What is a defogger in a car and how does it work?


Defogger in a car:

It is a device that removes the condensed water droplets or ice from the main windscreen, side windows and rear windscreen of a vehicle. Its main function is to clear the windscreen of condensed water thereby improving visibility. This device is also known as ‘defroster’ or ‘demister’. Defoggers are typically used during the rainy season.

Why use a defogger?

While driving during the rainy season, many times the drivers face the problem of water condensing on the windscreen and side windows. In some of the coldest regions, even the ice accumulates on the glass surfaces of the vehicle. Naturally, in these situations, it becomes very difficult for the driver to keep an eye on the road and other vehicles due to poor visibility. Hence, manufacturers employ defoggers in cars. This device removes the condensed water and ice from the glass surfaces and restores visibility. As a result, it significantly reduces the chances of an accident and improves road safety.

Classification of de-foggers:

Based on the principle of working, they are classified into two main types:

  1. Primary
  2. Secondary

Working of defogger:


primary defogger construction
Construction of primary defogger

These type of defoggers works with the help of heating and air-conditioning system (HVAC) of a vehicle. At first, fresh air is drawn into the air conditioning system where it is dehumidified. The dehumidified air then passes to the heater core. The heater core draws heat from the hot cooling water circulating around the engine and passes it to air. Thus, dry and hot air comes out of the heating core which is then passed through different vents and carried to the desired location such as windscreen.

Generally, vehicle manufacturers provide primary defogger on a vehicle that has an AC unit; as it acts as a supplementary system. Primary-defoggers mainly function for front windscreens.


Secondary defogger
Construction of Secondary defogger

Furthermore, these defoggers work on the principle of restive electric heating. The manufacturers paste the filaments of a heating material on the surface of a glass. After switching on the system, electric current passes through the filaments and generates heat. Thus, it melts the condensed material and clears the glass.

Manufacturers employ secondary type mainly to clear condensed water on rear windscreen. Some luxury vehicles also use this system for clearing external rear view mirrors.


  1. It clears the condensed water or ice from the parts such as front and rear screen, windows etc. of the vehicle.
  2. Improves visibility.
  3. Improves road safety.

Watch how a car defogger works here:

For more information, please click here.

Read More: How a car’s windscreen wiper works?>>

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