A demister is a thin heating pad bonded to the back of the glass. It warms the mirror a few degrees above the room, so steam cannot condense on it. The centre of the mirror stays clear while the rest of the bathroom fogs up.
It switches on with the light and needs no separate control. Turn the mirror on before you shower rather than after, so the glass never fogs in the first place. It draws about as much power as a small LED bulb.
The pad covers the centre of the glass, roughly the area you look into at a basin. On the largest sizes the far edges may still fog for a minute after a long, hot shower.
Built in at no extra cost. Each product page says whether that model has one
Covered by the warranty like the LED and driver
