The temperature wouldn't be 100 C out of the boiler as that is when water turns to steam, probably more like 70 C at the brush head at the very most. I can understand having the water as hot as you can get it though as it makes cleaning so much easier, I find about 50ish good in this weather.
It also depends how long you are going to be on a window for as well. If spending minutes on a pane then the 20C above ambient temperature is a good rule, if only seconds the keep it hot.
Simon.