EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew FLC Overrode Automation
Aircraft; Weather
Descending through precipitation; we accumulated moderate rime ice on the windscreen. I looked up at the anti-ice valves and saw they were closed. I waited approximately a minute and a half for them to open [automatically]; then I manually selected them on; after which we received the Anti-Ice On No Icing message on the EICAS. The ice detectors did not detect ice for two to three minutes after overriding the automatic function. After the anti-ice system automatically turned on it stayed on for five consecutive minutes. The start of the event was around 17;000 FT and a TAT -16C. The event ended around 11;000 FT and a TAT -11C. We had no further issues.
An EMB-145 Captain noted that; although they had been accumulating rime ice on the windshield the icing valves had not automatically opened.  When he opened them manually the EICAS opined 'Anti-ice On/No Icing.'