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 …

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 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 an…

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.

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 …

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…

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…

Defroster/Cracked windshield

Maintenance/Repairs

shadowfax September 17, 2010, 5:47pm 7

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.

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Defroster/Cracked windshield

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