In message , Johny B Good
writes
On Thu, 27 Nov 2014 09:38:21 +0000 (GMT), "Dave Liquorice"
wrote:

On Wed, 26 Nov 2014 14:13:23 +0000, Johny B Good wrote:

Back in the days when I was a wage slave, I found the most effective
way to de-ice the glass on early morning winter journeys was to use a
1 litre jug of warm water (about 40 deg C).

Depends how cold it has been. Air temp approaching -10 C under clear
skies and the windscreen and frost will be approaching -20 C. Luke
warm water has been known to freeze and with the glass at -20 ish you
have a 60 C temperature differance. The thermal shock might not be
healthy for the windscreen...

Thermal shock was one of my concerns. Normally I'd pour a gentle
trickle across the top of the glass to limit the heat transfer rate,
allowing more water to pour onto the glass after the first few seconds.

If it had been an extremely cold night, I'd start the pour from just
above the windscreen so it hit the metalwork of the roof first before
going onto the glass itself to reduce the initial temperature of the
water as it hit the glass (I tended to use slightly hotter water on
such early mornings to compensate so I knew to avoid applying the water
immediately to the glass.

I must have been doing it right since in the several years I'd been
practicing this de-icing technique, I enjoyed the good fortune of not
shattering the glass by thermal shock (I'l admit to this possibly being
simply my 'dumb luck' :-)


I have used that technique for some years on different cars (not bother
on the heated windscreen.

Never had any problems with screens cracking or anything (even with
existing chips etc.).

I have had the water refreeze when the windscreen was esp. cold, or the
water not warm enough but another jug of warm water sorted it..

I think my FIL does the same and he has never had a problem
--
Chris French