Air circulation option

My 2001 330ci has an air circulation option. One is supposedly used when the AC system is set to auto. The other is for "non" auto. I can't think of a
single reason to have this switch. When re-circulation the air in the cabin, what difference does it make if the system is set to auto or non-auto? TIA
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The concept is that in auto mode, the system will automatically switch to recirc when it encounters contaminated air (smoke, fumes, etc).
R / John
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So why have a switch at all? Why not just let it be "always on" and keep the smelly stuff out? Seems to be just one more thing that can go wrong.
Chris

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Perhaps that's why there is a switch? If it went wrong it might default to the setting you didn't want?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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How does it know the outside air is bad?
rtt

the
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It just noes.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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London SW

Even as far back as early E32's some BMWs have had particulate matter sensors that changed the airflow to recirc until either the incoming air was clean again or a certain number of minutes (8? 20? can't remember quite) had elapsed and the cabin air was going to be getting stale with CO2 from respiration.
-Russ.
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Bmw's like most high priced cars have an sensor mounted in the air intkae of your heating system that sniffs the air temp and air quality and adjust your recurculating flap to open of closed for economy and quality reasons thats a good thing
Dave Plowman (News) wrote:

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In Auto, it is supposed to go to Recirc by itself if it smells foul air. You can set the system to Auto, and then manually set Recirc to ON so the system Recircs all of the time. You may want to do this when it's raining or foggy because Recirc air will be dryer than outside air, and this will help the glass to clear.

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My experience is exactly the opposite. Inside air fogs the windshield and outside air clears it off.
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If the AC is on, Recirc should provide drier air than pulling from the outside. If the AC is off, then the moisture in the air will simply build and build if Recirs is on, then the windows will fog even worse. Setting the Air Conditioner on is key. Some cars, not BMW, automatically set the AC on and Recirc the air when Defrost is selected. I prefer the way BMW allows us to set the life support systems however we want, but all automakers are not so thoughtful.

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That's the qualifier you needed to make....because the original poster didn't distinguish between AC on or AC off.
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Thanks everyone.

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...so, it somehow knows a bad smell and keeps it from entering the cabin? If the answer is yes, how does it know???
face="Lucida Sans Unicode">...<BR>&gt;&gt; My 2001 330ci has an air circulation option. One is supposedly used when <BR>&gt;&gt; the AC system is set to auto. The other is for "non" auto. I can't think <BR>&gt;&gt; of a single reason to have this switch. When re-circulation the air in the <BR>&gt;&gt; cabin, what difference does it make if the system is set to auto or <BR>&gt;&gt; non-auto?&nbsp; TIA<BR>&gt;&gt; <BR>&gt; <BR>&gt;</FONT></BODY></HTML>
------=
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Two things, turn off the HTML (Rich Text), and I don't know how it knows that you are driving past a pasture or behind a bus. Certainly this is not rocket science though, and is a relatively simple thing to ascertain. Clean air has a certain electrical property, and if that property changes it is safe to assume that the air isn't so fresh anymore.

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On Fri, 29 Jul 2005 15:01:28 -0700, "Jeff Strickland"

Probably a bit like how your nose can recognize the smell of a fart in 100 milliseconds or so. The problem is making an electrical sensor/circuit that will do it. Evidently, the motor makers already did that. Now they need to develop the technology that will clearly identify the offender, open the door and push him/her out of the car.
--
Dan.

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wrote:

Or open the sunroof and launch the rockets ...
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Think you've misunderstood the meaning. Auto recirc has no link to auto on the climate control - it simply defaults to fresh air, but changes to recirc if it detects noisome air. It explains this in the driver's handbook.
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