Which is a more effective defroster: recirculated air or fresh air?

Dear Car Talk

Dear Car Talk | Apr 01, 2000

Dear Tom and Ray:

As soon as winter arrived, my sweet wife and I started our yearly battle over running the defroster. Do you recirculate the air within the car, or allow that fresh, freezing, outside air to come in? She claims the outside air prevents fogging and frosting up, but I disagree. Please end this dispute. Thanks. -- Bob

TOM: Oh, Bob, you lucky fellow. Your wife is not only sweet, but smart, too (you were afraid we were going to say that, weren't you?).

RAY: The reason you get fog on the inside of the windows is because moisture inside the car condenses on the glass. The water vapor -- from your breath, your wet shoes or your panting, 120-pound Bernese Mountain Dog -- hits the cold glass and turns back into water, which is hard to see through.

TOM: If you just leave your ventilation system on "recirculate," you are simply recirculating that same moist air. Whereas if you set the vent on "fresh air," you'll bring
in fresh, drier, outside air to replace the moist air that's condensing and fogging up your windows. And believe it or not, even when it's raining, the outside air is drier
because you're not adding your breath to it.

RAY: An even better approach is to add the air conditioner to the mix. The air conditioner "conditions" the air -- that is, it removes the moisture. That's one of the
reasons it makes you feel cooler and more comfortable in the summer. And if you use the air conditioner along with the defroster on the fresh-air setting, you'll do an
even better job of getting the moisture out of the air and, therefore, off the windows.

TOM: Many cars automatically turn on the air conditioner when you select "defrost." But if yours doesn't, go ahead and turn on the AC. You'll be amazed at how much
faster the windows clear.

RAY: And using the AC to clear the windows in the winter doesn't mean you have to freeze. You can turn the temperature selector all the way up to hot while the AC is
on. This will give you warm, dry air, which is exactly what you want for defrosting. But of course, your wife could've told you that!

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