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Is it safe to use your rear defroster in temps below 0 degrees?

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Dear Tom and Ray:



I have a great, wonderful, sweet co-worker who may be as wacky as a loon. It was 0 degrees Fahrenheit last week, and she refused to use her rear defroster, claiming it would shatter her rear window. Who is wacko here? Her, for this wacko theory, or me, for not believing her? -- Matt

RAY: It depends how cute she is, Matt.

TOM: Actually, we know she's wrong about the rear window. Car manufacturers are well aware that their vehicles will occasionally be used in temperatures below 0 degrees. And I've never seen a warning not to use a rear defroster under those conditions. In fact, those are exactly the kinds of conditions that the rear defroster is made for.

RAY: There are several reasons why the rear window won't break. First of all, the defroster's wires, which are embedded in the window, do not produce an enormous amount of heat. Remember, ice melts at any temperature above 32 degrees F, so the window doesn't have to get very hot.

TOM: If you don't believe us, unplug your freezer for a couple of hours, until everything's melting, then grab a bag of frozen peas and see how hot they are to the touch.

RAY: Second, the glass doesn't change temperature immediately. It's pretty thick. The heat has to work its way through. So it's not like taking a steaming-hot glass and thrusting it into ice water. The change in glass temperature happens gradually.

TOM: And finally, the tempered glass used in rear windows is designed to be able to expand and contract without cracking. Remember, there are other situations where the temperature of the glass changes rapidly. Like when you're driving on a hot, sunny day, and there's a sudden downpour.

RAY: Right. If the glass cracked every time its temperature changed, we'd get a lot of letters about wet interiors in the summer.
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