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But airplanes are still turning off their vents during the boarding process, to save money.

Is this asservation, which can be subdivided into two premises, true?

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    $\begingroup$ To quote the reddit comment, "But airplanes are still turning off their vents during the boarding process, to save money." That sentence has no meaning because airliners don't have "vents", they have air conditioning systems, recirculation fans and pressurization outflow valves (which are always open on the ground). Depending on the equipment present at the gate at a particualr airport pressurized air can be hooked up at the gate so that there is airflow to the air conditioning system. $\endgroup$ Nov 27 '21 at 8:20
  • $\begingroup$ @JetCityMatt, of course airliners have vents, there's one at every seat! A vent is just the opening where airflow from the air conditioning system enters the cabin. $\endgroup$ Nov 27 '21 at 16:51
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    $\begingroup$ @MichaelHall - that may be what they are referring to, but even then "airplanes are still turning off their vents" doesn't make sense - the vents I have seen so far are purely manual, so the only way to turn them off is to literally turn them by hand. $\endgroup$
    – rob74
    Nov 29 '21 at 14:54
  • $\begingroup$ I suspect (@JetCityMatt et al.) that "vents" is being - badly - used as an abbreviation for "ventilation (systems)". But this generous interpretation of the language probably shouldn't extend to the claims $\endgroup$
    – Chris H
    Nov 29 '21 at 16:06
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    $\begingroup$ @rob74, you are right, and if you want to split hairs over literal interpretations you could also argue that “airplanes” can’t “turn off” their own vents, it would require a person turning each one by hand to close them. But really, it doesn’t take a big stretch of the imagination to realize that the OP is referring to the stoppage of airflow through the vents, (the piece that pax see) via the pilot securing a valve within the system. $\endgroup$ Nov 29 '21 at 16:17

Do commercial airliners turn off vents during boarding?

No. First, as @rob74 highlighted, overhead vents cannot be remotely closed. To stop air from being refreshed, air-conditioners must be shut off, which is possible. However they are not.

Air is circulated for the purpose of regulating temperature and humidity within the cabin. If air weren't supplied, temperature in the empty cabin would be the same than outside and would increase above acceptable limits with passengers boarding. Air circulation, heating/cooling, relative humidity control, and pressurization are a single system based on air-conditioning packs. This system also regulates temperature in the cargo, in the electronics bays, in the cockpit and around the instruments and electronic devices behind the cockpit panels.

Packs (air-conditioning equipments) are fed with air from compressors. In flight this is bleed air from engine compressors, when engines are off, this is air from the auxiliary power unit (APU), which is a small turbine engine producing no thrust. Some recent aircraft have electric compressors to not use bleed air.

enter image description here

Air flow in the cabin, adapted from source

To save money?

While compressed air and electricity on the ground can be delivered by the APU, operators often prefer to use preconditioned air (PCA) and electricity delivered by airport units. This is a cheaper solution, which doesn't involve aircraft systems and reduces the need for maintenance.

PCA units can be individual units located on jet bridges, centralized industrial units accessed using ground outlets at gates, or mobile PCA units:

enter image description here

Image sources: left, right, bottom

PCA units size increases over time with aircraft size, they are easy to spot with their yellow pipes.

IATA recommendation regarding AC related to COVID

IATA, the union of air transport operators, themselves insists in measures to ensure a good circulation of air, prior and during the boarding and disembarkation:

For those aircraft with air conditioning, run the air conditioning packs (with bleed air provided by the APU) or supply air via external Pre-Conditioned Air (PCA) source at least 10 minutes prior to the boarding process, throughout boarding and during disembarkation. Avoid operations without the air-conditioning.

More: Guidance for Flight Operations During and Post Pandemic

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    $\begingroup$ Without cooled air, the temperature inside the cabin would warm up above the outside temperature if you have a lot of warm, breathing people in the cabin. Even on a winter day, without airflow, a full cabin quickly becomes hot & stuffy. An empty cabin is a different case. $\endgroup$
    – Ralph J
    Nov 27 '21 at 16:56
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    $\begingroup$ Those mobile units are huge! Amazing to think that all that equipment is carried around on the aircraft, as well. Of course, a big portion of that mobile PCA is an engine running a compressor and, well, those are kinda built into the engines that make the plane fly, but still, that's a lot of equipment. $\endgroup$
    – FreeMan
    Nov 29 '21 at 12:57
  • $\begingroup$ @FreeMan: Also note there is a PCA unit delivering air at 2°C, followed by a cooling unit to reach -9°C. This cooling unit is specific to wide bodies in hot places, to accelerate cooling. $\endgroup$
    – mins
    Nov 29 '21 at 13:09
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    $\begingroup$ @OrganicMarble: I detailed the pneumatic system of the A320 in this answer. From it this schematics, Air from ground supply enters a mixing and distribution unit like air extracted from the cargo holds by recirculation fans. Actual design varies by aircraft. The principle schematics in the current answer is not entirely representative of an actual pneumatic system. $\endgroup$
    – mins
    Nov 29 '21 at 23:00
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    $\begingroup$ Thanks for the informative links! $\endgroup$ Nov 29 '21 at 23:41

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