Narrative:

I was the captain flying on the first leg of our 3 day trip. WX was VMC and we were flying the SID out of ZZZ. On climb out; we heard the cabin altitude 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 position. The first officer then began to close the outflow valve and I called for the cabin altitude warning horn/abnormal pressurization checklist while I leveled the aircraft and began a descent to 10000 ft. I then gave the first officer the aircraft and talked to the flight attendants who informed me that the oxygenerator masks had deployed in the passenger 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 passenger that we were going to make a normal landing at ZZZ. We returned for an uneventful landing. During his preflight; the first officer had been distracted while doing the pressurization check and inadvertently left the outflow valve in the man position. I did not notice it during the before start originating checklist. My normal scan is to look for the correct cruise altitude; the destination field altitude and that the outflow valve is in automatic. Even though I believe I have heard that the cabin altitude 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 information from acn 696643: one contributing factor is that when I was doing my cockpit preflight; I was asked by the flight attendant if I needed anything to drink. This happened to be when I was doing the pressurization panel check with the selector in the 'man' position. I told the flight attendant that I wanted a water. He got my water and I continued my preflight -- never getting back to the pressurization panel.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.