Can Windshield Glass Withstand Hot Water?

Dear Car Talk

Dear Car Talk | Aug 18, 2016

Dear Car Talk:

For over 50 years, I have been using hot water to remove snow and ice from my windshield. It not only removes the ice quickly, but slightly warms up the glass, so it fogs less when I start driving. It seems that everyone who hears that I do this says it will crack the glass. But it has never cracked my glass, even in 5-degree weather when I lived in South Dakota. Not even when there were chips in the glass. So, are the naysayers wrong, or have I just been lucky in the hundreds of times I have done this?

-- Mike

I think they're wrong, Mike.

I've found automotive glass to be extremely tough. Windshields have expansion zones in them -- the area with all those dots, around the edges of the glass. So they're designed to expand and contract.

I've never had occasion to pour boiling water on a windshield (and you just say the water is hot, not boiling), but I've poured hot water on a car many times, and I've never seen a windshield break.

And I can think of some real-life examples where windshields are subject to quick temperature changes: In the winter when you go to the car wash and your cold windshield gets blasted with hot water and soap, and a blow dryer. Or in the summer, when a sudden thunderstorm rains 50-degree water down on a 130-degree windshield.

So I think you're OK, Mike. Just be careful with the hot water. The bigger danger is slipping on the ice on your way out to the car, and dumping it on your lap.

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