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The car defroster serves two purposes: to melt and to defog.

In wet winter weather, frost can accumulate on the outside of the windshield and obstruct vision. The defrost uses a blower motor and a fan to blow hot air through a vent onto the windshield, heating the glass and melting the ice.

Fog can form on the inside of the windows when it is warmer inside of the car than the air outside. When moisture in the air of the heated vehicle comes into contact with colder surfaces, like the windows, the vapor condenses into tiny droplets that form the fog. All you need is warm, humid air and a cool surface. This is the reason that within minutes of picking up your sweaty kid from basketball practice in the winter, you can no longer see out your windshield.  Your defroster combats condensation by reducing the temperature disparity between the car air and windshield by blowing warm air onto the glass and heating it up a bit.

The defroster sure is handy. But here’s something you may not know: most modern defrost systems automatically activate the air conditioning compressor when turned on. The air compressor requires some extra energy to run and can decrease your gas mileage.

The defroster can function just fine on its own by blowing outside air heated by the engine through the vent. The air conditioning system is rigged up to the defroster so that you end up running it in the winter, which keeps the refrigerant circulating and is ultimately good for the system. Granted, the hot A/C can have dehumidifying effects and defog your windows more efficiently. But, generally, it is an unnecessary use of fuel for the air conditioning to automatically kick in when the defrost is switched on.

So use your defrost judiciously, just as you would the air conditioning. Don’t be afraid to break out the ice scraper instead of letting your car idle on defrost. These little things add up to help you get better fuel efficiency and reduce emissions.

Of course, this doesn’t mean you should refrain from using the defroster when you really need it!  You shouldn’t sacrifice your safety or comfort for a relatively small amount of fuel. And don’t forget to check your car manual to see if there is a way for you to manually decide if you want to run the air conditioner when you turn on the defroster!

Any suggestions out there?

Earthgarage – Greener Car. Fatter Wallet.