Matt
The device is meant to be used to "defrost" the rear window.  Frost as
in ice as in cold as in needs clearing to be able to see through.
It's not as though you could use it to boil water or melt glass. It
slowly warms up the filaments and the surrounding glass.  It does not
"shock" the frost and glass.

Use it for what it was installed to do.

It's -32 C ( - 25.6 F) here today and I would not hesitate to use
mine, if it would start.  : >

Don
On 12-Dec-09, at 11:48 AM, Automatic digest processor wrote:

> Date:    Fri, 11 Dec 2009 22:15:27 -0700
> From:    Bob Stevens <mtbiker62@GMAIL.COM>
> Subject: Re: [WetWesties] Friday question- will this break your
> window??
>
> They are mis-applying the science. If you were to pour hot water
> on a very chilled windshield, likely you'd crack it where there are
> chips.
> It
> happens when volks are having their cars worked on and already
> have a chip or small/short crack. The shop people pull the vehicle
> into a very warm garage, out of very cold weather, and boing....
> the crack-runner appears. Happened to a Nissan Pathfinder of mine
> once long ago. The molecules change size quickly and the weak
> spot is exposed.
>
> bob
>
> On Fri, Dec 11, 2009 at 6:36 PM, Rob <becida@comcast.net> wrote:
>
>> At  12/11/2009 02:08 PM,Matthew wrote:
>>
>> it was around zero degrees f here this week. A coworker claims that
>>> if i use my rear defrost it will break the rear window? Before i
>>> call Tom and Ray, will someone chime in??  it seems a bit
>>> improbable.
>>>
>>> thanks
>>> Skeptic Matt
>>