Date: 201808
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Daylight
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: Aircraft Cooling System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Upon acceptance of the Dispatch Release we noticed there was a [Maintenance Release] Defect Report stating that the cabin temperature was too warm; and that the crew was unable to control the temperature through normal procedures. The discrepancy report did not indicate whether the aircraft was on the ground; or airborne at the time of the malfunction. The corrective action indicated the fault was corrected and that all temperature perimeters were within the normal range; and that the aircraft was returned to service.With this particular [Maintenance Release] Defect reported the day before; as well as the high surface temperatures; I elected to start both engines and assess cabin temperature from air supplied to the PACKS directly from the engine-bleeds prior to departure. Since the temperature of the conditioned air as supplied from the APU Bleed; as well as the Engine Bleeds was satisfactory to the entire crew on the ground; we departed on time.At approximately 10;000 feet MSL; we noticed the temperature of the air leaving the vents on the flight-deck becoming uncomfortably warm. Soon afterward the [Lead Flight Attendant] called and stated same problem was occurring in the cabin.As we were unable to control the temperature normally; I directed the execution of the non-normal checklist for Cabin Temperature Hot. If the Cabin Temperature cannot be controlled with the non-normal checklist; the procedure eventually requires descent to 10;000 feet MSL or below and opening the outflow-valve to allow better ventilation. It also requires an immediate landing at the nearest suitable airport.As we were complying with the non-normal checklist; I immediately [advised ATC] in accordance with FOM (Flight Operations Manual) 2.10.1 as well as the fact that we would perform an overweight landing. I completed the TEST briefing with the [Lead Flight Attendant] and the cabin was not prepared for evacuation. We informed the company of our plan to air-return and the passengers were briefed on the situation.We landed at [the departure airport] without incident and taxied to the gate without further assistance. Although temperature within the aircraft was uncomfortably warm; at no time did any crew member; and/or passenger ask for or require medical assistance.We debriefed maintenance personnel in great detail about the malfunction. I also conducted a de-brief in accordance with 'The Debrief' checklist card and FOM 3.70 with my First Officer. Additionally; I called and debriefed the [Chief Pilot].I would like to take this opportunity to recognize my First Officer and my [Lead Flight Attendant] for their professionalism and proficiency during this event. In addition; TRACON; as well as ARFF were outstanding.After a period of 20-minutes after my initial de-brief with the [Chief Pilot]; I called the [Chief Pilot] back and informed him that we were Fit For Duty and elected to continue the flight with another aircraft. We eventually completed the mission and safely delivered our passengers to [their destination].




Synopsis: B737NG Captain reported that the temperature in the cabin and cockpit was uncontrollable and it became too hot.