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What Is a Defogger, And How Does It Work?

Educational  ·  April 14, 2023

What Is a Defogger, And How Does It Work?

Compared to other standard car features on today's models, the defogger does not get much attention. That's because it's primarily used when temperatures are low outside and car windows quickly get fogged up. But what is a defogger in a car, and how does it work?

The car defroster is a feature that clears fogged-up windscreens. The primary defogger consists of AC-connected vents that blow air to remove moisture and clear the fog on the front windscreen. The secondary defogger consists of wires that generate heat to melt ice and frost from the rear windscreen.

Car defoggers are very straightforward features, and this guide will help you understand why they matter. Keep reading to learn how defoggers work and why they’re necessary!

What Defoggers Are, Different Types, and How They Work

A defogger is exactly what it sounds like. It’s a vehicle feature that removes fog from your car’s front and back windscreens so you can see through them clearly.

The defogger is a standard feature in all vehicles, so you can bet that all cars on the road have them. They play a critical role in maximizing visibility, especially when the weather is cold, as they help drivers keep their windscreens clear.

Did you know that your car has two kinds of defoggers onboard? Yes, and they are the primary or front defogger and the secondary defogger for the rear windscreen.

Let’s take a closer look at their differences.

Primary or Front Defogger

The primary car window defogger consists of special vents directly under the front windscreen. Those vents connect to your vehicle’s air conditioning system and are meant to channel some of that air directly against the windscreen.

Air normally comes from the vents pointing directly at your face, so you’ll have to change the vent settings to redirect air out through the defogger instead. Most cars still use a selector knob that you can turn to make that happen.

When selected, the flowing air will defog the front windscreen and keep it clear, improving visibility.

Secondary or Rear Defogger

The other type is the rear window defogger, which functions differently. Instead of air vents, the rear defogger consists of a thin grid of wires running across the rear windscreen. Those wires are controlled at the vehicle's front, typically with an ON/OFF button on your dashboard.

When you switch on the rear defogger, an electrical current will flow through those wires and generate a small amount of heat. This is sufficient to melt away ice and frost on the rear windscreen within minutes.

Why Do Car Windows Fog Up?

What makes car defoggers necessary in the first place? Well, they’re critical for your car because car windows fog up due to a process called condensation. That’s when moisture in the air turns into water.

Unfortunately, plenty of condensation happens inside a car, especially when temperatures are low outside. That’s because the low temperatures make your front and rear windscreens cold. Then, moisture in the air turns to water when it comes into contact with the cold surface.

Condensation is more significant when it’s cold outside, like early in the morning or during the winter months.

That might not sound much at first glance, but the water droplets cause your windscreen to fog up, making it difficult for you to see clearly and putting you at greater risk on the road.

How Do You Defog Side Windows?

Unfortunately, side windows do not have defoggers. So, you’ll have to defog them manually if necessary. You can wipe the fog off the car window with a piece of cloth or roll the window down.

Defogger Vs Defroster Vs Demister - The Difference

You may be confused after reading about defoggers, defrosters, and demisters – how do they differ? The truth is that they're all the same thing. 

The only difference you need to remember is the one between the front and rear windscreens. Whether you call them defoggers, defrosters, or demisters, the front one uses special air vents, while the rear one uses heat-generating wire grids.

Does the Defogger Use the AC?

Yes, the primary or front defogger uses your car's air conditioning system to function, but that's not the case with the secondary or rear defogger. Instead, wire grids generate heat to melt frost and ice on the rear windscreen.

Some drivers insist on using the car defroster heater to clear the front windscreen, while others prefer using the cold air settings on the AC system.

Setting your air conditioner to either the cold or hot option will defog the front windscreen effectively. However, the cold air setting works faster, contrary to what many believe. That's because cold air can carry more moisture away from the windscreen, clearing the fog more quickly.

Having trouble with your car’s defogger? Bring it to an expert who can fix it for you! Check out the CarpartAu Directory to locate automotive businesses in your area. Grab their contact details through the Directory and contact them quickly to get your problem diagnosed and resolved!


By Ray Hasbollah

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