How Do Car Defrosters Work?

When it's cold out and you can't see through your car's windshield, you'll probably reach for the defroster button. But how does the defroster work—and why does it always seem to take forever to clear ice, frost, mist, or fog from the windshield?

Learn all about how car defrosters, defoggers, and demisters work.

Types of Car Defrosters

There are two primary kinds of defrosters. The first type uses a vehicle's heating, ventilation, and air conditioning (HVAC) systems to blow warm, dehumidified air directly onto the foggy or iced-over windshield. The other type of defrosting system defogs and de-ices through a mechanism known as resistive heating.

How Do Primary Car Defrosters Work?

Defrosters that use a vehicle's HVAC system are sometimes referred to as "primary" defrosters, since they are designed to clear up the front and side windows, and they work via two main principles.

In order to melt ice that has accumulated on the windshield, the HVAC system activates the primary defroster to draw in fresh air, passing it through the vehicle's heater core. It then directs the warm air through dashboard vents toward the front windshield and side windows.

In addition to defrosting windows, these primary systems can also defog windows by removing condensation from the inner surface. To accomplish this, a front window defroster will typically pass air through the air conditioning system to remove moisture. When this dehumidified air reaches a fogged windshield, it absorbs moisture and removes the condensation.

Warm air can also hold more moisture than cold air, which is what makes primary defrosters effective when these two systems work in concert. Although it is possible to accomplish the same moisture removal process by physically wiping the condensation away, doing so may leave smudges that can result in glare; it may also make it difficult to see through the windshield at times.

How Do Secondary Car Defrosters Work?

Defrosters that don't use a car's HVAC system are sometimes referred to as secondary systems since they are designed to defrost things like rear windshields and mirrors. These systems typically use wire grids and resistive heating to physically warm up the surface of the glass, which can effectively melt ice and remove condensation.

Rear windshield defrosters typically use surface-mounted grids that are easily identifiable, while heated mirrors usually have internal wires that you can't see. However, both systems use the same basic mechanism of resistive heating. An electrical current is applied to the wire grid when you activate the system, and the resistance of the grid causes heat generation.

How Do You Defog a Windshield With No Primary Defroster?

If your car has air conditioning, but it doesn't have a button you can push to automatically defrost and defog the front windshield, you can accomplish the same task manually:

1. Start your car and turn on the heater.

2. Set the heater to the highest setting.

   Changing the vent selector to the dash vents that point at the windshield helps defrost the windshield, but warming up the air inside the car is the most important factor in defogging.

3. Change the HVAC circulation setting to draw air in from outside.

4. Turn on your air conditioning.

5. Open the windows a little bit.

Aftermarket Car Defrosters

Since OEM systems make use of both primary and secondary defrosters, aftermarket replacements and alternatives are also available for both types. In particular, grid-style rear defrosters can be repaired by conductive paint and adhesive materials, or scraped off and replaced altogether by aftermarket defroster grids.

Although no direct replacement exists for primary defrosters, 12V car defrosters operate via the same basic mechanism of action as OEM HVAC defrosters. These devices cannot heat up the same volume of air as a traditional HVAC system, but they still work by directing warm air at a fogged or iced over windshield, and they are a viable alternative to a broken defroster in some cases.