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The other day I made a question about the External Power Hatch. Now I would want to know what's that hole below the door.

737

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That is the outflow valve. The outflow valve allows air to leave the pressurized section of the fuselage. The aircraft's pressurization system controls the valve to keep the desired pressure, balancing the air coming in through the bleed system.

Outflow valve closeup
Source

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Outflow valve, controls cabin pressurisation. To pressurise the cabin, close this valve. Pump air into the cabin, stop the air flowing out, cabin pressurises. The valve doesn't completely close, but is about 98% closed. You still need some airflow going through the cabin. The cabin is pressurised to give an approximate equivalent altitude of 8,000 ft, whilst the aircraft is flying at 30,000 ft+.

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Looks like the potable water supply point.

B737-700

Image from 737 Airport planning document

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    $\begingroup$ No ... Potable water is on the captain's side, while the outflow valve - referenced in the OP - is on the FO side. $\endgroup$ – Ralph J Jan 30 '16 at 2:06

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