AN EMB145'S AUTOMATIC PRESSURIZATION CONTROL UNIT FAILED AT CRUISE. AN IMMEDIATE EMERGENCY DESCENT WAS BEGUN BUT THE CREW DID NOT DECLARE AN EMERGENCY. THE DUMP AND MANUAL PUSHBUTTONS ARE NOT IDENTIFIED ON THE PRESSURIZATION CONTROL PANEL.

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I WAS THE FO; AND PF FOR FLT FROM ZZZ TO ZZZ1 ON NOV/XA/08. APPROX 100 MI SE OF ABC INTXN WE ENCOUNTERED THE FOLLOWING MESSAGES ON THE EICAS: PRESN AUTO FAIL. IMMEDIATELY; WE COULD FEEL THE EFFECTS OF CABIN DEPRESSURIZATION IN OUR EARS; AND NOTICED THAT THE CABIN WAS CLBING AT AN ALARMING RATE (APPROX 3000 FPM). THE MOST IMMEDIATE THOUGHT THAT CAME TO BOTH ME AND THE CAPT WAS TO DSND. WE QUERIED ATC IMMEDIATELY AND NOTIFIED THEM THAT WE NEEDED A LOWER ALT IMMEDIATELY. UNFORTUNATELY; ATC DIDN'T RESPOND IMMEDIATELY; AND THE CABIN CONTINUED TO CLB. WE STARTED TO DSND AT A RAPID RATE (4000 FPM) AS WE OBTAINED CLRNC FROM ATC TO DSND FROM FL350 TO FL250. THE APPROX TIME WE DSNDED WITHOUT CLRNC WAS NO MORE THAN 5-10 SECONDS. DURING THE DSCNT I CALLED FOR THE PRESN AUTO FAIL CHKLIST; AND MY CAPT PROCEEDED TO RUN THE CHKLIST. THE PRESSURIZATION SYS WAS SWITCHED INTO MANUAL MODE AND THE CABIN PRESSURIZATION WAS STABILIZED. ATC NOTIFIED US THAT THE SUPVR WOULD LIKE TO SPEAK TO THE CAPT AFTER WE ARRIVED; AND PROCEEDED TO GIVE US A PHONE NUMBER. UPON TALKING TO THEM ONCE REACHING ZZZ1; IT WAS DETERMINED THAT WE SHOULD HAVE DECLARED AN EMER IN ORDER TO JUSTIFY OUR ALTDEV PRIOR TO RECEIVING A CLRNC. WE WERE TOLD THAT THE ATC WAS NOT GOING TO FILE ANY PAPERWORK; AS THERE WERE NO ACFT WITHIN OUR VICINITY. THE SUPVR WAS VERY KIND AND UNDERSTANDING; AND BOTH THE CAPT AND I AGREED THAT DECLARING AN EMER WOULD HAVE BEEN THE BEST THING TO DO. LOOKING BACK IS JUST TOO EASY. IT WAS DETERMINED THAT THE AUTO PRESSURIZATION UNIT FAILED WHEN WE RECEIVED THE EICAS MESSAGE: PRESN AUTO FAIL; AND WE FELT THE RAPID ASCENSION OF THE CABIN ALT IN OUR EARS. IT WAS EVIDENT THAT WE DEVIATED ALT AS SOON AS I INITIATED A DSCNT. ONCE WE RECEIVED THE EICAS MESSAGE; WE DETERMINED THE BEST CORRECTIVE ACTION WAS A RAPID DSCNT. I INITIATED A RAPID DSCNT BY SELECTING A 4000 FPM DSCNT IN THE FGCU; THE AUTOPLT WAS ENGAGED THROUGHOUT THE ENTIRE MANEUVER. DURING THE DSCNT I CALLED FOR THE CHKLIST PROC FOR AUTO PRESSURIZATION FAILURE; AND THE CAPT READ AND COMPLIED WITH THE LISTED MATERIAL. THE FAILURE OF THE AUTO PRESSURIZATION UNIT WAS OUT OF OUR CTL. THE ALTDEV WAS A RESULT OF OUR OWN CORRECTIVE ACTION WITHOUT FIRST CONSULTING THE APPROPRIATE CHKLIST. WE GOT A BIT TOO EXCITED AND DID NOT WAIT FOR THE APPROPRIATE ATC CLRNC BEFORE VACATING OUR ASSIGNED ALT. AFTER REVIEWING THE CHKLIST; IT IS EVIDENT THAT A DSCNT IS ONLY REQUIRED FOR THE SECOND PART OF THE CHKLIST; NOT THE FIRST; WHICH IS WHAT WE DID. THE PROB WITH THIS SITUATION WAS THAT THE CAPT AND I PANICKED A BIT AT THE RAPID RATE AT WHICH THE CABIN ALT WAS ASCENDING. RATHER THAN CALLING FOR THE CHKLIST IMMEDIATELY AS I SHOULD HAVE; WE INITIATED A RAPID DSCNT. THE CAPT AND I WERE AFRAID THAT THE RATE WAS GOING TO CONTINUE TO INCREASE; THEREFORE LEADING TO THE RAPID MANEUVER. I THINK THAT THIS PARTICULAR EVENT AND CHKLIST SHOULD BE STRESSED DURING THE TRAINING PROCESS IN ORDER TO PREVENT WHAT WE DID. ALSO; IN COMPLYING WITH THE CHKLIST; THE CAPT AND I DISCOVERED THAT THERE IS A PROB IN THE EMB145; IN THAT THE 2 GUARDED SWITCHES (DUMP PUSHBUTTON; AND MANUAL PUSHBUTTON) ON THE AUTO PRESSURIZATION SYS ARE UNIDENTIFIABLE. IT IS REQUIRED IN THE CHKLIST FOR THIS EVENT THAT THE MANUAL PUSHBUTTON BE SELECTED; BUT THE CAPT AND I COULD NOT ASCERTAIN WHICH BUTTON WAS THE DUMP BUTTON OR THE MANUAL BUTTON. WE DID NOT WANT TO PUSH EITHER WITHOUT KNOWING WHICH WAS WHICH. IT TOOK A CONVERSATION WITH MAINT CTL TO ASCERTAIN WHICH BUTTON WAS WHICH. WHEN THE BUTTON IS PRESSED; THE FUNCTION OF THE BUTTON IS THEN ILLUMINATED. THIS NEEDS TO BE VISUAL BEFORE THE BUTTON IS PUSHED! I STRONGLY RECOMMEND THAT THE CHKLIST BE EDITED TO SAY WHICH BUTTON IS WHICH.