B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 39; Flight Crew Total 3248; Flight Crew Type 3248
Communication Breakdown
Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Fuel Issue; Ground Event / Encounter Ground Equipment Issue
General None Reported / Taken
Human Factors; Procedure
We arrived in ZZZ at XA:00 local time. The aircraft was to remain overnight and we were asked to leave the APU up and packs on due to cold overnight temperatures. This method was used by the ground personnel in lieu of a ground heat cart since one was not available. We left the APU running as requested and left the airport. Upon returning to the airplane the next day we found the left wing tank which the APU was being feed from only containing 200 pounds of fuel. This is a concern as there is no one on the aircraft overnight to monitor the fuel low pressure lights. If there was a delay or other situation this aircraft would have run the left tank dry causing the APU to shut down. Most importantly there would be an unmonitored low pressure fuel low pressure light. The other concern for this configuration was the outflow valve was closed manually with the APU and packs on. I understand this configuration with an external air cart on the aircraft; but not certain it is appropriate for the APU configuration. If the main cabin door were to be closed this would pressurize the aircraft completely making it impossible to access unless the fire handle is pulled in the wheel well to shut down the APU.
B737 Captain reported ground personnel requested APU on overnight; but failed to monitor low fuel pressure.