts to doors that are opened and closed to direct the air to the desired outlet.
Depending on the vehicle, there may be modes to direct the flow towards the face or the feet, to the front or the rear of the compartment, and others, but there is always a position for screen demisting, which directs air onto the inside of the windscreen to clear any fogging that may occur.
The Fan Speed Switch controls an electric motor that has a barrel type fan attached. When operating, it forces air at the chosen speed through the ducting into the passenger compartment, via the heating and cooling components.
In some vehicles it is up to the occupants to manually select and adjust the combination of fan speed and heating or cooling to provide their preferred comfort level. In other vehicles, the occupants simply choose their preferred temperature, and the control system will automatically heat or cool the air, and boost the fan speed temporarily, until the vehicle has reached the target comfort level, and then maintain it in that condition.
Many vehicles also separately duct air through to the rear passengers. Normally these passengers can only determine the direction of the airflow, but on some vehicles additional temperature and mode switches are fitted for their convenience.
All vehicles can direct air over a heater core to warm the air entering the cabin. Air conditioning to cool the air, although becoming very common as a standard fitting in many countries, is still an optional extra on many makes and models of vehicle.
When fitted, the air conditioning evaporator will chill the incoming air to cool the occupants if required. This is highly desirable in warmer climates, as the interior of a vehicle in direct sunlight can heat up very quickly, and safe driving is best performed when the driver is comfortable, and not stressed by excessive heat or cold.
Some vehicles automatically activate the air conditioning when the screen demister is selected, as the air exiting the evaporator is very dry, speeding the demisting process.