I’ve heard that pouring hot water on a cold windshield will shatter it, but I’ve seen it happen or talked to anyone who actually experienced it. You know our disdain for anecdotal evidence, so I come here for the Straight Dope. 
I, and a lot of people I’ve known, have carried pichers of hot tap water out and poured it on frosty windshields with zero ill effects. But I’ve never lived in a truly frozen part of the country.
I ask 'cause my lazy old (64 yrs) butt is looking at getting one of these babies.
Peace,
mangeorge
BTW; It heats the fluid to 125-135F
One frigid desert morning I started the car and turned on the defroster. I went inside to do something, and I heard what sounded like a .22 shot. When I went outside there was a long crack along the bottom of my windshield. I surmised that the warm air was enough to cause a temperature differential sufficient to crack it. Pouring water on an icy windshield won’t shatter it, as they’re designed not to shatter. But it may well crack it.
It’s happened to me before, when I lived in Colorado. Didn’t shatter it, but significantly cracked the windshield.
I’ve also had windshields crack just from the defroster (after letting the car warm up without the defroster on and then as soon as I turned on the defroster full-hot: crrrrrack), but it was in subzero temperatures-- not like anything you’re going to get in the Bay area.
Story I was told by co-worker who was from Mexico and married into Chicago [I’m sure many of us can sympathize; I myself married into Chicago from the lower midwest]:
First winter in the states, he went out one morning to find the windshield of the car he and his wife shared all iced up. He had no idea what the normal procedure was in this situation, but there seemed a logical solution. His wife was still showering, oblivious, as he heated the pan of water. Wouldn’t she be surprised that he figured out what to do! Wouldn’t she be thrilled with his thoughtfulness!
…
Us, wincing, “So, did it crack the windshield?”
Him, shrugging, “No. It just made more ice.”
Well, speaking from experience, the opposite (a summer hot interior to a car) plus cold water from a hose (meant to wash said car) led to a huge crack in my first car’s windshield. I imagine the opposite conditions leading to a cracked windshield is equally true under at least some circumstance. But I’ve never heard of a windshield shattering.
I had a windshield crack as Johnny L.A. and GreasyJack describe. I swept the snow off the windshield itself, but didn’t clear it away from the base, where the windshield wipers are, because I could see well enough.
Then I turned the heater on high.
Cracked all along the snowline at the base of the windshield.
On the other hand, I’ve seen water poured on a frosted up windshield, and it didn’t crack, but the surrounding temperature was only a bit below freezing (this was in South Australia in their winter).
So, I wouldn’t be surprised if cold water could cause a windshield to crack, in the right circumstances. I certainly wouldn’t use it in our winters up here in Saskatchewan. That’s what scrapers are for.
One typical Arizona summer day, I used my windshield “squirter” to clean the windshield. The result was an immediate crack from top to bottom. I don’t do that any more…
It seems that many luxury cars have heaters for their winshield washers.
Some GM vehicles did, but discontinued them because of shorts (electrical, not Hanes) in the wiring and not related to the water temp.
I’m thinking the problem might be in the volume of warm water.
Also, some windshields are improperly installed (not bedded?) and are under stress all the time.
It’s the same thing that happened to another SDMB poster. Except his was home cooking and not hot water that cracked the glass.
I suspect the likelihood of this happening goes up orders of magnitude if you already have an imperfection in your windshield glass, like a rock chip. The cracks will usually emanate from there.
As a Canadian I am surprised someone would use hot water to clear ice off a windshield. Seems a lot of bother, if no ice scraper is handy, a cd, or cassette case case or any thing with 2+ inches of edge (preferably plastic) will do the trick. Start the car, set it to defrost and crank the fan, scrape your window and go. If its truly cold start your car five minutes before you have to leave.
I have to assume people only would use hot water if they are really unfamiliar with the problem, because really all you need is five minutes or almost any somewhat rigid plastic object.
That’s been my experience. I’ve poured warm (not hot) water on iced-up windshields – no cracks. But I had a chip in a windshield and the first cold day when I turned on the defroster, the chip became a crack.
Now I keep a respectable distance when I’m behind a truck loaded with gravel, and semis.
In the dead of winter (probably -15C) I had taken the car to a car wash. It was filthy with salt and road grime and the windows (all of them) were difficult to see out of. The very small crack just above the rear view mirror turned into a foot long crack as the wash started. The film between the layers of glass kept the inside layer intact, but the crack was obvious and we had to replace the windshield.
They sell those heaters here as well, so the likelyhood of damage must be limited to glass that already has minor damage. Also the temperature difference will also determine the chance of cracking. I would doubt that the Bay area would require too much caution. By the same token, I’m not sure how much benefit would be realized over simple winter washer fluid (not water).
I’ve had it happen more the other way: warm to cold. One of my first Canadian winters, there was a freak-day of mild weather with temperatures getting almost balmy. Overnight there was a very sudden cold snap that dropped temperatures very fast. My rear car window shattered. “Shattered” as in the full spider-web grid thing happened and most of the window had dropped into the backseat like translucent gravel.
My buddy and I looked at what was left still attached to the window frame, and it kind of looked like the glass was shaped in a bit of a wave. The top three inches that were left of my window in corss-section looked undulated, sort of like: ~~~~ when it should have looked like: ------.
Our best guess was that the metal strip that held the window in place had contracted way faster than the glass itself. If that’swhat happened, I can see how throwing hot water on the glass could have a similar effect, only by expanding the glass faster than the cold frame.
ETA: :mad: the “undulating” part is supposed to look like a bunch of squiggles. WHen I see it in the Dope font, it looks all straight.
I suspect a neighbour’s riding lawnmower kicked up a piece of gravel, which then his the lower-rear window of my Prius. It also shattered and fell apart. (Why this guy felt the need to ride his mower out of his yard, drive on the road the length of my property, and then turn around in some gravel across the street I’ll never know.) I suspect that on many or most cars the rear glass is not laminated, as the laminated glass (‘shatterproof glass’) is intended to be a safety measure in a forward crash.
I was a passenger in the back seat of my parent’s Horizon one subzero night when the rear windshield shattered as a result of the electric defroster being turned on. Glass gravel ended up all over the back deck of the car, down my neck, etc. I suspect that there was a narrow spot in the wiring that heated up faster than the other sections and led to the break; once it started, though, it was catastrophic.
Hm, well, here’s one from the opposite experience.
My husband warmed water and poured it on the windshield of both the car and the truck a number of times last winter (Canada) with no problems at all. I’d say he did this around 50 or 60 times over the winter season.
Hi, neighbor. 
Actually, as an USAian, it’s not a lot of bother to turn on the windshield washer which sprays a mist of hot (warm by the time it hits the glass) water.
And I think that might be key to this discussion, that the volume of heated mist compared to the mass of the ice and glass is so small that it quickly gives up it’s heat, melting the ice.
Warmed air from the defroster keeps coming at a high temperature, and is concentrated at a relatively small point on the glass. You can see that as it clears the frost on the inside of your windshield.
Make sense?
A few years back, a crew of us from Minnesota were on a business trip to Alabama, and at a convenience store, saw a miniature windshield scraper, with about a 1/4 inch blade! We were astonished, both that they ever had to scrape ice from their windshields in Alabama, and that they used such a tiny scraper to do it (but maybe they only got little patches of ice?).
Turned out those were for scraping off the covering on scratch-off lottery tickets. (Apparently lots of Alabamians have big nails, that they don’t want to damage by actually scratching off the scratch-off lottery tickets.)
We persuaded the store clerk to give us several of them. They made neat gag Xmas gifts for relatives back in Minnesota!