The main problem is how quick it freezes.
If you turns to ice before all the dirt is removed the window will still
be dirty when it melts.
Also ice can scratch the glass.
Nothing you can do unless you heat the water. Turning the flow up with cold water will not make it any better- the people who say different live down south & don't experience the temps you do on a 24 hour basis!
Heated water comes into it's own between 0 & -5 (approx) Less than -5 & you're stuffed anyway.
This is my third winter with an immersion heater & it's been priceless!!
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Spot on I'd say. Once the temperature is that low, adding more cold water will only compound the problem on the glass.
Although I've never heard of ice scratching glass.
