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Does the air of the fan in the interior of the car comes from outside air or is it the same air that circulate inside, when the air conditioner is off and heat is also off (just the fan) ? or is it an option available in all vehicles to choose between the 2 options ?

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    There is usually a button to select between 'exterior air' and 'recirculated air'. Some a/c settings will set it automatically. The 'default' is external air, as people do not want to breathe stale air, which will also mist up the interior glass very readily. In most vehicles I have driven, when the vehicle is moving, the 'ram' effect is enough to introduce fresh air even when the fan is off.
    – Weather Vane
    Jun 22, 2021 at 12:20
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    Jun 22, 2021 at 19:56

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By default most cars have an outside air inlet which supplies fresh air to the passenger compartment. There is usually a control on the HVAC panel that allows you to select air recirculation which closes the outside air inlet and just reuses the existing air.

Usually you won't want to do this for very long as moisture quickly accumulates and will fog your windows.

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  • This may be an OSHA ( or ??? ) rule . Bring in outside air to flush any possible exhaust gas leak into the passenger area.
    – blacksmith37
    Jun 22, 2021 at 16:33
  • @blacksmith37 on my previous Mk I Octavia, the air source would automatically change to 'recirculate' when put in reverse gear, presumably so that driving back though the exhaust gases would not allow them to enter the cabin. I have always wondered why the exhaust of cars is at ground level, though: in traffic the exhaust from the car in front comes straight in.
    – Weather Vane
    Jun 22, 2021 at 16:59
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    Here in the US, even if set to "recirculate," some outside air is allowed to enter the passenger compartment. I'll bet it's the same in other countries.
    – DavidRecallsMonica
    Jun 22, 2021 at 19:46
  • Yes @ David... , that is what I was trying to say . And very likely likely the manufacturers use the same heat/AC plenum for all countries.
    – blacksmith37
    Nov 20, 2021 at 15:22

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