Defrosters remove condensation from windows and melt frost, ice and snow. They work differently depending on whether they are for the windshield or the rear window.
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In most air-conditioned vehicles, the air-conditioning automatically turns on when the defrost mode is selected to dehumidify the air. Drier air helps remove condensation from the inside of the windshield and the front side windows more quickly. Vehicle manufacturers often recommend switching the air recirculation mode off to speed up defogging, as the air inside the vehicle often contains more moisture than the outside air.
Front defroster
The front defroster is controlled by the heating and ventilation system. Windshield defrosters blow air from the heating and ventilation system through openings on the front edge of the dashboard and onto the windshield.
Most vehicles have a setting that directs all air to the windshield for maximum defrosting or division of the air between the windshield and the ground. The driver can also choose the temperature setting and fan speed for the defroster.
In winter, the defrost mode with the hot temperature setting does two things: The hot, dry air fogges up the windshield and melts frost and snow, which makes it easier to scrape the windshield and prevents ice from forming while driving.
Rear defroster
The rear window heater is electrically operated and operated via a switch on the dashboard. When switched on, a grid of wires, visible as thin lines on the inside of the glass, is activated and heats the rear window to fog up the window and melt frost, snow and ice.
Most rear defrosters have a timer that automatically turns the defroster off after about 10 minutes. On vehicles that are equipped with heated exterior mirrors, the mirror heating is also activated when the rear window heating is switched on.
Rear defrosters start working as soon as they are turned on, but in cold weather the front defrosters take more time to be effective as the engine needs to warm up first. Like the rest of the heating system, the front defroster requires warm engine coolant to circulate through the radiator before warm air can be blown onto the windshield.
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