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Sitting on the plane yesterday, wanting some fresh air and boom it hit me [literally]. What would happen, let's say both static and mid flight, if everyone turned on there air vents at the same time?

Could the unit handle all of it simultaneously, would it effect pressure, I'm not privy to the working of air units of aircraft so it may be a silly question but it's still a question that I'm interested in knowing the answer to.

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    $\begingroup$ Absolutely nothing will happen. The vents have no effect on the air cycle machines or circulation fans. $\endgroup$ – casey Nov 4 '15 at 23:20
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@Casey is correct. Nothing will happen under normal operating conditions. The FAR §25.831 Ventilation requires,

... the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.

In general, the amount of air available is far greater than the requirement. On the ground, compressed air for the Environmental Control System, which supplies air to crew and passengers can be obtained from an APU, a GCU, airport high-pressure hydrants, or the aircraft engines. In flight, however, compressed air is obtained almost exclusively from the compressor stages of the aircraft engines.

The pressure would not be affected by this. According to FAR §25.843, Tests for pressurized cabins, the pressurized cabins are tested to show the performance of

... the pressure supply, pressure and flow regulators, indicators, and warning signals, in steady and stepped climbs and descents at rates corresponding to the maximum attainable within the operating limitations of the airplane, up to the maximum altitude for which certification is requested.

Note that in some aircraft the air conditioning system is switched off during climb- this is to do with the load on engine and not the air supply.

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    $\begingroup$ New aircraft designs may also use an independent compressed air source rather than the engines. This is the case of the B787 where Boeing switched from engine air to electrically-powered pressure supply to increase engines efficiency (and possibly air quality) and decrease weight and pneumatic system complexity. $\endgroup$ – mins Nov 5 '15 at 6:57
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To add to @aeroalias' excellent answer: compressed outside air (whether from the engines or separate compressors) is fed into the plane's air conditioning system, from where it's distributed to the cabin, the cockpit, etc. The air destined for the cabin is routed to air outlets (sometimes called diffusers), installed above and/or below the hatracks (picture - look for some long, thin openings near the hatracks next time you fly), which blow the air into the cabin. Air is blown into the cabin to control the temperature in the cabin and to ensure a constant supply of fresh air for the passengers.

The air gaspers (what you call "air vents") above the passengers are (usually) connected to the same ducts which feed the air outlets, which means that when you open a gasper, a tiny amount of the air which would normally enter the cabin through an air outlet enters it through the open gasper instead. Therefore, even if everyone opened their gaspers, the total amount of air going to the cabin wouldn't change - except that some of it would be routed via the gaspers instead of the "normal" air outlets. However, even in this case, the majority of the air would still be routed to the air outlets.

(Note that when I say "routed", it's all done passively via the design of the air conditioning ducts and manifolds; there is no "active" routing (via valves or suchlike) between the air outlets and the gaspers.)

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