A CRJ 200 CREW DIVERTED BECAUSE OF AN UNUSUAL BLEED CONFIGURATION DURING DE ICE PROCEDURES. THE DIVERSION WAS UNNECESSARY IF THEY LANDED UNPRESSURIZED.

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CAPT AND I WERE IN THE DSCNT PHASE ON VECTORS TO SHOOT AN ILS RWY 33 INTO ZZZ1. THE WX ON THE FIELD WAS LOW VISIBILITY AND OVCST; SNOW WITH POOR BRAKING ACTION ON ALL SURFACES. I STARTED THE APU ABOUT 30 NM OUT AND CONTINUED SETTING UP FOR THE ILS. SOON AFTER; WE RECEIVED A TRIPLE-CHIME WITH AN APU OVERTEMP WARNING MESSAGE. WE REALIZED THE APU DOOR HAD NEVER FULLY OPENED. WE SHUT DOWN THE APU IN ACCORDANCE WITH THE QRH AND PROCEEDED TO DISCUSS HOW IT AFFECTED THE APCH. I INFORMED ZZZ1 APCH OF THE SITUATION AND REQUESTED DELAYING VECTORS WHILE WE GOT IT SORTED OUT. WE BOTH KNEW WE WERE UNAUTH TO LAND THE ACFT WITH A BLEED MISCONFIGN CAUTION MESSAGE THAT WOULD OCCUR WHEN USING THE ANTI-ICE WITHOUT AN APU. WE AGREED THAT BECAUSE OF THIS; WE WOULD NEED TO DIVERT TO A FIELD WHERE ANTI-ICE WASN'T REQUIRED. I THEN HAD A RADIO PATCH TO OUR DISPATCHER TO RELAY OUR SITUATION. AS THAT WAS BEING DONE; I GRABBED MY PLT HANDBOOK AND FOUND THE AREA COVERING THE BLEED MISCONFIGN MESSAGE AND READ THROUGH IT. AT THE POINT IN TIME IT SEEMED TO CONFIRM OUR INITIAL THOUGHTS THAT WE WERE UNAUTH TO LAND IN ICING CONDITIONS WITHOUT AN APU. OUR DISPATCHER CAME ON; AND AFTER HEARING THE SITUATION; RELAYED THE WX AND BURN FOR A DIVERSION INTO ZZZ2. I TOLD APCH OF OUR INTENTION TO DIVERT TO ZZZ2 AND HE STARTED CLB AND PASSED US TO CTR. I GAVE CAPT THE RADIOS AND CALLED THE FLT ATTENDANT TO BRIEF HER. I THEN BRIEFED THE PAX; AND CHKED BACK WITH THE FO. CTR THEN RELAYED THAT DISPATCH WANTED TO GET A HOLD OF US AGAIN; SO I WENT BACK ON COM #2 TO TALK TO THEM. THEY SAID THAT AFTER TALKING WITH MAINT THEY WEREN'T SURE WHY A DIVERSION WAS NECESSARY WITH THE APU INOP. I STILL HAD THE PLT HANDBOOK OPEN AND GUIDED THEM TO THE 'NOTE' REGARDING THE BLEED MISCONFIGN SCENARIO. THE DISPATCHER ACKNOWLEDGED THAT HE UNDERSTOOD NOW AND WOULD RADIO AHEAD TO ZZZ2 OPS AND MAINT. WE LANDED UNEVENTFULLY AND DEPLANED THE PAX. CAPT WENT INSIDE TO CALL THE CHIEF PLT AND RELAY THE SCENARIO TO HIM. WHEN CAPT CAME BACK; HE TOLD ME THE CHIEF PLT AGREED WITH OUR ACTIONS UNDER THE CIRCUMSTANCES; BUT NOTED THAT WE ARE ACTUALLY PERMITTED TO COMMENCE LNDG UNPRESSURIZED IF THE APU IS DOWN AND IN ICING CONDITIONS. NEITHER CAPT NOR MYSELF HAD EVER BEEN IN THAT PARTICULAR SITUATION AND BOTH ASSUMED THE ONLY WAY WE WERE PERMITTED TO CLR THE MISCONFIGN WAS TO EITHER XFER THE BLEEDS TO THE APU OR TURN OFF THE ANTI-ICE. IT WASN'T UNTIL CAPT RELAYED THE INFO FROM THE CHIEF PLT THAT I WENT BACK TO CAREFULLY RE-READ THE PLT HANDBOOK AND DISCOVERED THAT THE VERY LAST SENTENCE IN THE 'NOTE' SHOWS THAT AN UNPRESSURIZED LNDG IS PERMITTED IN SUCH A SCENARIO. I THINK THERE ARE A FEW FACTORS THAT CONTRIBUTED TO THIS DEV. FIRST; NEITHER CAPT (A SENIOR CAPT OF 17 YRS WITH ACR) NOR MYSELF (3.5 YRS HERE) HAD EVER EXPERIENCED THIS PARTICULAR SCENARIO. IN CONTRAST; I HAVE NEEDED TO DEPART A FIELD MAYBE TWICE UNPRESSURIZED DUE TO AN APU OUT AND LOW OVCST; HAD NEVER LANDED UNPRESSURIZED. IN HINDSIGHT; I NOW KNOW I SHOULD HAVE APPLIED THAT KNOWLEDGE TO THIS SCENARIO SINCE IT REALLY ISN'T THAT DIFFERENT A SITUATION. SECOND; THIS PHASE OF FLT WAS PARTICULARLY BUSY -- SETTING UP AN APCH TO NEAR MINIMUMS IS EXTREMELY UNFAVORABLE ICING AND FIELD CONDITIONS; WARNING MESSAGE; DELAYING VECTORS; DETERMINING OPTIONS; BRIEFING FLT ATTENDANT; PAX; OPS; ETC. KNOWING THIS; I BELIEVE I READ THE PLT HANDBOOK PARAGRAPH TOO QUICKLY AND MUST HAVE GLAZED THE LAST SENTENCE. THIRD; POSSIBLY IF WE HAD TALKED TO MAINT OURSELVES; WE MAY HAVE DISCOVERED OUR ERROR IN THINKING VERSUS LEARNING ABOUT IT ON THE GND IN ZZZ2. REGARDLESS; I DO BELIEVE WE TOOK THE SAFEST ACTION BASED ON THE ABOVE CONDITIONS.