Date: 201703
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We had planned takeoff on a different runway. While single engine taxi out; First Officer (FO) advised me he was going to run the numbers for the new runway along with changing our FMC. Once that was completed; I called for the number 1 engine to be started. Once that was completed and we approached the departure end of the runway; we were number two for departure. At that time I briefed the new departure procedure with waypoints and restrictions while the Pilot Monitoring (PM) followed along on the JeppFD-Pro charts. We also briefed a new emergency return to airport procedure (if needed) for the new runway assignment. Once complete I called for and we completed the Departure Plan Checklist. At that time I dinged the Flight Attendants (FA); did my configuration throttle check and armed the auto throttles; and called for the Before Takeoff Checklist. By that time the 737 in front of us was cleared into position and cleared for takeoff; he turned on his strobes which caused us to be temporarily affected by the brightness of the strobes into our eyes. We were partially through our checklist when we were given line up and wait instructions; which we replied; and then went back to our checklist repeating the last item covered on the checklist. We departed on the RNAV Departure to our assigned initial altitude of 10;000 feet. In our climb we both heard a non-normal rumbling noise and noticed an abnormal pressurization. We leveled off at 10;000 feet and requested initially to stay at this altitude while we assessed our situation. We immediately focused on the aircraft doors to make sure we had no unsafe door indications following this we focused on our pressurization controller. Still seeing no indication of a fault in the controller or the outflow valve we consulted the QRH unscheduled pressurization change checklist. While the PM ran through this checklist I spoke with the Flight Attendants and advised them of our situation and to make sure everything was ok in the back. One Flight Attendant said she was feeling a little light headed; so I requested to descend in order to lessen the effects of possible hypoxia. I then coordinated with Dispatch and local Operations for a return to ZZZ. I then [advised ATC of emergency condition] for our return to comply with Flight Operations Manual requirement for an overweight landing. I advised the Passengers of our situation and that we would be returning back to ZZZ for a precautionary landing and advised the Flight Attendants to secure the cabin for arrival. At this time I conferred with my First Officer on what we had already done and if there was anything else he could think we needed to accomplish for a return. Then I briefed the visual to Runway XYR and called for a Descent Checklist followed by the Approach Checklist. It was at that time we discovered that the pack switches were in the off position. We immediately placed the packs into the auto position and continued with the checklist and returned to ZZZ.Anytime and especially at night extra due diligence needs to be made crosschecking essential switch positions. I know the First Officer called packs on the checklist I know I looked over and in the lighting conditions and the lasting effect of the preceding aircraft's strobe lights on our night vision; I saw what I thought were in the auto position. Anything short of reaching over and visually and physically verifying the switch position I don't know what else more could have been done. Also in the unscheduled pressurization change checklist an earlier reference to pack switches perhaps in the same area as pressurization mode selector - auto the next step should read packs- auto/ high. This would cause us to reference the packs switches prior to the only time they are referred to in the Approach Checklist in the QRH procedure. The packs switches would normally be the logical and normal area I would look at in an aircraft not pressurizing situation; however; the unusual rumbling noise caused me to focus on other potential causes of our pressurization problem. A certain level of complacency crept in that was made worse with looking into strobe lights when checking pack switch position during before takeoff checklist. For future and especially in night conditions I will follow along checklist not only visually but also physically by touch each switch and knob on checklist to insure proper position.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737NG flight crew reported they did not turn on the pressurization pack valves and consequently the aircraft did not pressurize.