Date: 201308
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Exterior Pax/Crew Door
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: This was the second leg of a five-leg day. At about two minutes after takeoff and an altitude of 400 FT; we started noticing pressure in our ears. I checked the FLT/GND switch and it was correctly in the FLT position; however; the cabin rate of climb was around 1;000 FPM (unusual; but not completely abnormal for a -300 aircraft). We continued to climb and; approaching 10;000 FT; the 'A' Flight Attendant called to inform us of a loud noise coming from the forward entry door. We could hear through the service interphone what sounded like the door seal hadn't completely seated yet. We completed the Climb Checklist; noting that the cabin rate was still climbing higher than normal; but all other indications on the pressurization panel were normal. We talked about the possibility of the future use of the QRH for abnormal pressurization; but now the cabin altitude was only 5;000 FT. We continued our climb (cleared to FL230) and; at approximately 15;000 FT we heard a loud pop. We expected that the door seal had finally seated and I asked the First Officer to check with the flight attendants to see if the noise had stopped. While the First Officer was on the service interphone; I noticed that the cabin rate was now negative. This would be a normal indication if the door seal had indeed seated and the plane was now trying to adjust the cabin altitude and differential pressure down. I slowed the aircraft rate of climb to 500 FPM so the First Officer and I could discuss our indications on the flight deck; information from the flight attendants; use of QRH; and continued our climb and proceeded onward to our destination. We decided to not use the QRH for abnormal pressurization since the cabin altitude was only 6;000 FT and the pressurization controller was operating normally and placing the packs to high would make the negative cabin rate worse. We heard another loud pop from the door seal and leveled the aircraft at 18;000 FT. Even though the cabin altitude was still decreasing; we decided to return to [departure airport] and coordinated with ARTCC for a slow descent and return. We started our descent; retrieved the ATIS; and I talked to the passengers. We soon received an Off Schedule Descent light and complied with the QRH to reset the flight altitude indicator. We calculated OPC Landing data and sent a message to Dispatch letting them know we were returning. The rest of the flight was without incident. Once at the gate; the forward entry door couldn't be opened by normal means from the inside or outside. The door was finally opened simultaneously using the inside and outside handles. The passengers were deplaned and the aircraft was turned over to Maintenance.


Second narrative: [Narrative 2 had no additional information]


Synopsis: B737-300 flight crew reported pressurization difficulties involving forward entry door seal. They returned to departure airport where they found the door very difficult to open.