Both the AS 355 that we had and the EC 135 suffered from similar problems. The AS 355 heater did demist....eventually. The EC 135 has an unfortunate design flaw of a 'box' in the heater ducting which does seem to collect standing water. The best way to avoid the problem is (if you have moist clammy conditions), to put the heater on full blast prior to take-off, if the 120 is anything like the 135 then that should cause the screens to mist up almost instantly, after a minute or so the screens shopuld clear and then...problem gone (for that flight anyway)!