We’ve all been there. You slide behind the wheel, crank the engine and then stop. You realize that you really can’t go anywhere because your windshield is fogged up. Thankfully, you can just turn on the defroster and let your car do the work of clearing away that unwanted moisture for you.

How a defroster works

Your car’s defroster is tied into the air conditioning system. While that means it can get quite warm and very cold, it also means something else. If you’ve ever had to use a humidifier in your home during the winter because your furnace was removing too much moisture from the air, you’re already familiar with what’s going on here.

Your air conditioner (whether it’s set to cold or hot) condenses the moisture out of the air into water. This condensation is vented through a drain hose that runs from behind your glove box out the bottom of the car. The system then pumps dry air into the car. When you turn on the defroster, it pumps dry air up toward the windshield. This helps to evaporate the moisture.

The right temperature

Sometimes, different temperatures are needed. For example, you may notice that cold air works better during the summer and warm air works better during the winter. This is simply due to the outside ambient temperature. Your defroster (in addition to drying the humidity from the air) is also equalizing the temperature of the glass and the interior air to some extent.

Unfortunately, this also means that if your air conditioner isn’t working properly, your front defroster won’t work right either. It may either only slightly clear the moisture from the glass, or it may not work very well at all. Generally, this is caused by low refrigerant levels in the air conditioner.

This article originally appeared on YourMechanic.com as How Does a Defroster Work? and was authored by Joyce Morse.


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