Has anyone ever cracked a frozen windscreen

[mummybongo]

Has anyone ever cracked their frozen windscreen by pouring warm water on it to de-ice it rather than using spray de-icer or scraping?

Just wondering how likely it is to happen?

I use a windscreen cover - they work really well. For the side windows I use warm tap water in a ziplock bag and sort of "rub/sweep" it over, like a hot water bottle I suppose - I don't pour the water because it can then freeze on the driveway and cause a slip hazard.

I've been using warm water for 40 years. Half fill the kettle, then once boiled, fill up with cold water, a quick check that it's "hand warm" and pour it over the windscreen and all other windows. Never had any cracks.

We have a heater in one of our vehicles, you put it on remotely from indoors. Go out to clear windscreen, ready to drive.

But doesn’t pouring water in the car then leave a puddle of water to freeze?

Well I've just sterilised 8 large jam jars for pickled beetroots with boiling water. The jars were straight out of the conservatory where the ambient temperature is floating just above 0. Guess how many broke from having boiling water poured in them?

That's how many windscreens will break from using not even boiling water (frankly you could use boiling water on an undamaged windscreen easily enough).

It's all in the physics. The secret is actually how quickly the glass heats up as much as how much it warms up as it has a very low coefficient of thermal expansion, but rapid enough or large enough warming can create enough expansion to break it.

So the two keys are to reduce the temperature gradient (so I use boiling water half diluted with cold, so very roughly 50C), and to pour slowly, so that the glass doesn't warm too rapidly (the small volumes of water at a time cool quickly and glass has a very low coefficient of thermal conductivity, so heats up slowly).

You don’t actually say how many broke…

No, I use the still-warm water from the previous nights hot water bottle. Not caused an issue.

Yeah - me too. Always use cold water and it works just fine

Assigning an exact percentage to the likelihood of cracking a frozen windshield

A general assessment based on the common scenarios.

Low Risk (20-30%): If the temperature difference between the frozen windshield and the warm water is relatively small, and the windshield is in good condition without existing stress or damage, the risk of cracking may be lower.

Moderate Risk (30-50%): In cases where there is a significant temperature difference, or if the windshield has pre-existing stress, the likelihood of cracking increases. The risk is also influenced by how evenly the warm water is distributed across the surface.

High Risk (50% and above): Pouring very hot water on a severely frozen windshield, especially in extremely cold conditions, significantly raises the risk of cracking. If the windshield is already damaged or compromised, the likelihood of cracking is higher.

No because I don't use water to defrost but I did have a massive crack one summer totally at random and the man who came to fix it asked if I'd done this as he said that was what it looked like! Literally split overnight only spotted it as I was on my way for a sweep being 10 days overdue!

Never had or seen or heard from others. But then nobody i know would be stupid enough to use hot water. I use cool to tepid water. Sometimes have to throw it at the handle/drivers door so I can get in but usually it's for windows and the lights, front and back. Quick swish, use wipers, blower on, drive away. I use a 4pt milk carton so I can leave it next to the fence on the drive and not leave my car unattended. And no, there is no icy puddle to return to if you are careful.

I live somewhere that we get huge sheets of thick ice on our cars. I have never even heard of pouring water on the car to melt the ice. That sounds crazy.

i have scraped so many times. Bundle up and put on good gloves and a hat because it’s about -17c outside and the wind is whipping around. It may also be sleeting so the ice is building as you scrape. Turn on car and set the defrost to high, this is the biggest help. Start scraping with the biggest, baddest scraper you could find at the store.

I've used lukewarm water to defrost several times and it didn't crack my windscreen but it was once so cold that it just froze causing more ice. I bought a job lot of spray from Costco and always had a can in the front and a couple in the boot after that.

My daughter slipped and fell off a sheet of ice on a pavement- I assume this was the reason for a random sheet of thick ice on the pavement.

And I saw.multiple people slip on that bit of ice as well.

I don't use water because I park on the street and it would leave a big frozen puddle on the floor. I scrape and it works fine, even for my ancient car that needs a good 15 minutes to warm up.

My mum used to use hot kettle.water every winter to deice her car when I wax growing up. Never cracked a windscreen!

Not once in 25 years of driving nor do I know of anybody else who has.

I don't know anybody that pours water on their windshields but I am curious about this now and going to ask people.

Ooh please can you tell us more about this?

I can tell my Leaf to get the car ready for a particular time and it clears all the ice and heats the inside ready to go. Except it's much cheaper to fill the kettle with lukewarm water from the tap and throw it over the windscreen. 🤷 Never been a problem.

the heater in the vehicle that can be activated remotely from indoors is likely an engine block heater integrated with a remote start system.

Yep.

I don’t do any of this. I press the button that heats up both front and rear windscreens, wait 60 seconds, put on wipers and it’s gone.

the heater in the vehicle that can be activated remotely from indoors is likely an engine block heater integrated with a remote start system.

I just use an ordinary fan heater from Argos. I turn it from inside (activate it remotely) from the switch on the electricity socket.

@Floopdifloo it's a Mercedes, came as an optional extra