How Does a Car Defroster Work?

by Derek Odom


When the temperature inside an object is warmer than the temperature outside, this can cause the water molecules to go through a process called condensation. Moisture is everywhere, even in a car that is just sitting. And because of interior items such as carpets, seats and other cloth or vinyl upholstery items, the outside metal surface of the car will get much colder than the interior. This causes moisture to form on the windows, especially on a cold or rainy day. Defrosters combat this phenomenon so that drivers can see out the windows and operate the vehicle safely.

Forced Air or Blower Type

On the majority of production vehicles that exist today, the front defroster will consist of vents. It utilizes the same blower motor and fan that the heating and air conditioning systems use. The vents are placed directly below the windshield, facing up toward the roof of the car. When the vehicle's coolant reaches temperature and the thermostat opens, it allows the hot water to cycle through the heater core. The blower fan then directs this hot air up through the defroster vents (selected on the heater/AC panel) and warms up the temperature of the windows, removing any condensation that may have built up. The obvious drawback is that the vehicle must be warmed up first in order for the system to really work well.

Electric Type

The rear window on most automobiles has tiny lines going through it. These are actually electrical wires, which heat up when current flows through them. In this manner, the resulting heat is used to displace the condensation on the inside of the window, but can also work on the outside. The heating elements have very low heat compared to say, a burner, so humans are in no danger of getting burnt if they are touched while powered on. They put out just enough heat to combat the moisture built up on the window. They are usually wired along the roofline of the car, underneath the headliner. New model automobiles are starting to come out with heating elements in the front windshield, as well, which will negate the need to let the vehicle warm up before turning on the defroster.

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