B737-700 FAILS TO PRESSURIZE DURING CLB DUE TO OUTFLOW VALVE SELECTOR BEING LEFT IN THE MANUAL MODE DURING PREFLT.

Narrative

I WAS THE CAPT FLYING ON THE FIRST LEG OF OUR 3 DAY TRIP. WX WAS VMC AND WE WERE FLYING THE SID OUT OF ZZZ. ON CLB OUT; WE HEARD THE CABIN ALT WARNING HORN. IT TOOK US A SHORT TIME TO DIAGNOSE THE CABIN WAS NOT PRESSURIZING (I FIRST THOUGHT WE HAD A FAULTY CONFIGURATION WARNING HORN). WE BEGIN A LEVEL OFF SOON AFTER HEARING THE HORN. UPON REALIZING THE CABIN WAS NOT PRESSURIZING; I SAW THAT THE OUTFLOW VALVE SELECTOR WAS IN THE MAN POS. THE FO THEN BEGAN TO CLOSE THE OUTFLOW VALVE AND I CALLED FOR THE CABIN ALT WARNING HORN/ABNORMAL PRESSURIZATION CHKLIST WHILE I LEVELED THE ACFT AND BEGAN A DSCNT TO 10000 FT. I THEN GAVE THE FO THE ACFT AND TALKED TO THE FLT ATTENDANTS WHO INFORMED ME THAT THE OXYGENERATOR MASKS HAD DEPLOYED IN THE PAX CABIN. I HAD THOUGHT THAT WE NEVER ASCENDED ABOVE 13300 FT AND WAS A LITTLE SURPRISED THAT THE MASKS HAD DROPPED. AT THAT POINT; I COORDINATED WITH ATC TO RETURN TO ZZZ AND BRIEFED THE PAX THAT WE WERE GOING TO MAKE A NORMAL LNDG AT ZZZ. WE RETURNED FOR AN UNEVENTFUL LNDG. DURING HIS PREFLT; THE FO HAD BEEN DISTRACTED WHILE DOING THE PRESSURIZATION CHK AND INADVERTENTLY LEFT THE OUTFLOW VALVE IN THE MAN POS. I DID NOT NOTICE IT DURING THE BEFORE START ORIGINATING CHKLIST. MY NORMAL SCAN IS TO LOOK FOR THE CORRECT CRUISE ALT; THE DEST FIELD ALT AND THAT THE OUTFLOW VALVE IS IN AUTO. EVEN THOUGH I BELIEVE I HAVE HEARD THAT THE CABIN ALT WARNING HORN SOUNDS LIKE THE CONFIGURATION WARNING HORN; IT STILL TOOK ME TOO LONG TO DETERMINE WHAT WAS ACTUALLY HAPPENING. MY FIRST REACTION WAS THAT WE HAD A FAULTY CONFIGURATION INDICATION.SUPPLEMENTAL INFO FROM ACN 696643: ONE CONTRIBUTING FACTOR IS THAT WHEN I WAS DOING MY COCKPIT PREFLT; I WAS ASKED BY THE FLT ATTENDANT IF I NEEDED ANYTHING TO DRINK. THIS HAPPENED TO BE WHEN I WAS DOING THE PRESSURIZATION PANEL CHK WITH THE SELECTOR IN THE 'MAN' POS. I TOLD THE FLT ATTENDANT THAT I WANTED A WATER. HE GOT MY WATER AND I CONTINUED MY PREFLT -- NEVER GETTING BACK TO THE PRESSURIZATION PANEL.