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Defroster/Cracked windshield

I noticed, by accident, that the defrost setting keeps the car comfortably cool in the summer. I mentioned this to my teenage son, who then asked, "well, what does the defroster run off of?" I told him I really didn't know, but maybe it was the compressor for the A/C. So anyway, I tried it again on a rather warm day, and later noticed my windshield had a foot-long crack. I had the temperature set at the coolest setting, but did not have the A/C button pushed in. Was the crack in the windshield just a coincidence? Or did I do it by running a cool defrost on the windshield?
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Comments

  • edited September 2010
    The defroster uses the air conditioner compressor to condense moisture out of the air. On a cold winter day, if you blow warm moist air onto cold glass, the moisture will just condense out, fogging the glass. The defroster blows air first over the A/C coils, to condense out the moisture, then over the heat exchanger, to warm it back up. I won't say that the A/C compressor always runs when the defroster is on (it is probably not needed on really cold days for complex reasons) but the compressor certainly runs on warm days.

    So what you were doing was running the A/C, and blowing the cold air directly on the window glass instead of through the face or floor vents. This certainly could crack the glass, because a strong temperature gradient (like, the glass is 100 degrees on the outside due to the air temp and the hot sun, and you are blowing 40 degree air on the inside) causes a lot of stress on the glass.

    So, yes, you could have cracked the glass by doing this. But there is no way to prove it with certainty (other than examining the glass with an electron microscope).
  • edited September 2010
    The AC system is used by the defrost system to remove moisture from the air.

    You did not cause the fracture by using the defroster. You may have caused a chip to spread and become a crack, or the glass may mave been under unusual stress becaus of a prior accidennt, but a windshield in good shape and not under unusual stresses won't crack like that.
  • edited September 2010
    It's very possible. Glass is a liquid that appears solid. Vertically, it will over time thin on the top of a pane and thicken at the base of a window.

    Your A/C weakens the liquid properties of glass. You should run the A/C directly into the cabin and not onto the windshield.
  • edited September 2010
    Well, sort of.
    http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

    I still think it more likely that a chip or an anomolous stress is the root cause rather than the defroster. There's no real way for us to know, and it doesn't really matter, but I've never seen glass fracture solely as the result of a defroster.
  • edited September 2010
    I have personally broken glass by having it cold on one side (40F) and hot on the other (100F), but this was a small piece of plate glass, and not automotive laminated safety glass. The only way to be 100% sure is to keep running your defrost on really hot days....
  • edited September 2010
    I agree with TSM. The defroster can do this IF there is already a flaw in the glass. If it's good glass that's in good repair, it's extremely unlikely that you'll do this, especially in the cool air/warm summer scenario. Far more likely to happen on a -30 degree day when you're blasting max heat on it.
  • edited September 2010
    I don't see how...

    If AC could cause a window to crack we'd be seeing thousands...if not millions of cracked windows every year. People have been running the AC directly on a HOT window for years...I know I have....
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